Linguistic rights

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https://en.wikipedia.org/wiki/Linguistic_rights 

Linguistic rights are the human and civil rights concerning the individual and collective right to choose the language or languages for communication in a private or public atmosphere. Other parameters for analyzing linguistic rights include the degree of territoriality, amount of positivity, orientation in terms of assimilation or maintenance, and overtness.

Linguistic rights include, among others, the right to one's own language in legal, administrative and judicial acts, language education, and media in a language understood and freely chosen by those concerned.

Linguistic rights in international law are usually dealt in the broader framework of cultural and educational rights.

Important documents for linguistic rights include the Universal Declaration of Linguistic Rights (1996), the European Charter for Regional or Minority Languages (1992), the Convention on the Rights of the Child (1989) and the Framework Convention for the Protection of National Minorities (1988), as well as Convention against Discrimination in Education and the International Covenant on Civil and Political Rights (1966).

History

Linguistic rights became more and more prominent throughout the course of history as language came to be increasingly seen as a part of nationhood. Although policies and legislation involving language have been in effect in early European history, these were often cases where a language was being imposed upon people while other languages or dialects were neglected. Most of the initial literature on linguistic rights came from countries where linguistic and/or national divisions grounded in linguistic diversity have resulted in linguistic rights playing a vital role in maintaining stability. However, it was not until the 1900s that linguistic rights gained official status in politics and international accords.

Linguistic rights were first included as an international human right in the Universal Declaration of Human Rights in 1948.

Formal treaty-based language rights are mostly concerned with minority rights. The history of such language rights can be split into five phases.

1. Pre-1815. Language rights are covered in bilateral agreements, but not in international treaties, e.g. Treaty of Lausanne (1923).
2. Final Act of the Congress of Vienna (1815). The conclusion to Napoleon I's empire-building was signed by 7 European major powers. It granted the right to use Polish to Poles in Poznan alongside German for official business. Also, some national constitutions protects the language rights of national minorities, e.g. Austrian Constitutional Law of 1867 grants ethnic minorities the right to develop their nationality and language.
3. Between World I and World War II. Under the aegis of the League of Nations, Peace Treaties and major multilateral and international conventions carried clauses protecting minorities in Central and Eastern Europe, e.g., the right to private use of any language, and provision for instruction in primary schools through medium of own language. Many national constitutions followed this trend. But not all signatories provided rights to minority groups within their own borders such as United Kingdom, France, and US. Treaties also provided right of complaint to League of Nations and International Court of Justice.
4. 1945–1970s. International legislation for protection of human rights was undertaken within infrastructure of United Nations. Mainly for individual rights and collective rights to oppressed groups for self-determination.
5. Early 1970s onwards, there was a renewed interest in rights of minorities, including language rights of minorities. e.g. UN Declaration on the Rights of Persons Belonging to National or Ethnic, Religious and Linguistic Minorities.

Theoretical discussion

Language rights + human rights = linguistic human rights (LHR)

Some make a distinction between language rights and linguistic human rights because the former concept covers a much wider scope. Thus, not all language rights are LHR, although all LHR are language rights. One way of distinguishing language rights from LHR is between what is necessary, and what is enrichment-oriented. Necessary rights, as in human rights, are those needed for basic needs and for living a dignified life, e.g. language-related identity, access to mother tongue(s), right of access to an official language, no enforced language shift, access to formal primary education based on language, and the right for minority groups to perpetuate as a distinct group, with own languages. Enrichment rights are above basic needs, e.g. right to learn foreign languages.

Individual linguistic rights

The most basic definition of linguistic rights is the right of individuals to use their language with other members of their linguistic group, regardless of the status of their language. They evolve from general human rights, in particular: non-discrimination, freedom of expression, right to private life, and the right of members of a linguistic minority to use their language with other members of their community.

Individual linguistic rights are provided for in the Universal Declaration of Human Rights:

• Article 2 – all individuals are entitled to the rights declared without discrimination based on language.
• Article 10 – individuals are entitled to a fair trial, and this is generally recognized to involve the right to an interpreter if an individual does not understand the language used in criminal court proceedings, or in a criminal accusation. The individual has the right to have the interpreter translate the proceedings, including court documents.
• Article 19 – individuals have the right to freedom of expression, including the right to choose any language as the medium of expression.
• Article 26 – everyone has the right to education, with relevance to the language of medium of instruction.

Linguistic rights can be applied to the private arena and the public domain.

Private use of language

Most treaties or language rights documents distinguish between the private use of a language by individuals and the use of a language by public authorities. Existing international human rights mandate that all individuals have the right to private and family life, freedom of expression, non-discrimination and/or the right of persons belonging to a linguistic minority to use their language with other members of their group. The United Nations Human Rights Committee defines privacy as:

... the sphere of a person's life in which he or she can freely express his or her identity, be it by entering into relationships with others or alone. The Committee is of the view that a person's surname [and name] constitutes an important component of one's identity and that the protection against arbitrary or unlawful interference with one's privacy includes the protection against arbitrary or unlawful interference with the right to choose and change one's own name.

This means that individuals have the right to have their name or surname in their own language, regardless of whether the language is official or recognised, and state or public authorities cannot interfere with this right arbitrarily or unlawfully.

Linguistic rights in the public domain

The public domain, with respect to language use, can be divided into judicial proceedings and general use by public officials.

According to Article 10 of the Universal Declaration of Human Rights, individuals have the right to a fair trial. Therefore, in the name of fairness of judicial proceedings, it is an established linguistic right of an individual to an interpreter when he or she does not understand the language used in criminal court proceedings, or in a criminal accusation. The public authorities must either use the language which the individual understands, or hire an interpreter to translate the proceedings, including court cases.

General use by public officials can cover matters including public education, public radio and television broadcasting, the provision of services to the public, and so on. It is often accepted to be reasonable and justified for public officials to use the language of minorities, to an appropriate degree and level in their activities, when the numbers and geographic concentration of the speakers of a minority language are substantial enough. However, this is a contentious topic as the decision of substantiation is often arbitrary. The International Covenant on Civil and Political Rights, Article 26, does promise to protect all individuals from discrimination on the grounds of language. Following that, Article 27 declares, "minorities shall not be denied the right... to use their own language". The Convention against Discrimination in Education, Article 5, also does declares the rights for minorities to "use or teach their own language".

Collective linguistic rights

Collective linguistic rights are linguistic rights of a group, notably a language group or a state. Collective rights mean "the right of a linguistic group to ensure the survival of its language and to transmit the language to future generations". Language groups are complex and more difficult to demarcate than states. Part of this difficulty is that members within language groups assign different roles to their language, and there are also difficulties in defining a language. Some states have legal provisions for the safeguard of collective linguistic rights because there are clear-cut situations under particular historical and social circumstances.

Collective linguistic rights apply to states because they express themselves in one or more languages. Generally, the language régime of states, which is communicated through allocation of statuses to languages used within its boundaries, qualifies linguistic rights claimed by groups and individuals in the name of efficient governance, in the best interest of the common good. States are held in check by international conventions and the demands of the citizens. Linguistic rights translate to laws differently from country to country, as there is no generally accepted standard legal definition.

Territoriality vs. personality principles

The principle of territoriality refers to linguistic rights being focused solely within a territory, whereas the principle of personality depends on the linguistic status of the person(s) involved. An example of the application of territoriality is the case of Switzerland, where linguistic rights are defined within clearly divided language-based cantons. An example of the application of personality is in federal Canadian legislation, which grants the right to services in French or English regardless of territory.

Negative vs. positive rights

Negative linguistic rights mean the right for the exercise of language without the interference of the State. Positive linguistic rights require positive action by the State involving the use of public money, such as public education in a specific language, or state-provided services in a particular language.

Assimilation-oriented vs. maintenance-oriented

Assimilation-oriented types of language rights refer to the aim of the law to assimilate all citizens within the country, and range from prohibition to toleration. An example of prohibition type laws is the treatment of Kurds in Turkey as well as Turks in Iran, where they are forbidden to use the Kurdish and Turkish languages respectively. Assimilation-oriented approaches to language rights can also be seen as a form of focus on the individuals right to communicate with others inside a system. Many policies of linguistic assimilation being tied to the concept of nation building and facilitating communication between various groups inside of a singular state system.

Maintenance-oriented types of language rights refer to laws aiming to enable the maintenance of all languages within a country, and range from permission to promotion. An example of laws that promote language rights is the Basque Normalization Law, where the Basque language is promoted. Many maintenance-oriented approaches require both a framework of collective and positive rights and significant government funding in order to produce the desired outcomes of linguistic maintenance. In Wales and Quebec, for example, there is significant debate over funding and the use of collective rights in building an effective maintenance framework.

The neutral point between assimilation-orientation and maintenance-orientation is non-discrimination prescription, which forbids discrimination based on language. However the non-discrimination position has also been seen as just another form of assimilationist policy as its primarily just leads to a more extended period of assimilation into the majority language rather than a perpetual continuation of the minority language.

Overt vs. covert

Another dimension for analyzing language rights is with degree of overtness and covertness. Degree of overtness refers to the extent laws or covenants are explicit with respect to language rights, and covertness the reverse. For example, Indian laws are overt in promoting language rights, whereas the English Language Amendments to the US Constitution are overt prohibition. The Charter of the United Nations, the Universal Declaration of Human Rights, the International Covenant on Economic, Social and Cultural Rights, the International Covenant on Civil and Political Rights, and the UN Convention on the Rights of the Child all fall under covert toleration.

Criticisms of the framework of linguistic human rights

Some have criticized linguistic rights proponents for taking language to be a single coherent construct, pointing out instead the difference between language and speech communities, and putting too much concern on inter-language discrimination rather than intra-language discrimination.

Other issues pointed out are the assumptions that the collective aims of linguistic minority groups are uniform, and that the concept of collective rights is not without its problems.

There is also the protest against the framework of Linguistic Human Rights singling out minority languages for special treatment, causing limited resources to be distributed unfairly. This has led to a call for deeper ethnographic and historiographic study into the relationship between speakers' attitudes, speakers' meaning, language, power, and speech communities.

Practical application

Linguistic rights manifest as legislation (the passing of a law), subsequently becoming a statute to be enforced. Language legislation delimiting official usage can by grouped into official, institutionalizing, standardizing, and liberal language legislation, based on its function:

Official legislation makes languages official in the domains of legislation, justice, public administration, and education, [commonly according to territoriality and personality]. Various combinations of both principles are also used.... Institutionalizing legislation covers the unofficial domains of labour, communications, culture, commerce, and business....

In relation to legislation, a causal effect of linguistic rights is language policy. The field of language planning falls under language policy. There are three types of language planning: status planning (uses of language), acquisition planning (users of language), and corpus planning (language itself).

Language rights at international and regional levels

International platform

See also: Universal Declaration of Linguistic Rights, International Covenant on Civil and Political Rights, and Convention on the Rights of the Child

The Universal Declaration of Linguistic Rights was approved on 6 June 1996 in Barcelona, Spain. It was the culmination of work by a committee of 50 experts under the auspices of UNESCO. Signatories were 220 persons from over 90 states, representing NGOs and International PEN Clubs Centres. This Declaration was drawn up in response to calls for linguistic rights as a fundamental human right at the 12th Seminar of the International Association for the Development of Intercultural Communication and the Final Declaration of the General Assembly of the International Federation of Modern Language Teachers. Linguistic rights in this Declaration stems from the language community, i.e., collective rights, and explicitly includes both regional and immigrant minority languages.

Overall, this document is divided into sections including: Concepts, General Principles, Overall linguistic regime (which covers Public administration and official bodies, Education, Proper names, Communications media and new technologies, Culture, and The socioeconomic sphere), Additional Dispositions, and Final Dispositions. So for instance, linguistic rights are granted equally to all language communities under Article 10, and to everyone, the right to use any language of choice in the private and family sphere under Article 12. Other Articles details the right to use or choice of languages in education, public, and legal arenas.

There are a number of other documents on the international level granting linguistic rights. The UN International Covenant on Civil and Political Rights, adopted by the UN General Assembly in 1966 makes international law provision for protection of minorities. Article 27 states that individuals of linguistic minorities cannot be denied the right to use their own language.

The UN Declaration on the Rights of Persons Belonging to National or Ethnic, Religious and Linguistic Minorities was adopted by the UN General Assembly in 1992. Article 4 makes "certain modest obligations on states". It states that states should provide individuals belonging to minority groups with sufficient opportunities for education in their mother tongue, or instruction with their mother tongue as the medium of instruction. However, this Declaration is non-binding.

A third document adopted by the UN General Assembly in 1989, which makes provisions for linguistic rights is the Convention on the Rights of the Child. In this convention, Articles 29 and 30 declare respect for the child's own cultural identity, language and values, even when those are different from the country of residence, and the right for the child to use his or her own language, in spite of the child's minority or immigrant status.

Regional platform

Africa

See also: List of Linguistic Rights in Constitutions (Africa)

Linguistic rights in Africa have only come into focus in recent years. In 1963, the Organisation of African Unity (OAU) was formed to help defend the fundamental human rights of all Africans. It adopted in 1981 the African Charter on Human and Peoples' Rights, which aims to promote and protect fundamental human rights, including language rights, in Africa. In 2004, fifteen member states ratified the Protocol to the African Charter on Human and Peoples' Rights Establishing the African Court on Human and Peoples' Rights. The Court is a regional, legal platform that monitors and promotes the AU states' compliance with the African Charter on Human and Peoples' Rights. It is currently pending a merger with the Court of Justice of the African Union.

In 2001 the President of the Republic of Mali, in conjunction with the OAU, set up the foundation for the African Academy of Languages (ACALAN) to "work for the promotion and harmonisation of languages in Africa". Along with the inauguration of the Interim Governing Board of the ACALAN, the African Union declared 2006 as the Year of African Languages (YOAL).

In 2002, the OAU was disbanded and replaced by the African Union (AU). The AU adopted the Constitutive Act previously drawn up by the OAU in 2000. In Article 25, it is stated that the working languages of the Union and its institutions are Arabic, English, French and Portuguese, and if possible, all African languages. The AU also recognizes the national languages of each of its member institutions as stated in their national constitutions. In 2003, the AU adopted a protocol amending the Act such that working languages shall be renamed as official languages, and would encompass Spanish, Kiswahili and "any other African language" in addition to the four aforementioned languages . However, this Amendment has yet to be put into force, and the AU continues to use only the four working languages for its publications.

Europe

See also: European Charter for Regional or Minority Languages, Framework Convention for the Protection of National Minorities, and List of Linguistic Rights in Constitutions (Europe)

The Council of Europe adopted the European Convention on Human Rights in 1950, which makes some reference to linguistic rights. In Article 5.2, reasons for arrest and charges have to be communicated in a language understood by the person. Secondly, Article 6.3 grants an interpreter for free in a court, if the language used cannot be spoken or understood.

The Council for Local and Regional Authorities, part of the Council of Europe, formulated the European Charter for Regional or Minority Languages in 1992. This Charter grants recognition, protection, and promotion to regional and/or minority languages in European states, though explicitly not immigrant languages, in domains of "education, judicial authorities, administrative and public services, media, cultural activities, and socio-economic life" in Articles 8 to 13. Provisions under this Charter are enforced every three years by a committee. States choose which regional and/or minority languages to include.

The Framework Convention for the Protection of National Minorities was implemented by the Council of Europe in 1995 as a "parallel activity" to the Charter for Regional or Minority Languages. This Framework makes provisions for the right of national minorities to preserve their language in Article 5, for the encouragement of "mutual respect and understanding and co-operation among all persons living on their territory", regardless of language, especially in "fields of education, culture and the media" in Article 6. Article 6 also aims to protect persons from discrimination based on language.

Another document adopted by the Council of Europe's Parliamentary Assembly in 1998 is the Recommendation 1383 on Linguistic Diversification. It encourages a wider variety of languages taught in Council of Europe member states in Article 5. It also recommends language education to include languages of non-native groups in Article 8.

Language rights in different countries

Australia

See also: Australian Aboriginal languages

Zuckermann et al. (2014) proposed the enactment of "Native Tongue Title", an ex gratia compensation scheme for the loss of indigenous languages in Australia: "Although some Australian states have enacted ex gratia compensation schemes for the victims of the Stolen Generations policies, the victims of linguicide (language killing) are largely overlooked ... Existing grant schemes to support Aboriginal languages ... should be complemented with compensation schemes, which are based on a claim of right. The proposed compensation scheme for the loss of Aboriginal languages should support the effort to reclaim and revive the lost languages.

On October 11, 2017, the New South Wales (NSW) parliament passed a legislation that recognises and revives indigenous languages for the first time in Australia's history. "The NSW Government will appoint independent panel of Aboriginal language experts" and "establish languages centres".

Austria

See also: Constitution of Austria

Under the Austrian Constitutional Law (1867), Article 8(2) grants the right to maintenance and development of nationality and language to all ethnic minorities, equal rights to all languages used within the regions in domains of education, administration and public life, as well as the right to education in their own language for ethnic communities, without the necessity of acquiring a second language used in the province.

Canada

See also: Constitution of Canada

The Canadian Charter of Rights and Freedoms (1982) grants positive linguistic rights, by guaranteeing state responsibility to the French and English language communities. Section 23 declares three types of rights for Canadian citizens speaking French or English as their mother tongue and are minorities in a region. The first accords right of access to instruction in the medium of the mother tongue. The second assures educational facilities for minority languages. The third endows French and English language minorities the right to maintain and develop their own educational facilities. This control can take the form of "exclusive decision-making authority over the expenditure of funds, the appointment and direction of the administration, instructional programs, the recruitment of teachers and personnel, and the making of agreements for education and services". All of these rights apply to primary and secondary education, sustained on public funds, and depend on the numbers and circumstances.

China

Standard Chinese is promoted, which has been seen as damaging to the varieties of Chinese by some of the speakers of those languages. Efforts to protect the varieties of Chinese have been made.

Croatia

Main article: Minority languages of Croatia

 

Minority languages in Croatia (official use at local level)

Croatian language is stated to be the official language of Croatia in Article 3 of the Croatian constitution. The same Article of Constitution stipulates that in some of local units, with the Croatian language and Latin script, in official use may be introduced another language or another writing script under the conditions prescribed by law. The only example of the use of minority language at the regional level currently is Istria County where official languages are Croatian and Italian. In eastern Croatia, in Joint Council of Municipalities, at local (municipal) level is introduced Serbian and its Cyrillic script as a co-official language. Each municipality, where a certain minority has more than one third of the population, can if it wants to introduce a minority language in official use.

The only currently excluded minority language in the country is Romani, a non-territorial language, although the reservation is said to be in a process of withdrawal.

Finland

See also: Constitution of Finland

Finland has one of the most overt linguistic rights frameworks. Discrimination based on language is forbidden under the basic rights for all citizens in Finland. Section 17 of the Constitution of Finland explicitly details the right to one's language and culture, although these languages are stated as either Finnish or Swedish. This right applies to in courts of law and other authorities, as well as translated official documents. There is also overt obligation of the state to provide for the "cultural and societal needs of the Finnish-speaking and Swedish-speaking populations of the country on an equal basis". In addition, the Sámi, as an indigenous group, the Roma, and other language communities have the right to maintain and develop their own language. The deaf community is also granted the right to sign language and interpretation or translation. The linguistic rights of the Sámi, the deaf community, and immigrants is further described in separate acts for each group.

Regulations regarding the rights of linguistic minorities in Finland, insist on the forming of a district for the first 9 years of comprehensive school education in each language, in municipalities with both Finnish- and Swedish-speaking children, as long as there is a minimum of 13 students from the language community of that mother tongue.

India

See also: Constitution of India

The constitution of India was first drafted on January 26, 1950. It is estimated that there are about 1500 languages in India. Article 343–345 declared that the official languages of India for communication with centre will be Hindi and English. There are 22 official languages identified by constitution. Article 345 states that "the Legislature of a state may by law adopt any one or more of the languages in use in the State or Hindi as the language or languages to be used for all or any of the official purposes of that State: Provided that, until the Legislature of the State otherwise provides by law, the English language shall continue to be used for those official purposes within the State for which it was being used immediately before the commencement of this Constitution".

Ireland

See also: Languages of Ireland

Language rights in Ireland are recognised in the Constitution of Ireland and in the Official Languages Act.

Irish is the national and first official language according to the Constitution (with English being a second official language). The Constitution permits the public to conduct its business – and every part of its business – with the state solely through Irish.

On 14 July 2003, the President of Ireland signed the Official Languages Act 2003 into law and the provisions of the Act were gradually brought into force over a three-year period. The Act sets out the duties of public bodies regarding the provision of services in Irish and the rights of the public to avail of those services.

The use of Irish on the country's traffic signs is the most visible illustration of the state's policy regarding the official languages. It is a statutory requirement that placenames on signs be in both Irish and English except in the Gaeltacht, where signs are in Irish only.

Mexico

See also: Languages of Mexico

Language rights were recognized in Mexico in 2003 with the General Law of Linguistic Rights for the Indigenous Peoples which established a framework for the conservation, nurturing and development of indigenous languages. It recognizes the countries Many indigenous languages as coofficial National languages, and obligates government to offer all public services in indigenous languages. As of 2014 the goal of offering most public services in indigenous languages has not been met.

Pakistan

See also: Languages of Pakistan

Pakistan uses English (Pakistani English) and Urdu as official languages. Although Urdu serves as the national language and lingua franca and is understood by most of the population, it is natively spoken by only 8% of the population. English is not natively used as a first language, but, for official purposes, about 49% of the population is able to communicate in some form of English. However, major regional languages like Punjabi (spoken by the majority of the population), Sindhi, Pashto, Saraiki, Hindko, Balochi, Brahui and Shina have no official status at the federal level.

Philippines

See also: Languages of the Philippines

Article XIV, Sections 6–9 of the 1987 Philippine constitution mandate the following:

• SECTION 6. The national language of the Philippines is Filipino. As it evolves, it shall be further developed and enriched on the basis of existing Philippine and other languages.

Subject to provisions of law and as the Congress may deem appropriate, the Government shall take steps to initiate and sustain the use of Filipino as a medium of official communication and as language of instruction in the educational system.

• SECTION 7. For purposes of communication and instruction, the official languages of the Philippines are Filipino and, until otherwise provided by law, English.

The regional languages are the auxiliary official languages in the regions and shall serve as auxiliary media of instruction therein.

Spanish and Arabic shall be promoted on a voluntary and optional basis.

• SECTION 8. This Constitution shall be promulgated in Filipino and English and shall be translated into major regional languages, Arabic, and Spanish.

• SECTION 9. The Congress shall establish a national language commission composed of representatives of various regions and disciplines which shall undertake, coordinate, and promote researches for the development, propagation, and preservation of Filipino and other languages.

Spain

See also: Constitution of Spain

The Spanish language is the stated to be the official language of Spain in Article 3 of the Spanish constitution, being the learning of this language compulsory by this same article. However, the constitution makes provisions for other languages of Spain to be official in their respective communities. An example would be the use of the Basque language in the Basque Autonomous Community (BAC). Apart from Spanish, the other co-official languages are Basque, Catalan and Galician.

Sweden

In ratifying the European Charter for Regional or Minority Languages, Sweden declared five national minority languages: Saami, Finnish, Meänkieli, Romani, and Yiddish. Romani and Yiddish are non-territorial minority languages in Sweden and thus their speakers were granted more limited rights than speakers of the other three. After a decade of political debate, Sweden declared Swedish the main language of Sweden with its 2009 Language Act.

United States

See also: Constitution of the United States

Language rights in the United States are usually derived from the Fourteenth Amendment, with its Equal Protection and Due Process Clauses, because they forbid racial and ethnic discrimination, allowing language minorities to use this Amendment to claim their language rights. One example of use of the Due Process Clauses is the Meyer v. Nebraska case which held that a 1919 Nebraska law restricting foreign-language education violated the Due Process clause of the Fourteenth Amendment. Two other cases of major importance to linguistic rights were the Yu Cong Eng v. Trinidad case, which overturned a language-restrictive legislation in the Philippines, declaring that piece of legislation to be "violative of the Due Process and Equal Protection Clauses of the Philippine Autonomy Act of Congress", as well as the Farrington v. Tokushige case, which ruled that the governmental regulation of private schools, particularly to restrict the teaching of languages other than English and Hawaiian, as damaging to the migrant population of Hawaii. Both of these cases were influenced by the Meyer case, which was a precedent.

Disputes over linguistic rights

Basque, Spain

See also: Basque language

The linguistic situation for Basque is a precarious one. The Basque language is considered to be a low language in Spain, where, until about 1982, the Basque language was not used in administration. In 1978, a law was passed allowing for Basque to be used in administration side by side with Spanish in the Basque autonomous communities.

Between 1935 and 1975, the period of Franco's régime, the use of Basque was strictly prohibited, and thus language decline begun to occur as well. However, following the death of Franco, many Basque nationalists demanded that the Basque language be recognized. One of these groups was Euskadi Ta Askatasun (ETA). ETA had initially begun as a nonviolent group to promote Basque language and culture. However, when its demands were not met, it turned violent and evolved into violent separatist groups. Today, ETA's demands for a separate state stem partially from the problem of perceived linguistic discrimination. However, ETA called a permanent cease-fire in October 2011.

Faroe Islands

See also: Faroese language conflict

The Faroese language conflict, which occurred roughly between 1908 and 1938, has been described as political and cultural in nature. The two languages competing to become the official language of the Faroe Islands were Faroese and Danish. In the late 19th and early 20th century, the language of the government, education and Church was Danish, whereas Faroese was the language of the people. The movement towards Faroese language rights and preservation was begun in the 1880s by a group of students. This spread from 1920 onwards to a movement towards using Faroese in the religious and government sector. Faroese and Danish are now both official languages in the Faroe Islands.

Nepal

See also: Nepal Bhasa movement

The Newars of Nepal have been struggling to save their Nepal Bhasa language, culture and identity since the 1920s. Nepal Bhasa was suppressed during the Rana (1846–1951) and Panchayat (1960–1990) regimes leading to language decline. The Ranas forbade writing in Nepal Bhasa and authors were jailed or exiled. Beginning in 1965, the Panchayat system eased out regional languages from the radio and educational institutions, and protestors were put in prison.

After the reinstatement of democracy in 1990, restrictions on publishing were relaxed; but attempts to gain usage in local state entities side by side with Nepali failed. On 1 June 1999, the Supreme Court forbade Kathmandu Metropolitan City from giving official recognition to Nepal Bhasa, and Rajbiraj Municipality and Dhanusa District Development Committee from recognizing Maithili.

The Interim Constitution of Nepal 2007 recognizes all the languages spoken as mother tongues in Nepal as the national languages of Nepal. It says that Nepali in Devanagari script shall be the language of official business, however, the use of mother tongues in local bodies or offices shall not be considered a barrier. The use of national languages in local government bodies has not happened in practice, and discouragement in their use and discrimination in allocation of resources persist. Some analysts have stated that one of the chief causes of the Maoist insurgency, or the Nepalese Civil War (1996–2006), was the denial of language rights and marginalization of ethnic groups.

Sri Lanka

See also: Sinhala Only Act

The start of the conflict regarding languages in Sri Lanka goes as far back as the rule of the British. During the colonial period, English had a special and powerful position in Sri Lanka. The British ruled in Sri Lanka from the late eighteenth century to 1948. English was the official language of administration then. Just before the departure of the British, a "swabhasha" (your own language) movement was launched in a bid to phase out English slowly, replacing it with Sinhala or Tamil. However, shortly after the departure of the British the campaign, for various political reasons, evolved from advocating Sinhala and Tamil replacing English to just Sinhala replacing English.

In 1956, the first election after independence, the opposition won and the official language was declared to be Sinhala. The Tamil people were unhappy, feeling that they were greatly disadvantaged. Because Sinhala was now the official language, it made it easier for the people whose mother tongue was Sinhala to enter into government sector and also provided them with an unfair advantage in the education system. Tamils who also did not understand Sinhala felt greatly inconvenienced as they had to depend on others to translate official documents for them.

Both the Tamil and Sinhala-speaking people felt that language was crucial to their identity. The Sinhala people associated the language with their rich heritage. They were also afraid that, given that there were only 9 million speakers of the language at that time, if Sinhala was not the only official language it would eventually be slowly lost. The Tamil people felt that the Sinhala-only policy would assert the dominance of the Sinhalese people and as such they might lose their language, culture and identity.

Despite the unhappiness of the Tamil people, no big political movement was undertaken till the early 1970s. Eventually in May 1976, there was a public demand for a Tamil state. During the 1956 election the Federal party had replaced the Tamil congress. The party was bent on "the attainment of freedom for the Tamil-speaking people of Ceylon by the establishment of an autonomous Tamil state on the linguistic basis within the framework of a Federal Union of Ceylon". However it did not have much success. Thus in 1972, the Federal Party, Tamil Congress and other organizations banded together into a new party called the "Tamil United Front".

One of the catalysts for Tamil separation arose in 1972 when the Sinhala government made amendments to the constitution. The Sinhala government decided to promote Buddhism as the official religion, claiming that "it shall be the duty of the State to protect and foster Buddhism". Given that the majority of the Tamils were Hindus, this created unease. There was then a fear among the Tamils that people belonging to the "untouchable castes" would be encouraged to convert to Buddhism and then "brainwashed" to learn Sinhala as well.

Another spur was also the impatience of Tamil youth in Sri Lanka. Veteran politicians noted that current youths were more ready to engage in violence, and some of them even had ties to certain rebel groups in South India. Also in 1974, there was conference of Tamil studies organized in Jaffna. The conference turned violent. This resulted in the deaths of seven people. Consequently, about 40 – 50 Tamil youths in between the years of 1972 and 1975 were detained without being properly charged, further increasing tension.

A third stimulus was the changes in the criteria for University examinations in the early 1970s. The government decided that they wanted to standardize the university admission criteria, based on the language the entrance exams were taken in. It was noted that students who took the exams in Tamil scored better than the students who took it in Sinhala. Thus the government decided that Tamil students had to achieve a higher score than the students who took the exam in Sinhala to enter the universities. As a result, the number of Tamil students entering universities fell.

After the July 1977 election, relations between the Sinhalese and the Ceylon Tamil people became worse. There was flash violence in parts of the country. It is estimated about 100 people were killed and thousands of people fled from their homes. Among all these tensions, the call for a separate state among Tamil people grew louder.

Polarization (waves)

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Polarization_(waves)

Circular polarization on rubber thread, converted to linear polarization

Polarization (also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. A simple example of a polarized transverse wave is vibrations traveling along a taut string (see image); for example, in a musical instrument like a guitar string. Depending on how the string is plucked, the vibrations can be in a vertical direction, horizontal direction, or at any angle perpendicular to the string. In contrast, in longitudinal waves, such as sound waves in a liquid or gas, the displacement of the particles in the oscillation is always in the direction of propagation, so these waves do not exhibit polarization. Transverse waves that exhibit polarization include electromagnetic waves such as light and radio waves, gravitational waves, and transverse sound waves (shear waves) in solids.

An electromagnetic wave such as light consists of a coupled oscillating electric field and magnetic field which are always perpendicular to each other; by convention, the "polarization" of electromagnetic waves refers to the direction of the electric field. In linear polarization, the fields oscillate in a single direction. In circular or elliptical polarization, the fields rotate at a constant rate in a plane as the wave travels. The rotation can have two possible directions; if the fields rotate in a right hand sense with respect to the direction of wave travel, it is called right circular polarization, while if the fields rotate in a left hand sense, it is called left circular polarization.

Light or other electromagnetic radiation from many sources, such as the sun, flames, and incandescent lamps, consists of short wave trains with an equal mixture of polarizations; this is called unpolarized light. Polarized light can be produced by passing unpolarized light through a polarizer, which allows waves of only one polarization to pass through. The most common optical materials do not affect the polarization of light, however, some materials—those that exhibit birefringence, dichroism, or optical activity—affect light differently depending on its polarization. Some of these are used to make polarizing filters. Light also becomes partially polarized when it reflects at an angle from a surface.

According to quantum mechanics, electromagnetic waves can also be viewed as streams of particles called photons. When viewed in this way, the polarization of an electromagnetic wave is determined by a quantum mechanical property of photons called their spin. A photon has one of two possible spins: it can either spin in a right hand sense or a left hand sense about its direction of travel. Circularly polarized electromagnetic waves are composed of photons with only one type of spin, either right- or left-hand. Linearly polarized waves consist of photons that are in a superposition of right and left circularly polarized states, with equal amplitude and phases synchronized to give oscillation in a plane.

Polarization is an important parameter in areas of science dealing with transverse waves, such as optics, seismology, radio, and microwaves. Especially impacted are technologies such as lasers, wireless and optical fiber telecommunications, and radar.

Introduction

Wave propagation and polarization

cross linear polarized

Most sources of light are classified as incoherent and unpolarized (or only "partially polarized") because they consist of a random mixture of waves having different spatial characteristics, frequencies (wavelengths), phases, and polarization states. However, for understanding electromagnetic waves and polarization in particular, it is easier to just consider coherent plane waves; these are sinusoidal waves of one particular direction (or wavevector), frequency, phase, and polarization state. Characterizing an optical system in relation to a plane wave with those given parameters can then be used to predict its response to a more general case, since a wave with any specified spatial structure can be decomposed into a combination of plane waves (its so-called angular spectrum). Incoherent states can be modeled stochastically as a weighted combination of such uncorrelated waves with some distribution of frequencies (its spectrum), phases, and polarizations.

Transverse electromagnetic waves

A "vertically polarized" electromagnetic wave of wavelength λ has its electric field vector E (red) oscillating in the vertical direction. The magnetic field B (or H) is always at right angles to it (blue), and both are perpendicular to the direction of propagation (z).

Electromagnetic waves (such as light), traveling in free space or another homogeneous isotropic non-attenuating medium, are properly described as transverse waves, meaning that a plane wave's electric field vector E and magnetic field H are each in some direction perpendicular to (or "transverse" to) the direction of wave propagation; E and H are also perpendicular to each other. By convention, the "polarization" direction of an electromagnetic wave is given by its electric field vector. Considering a monochromatic plane wave of optical frequency f (light of vacuum wavelength λ has a frequency of f = c/λ where c is the speed of light), let us take the direction of propagation as the z axis. Being a transverse wave the E and H fields must then contain components only in the x and y directions whereas E_z = H_z = 0. Using complex (or phasor) notation, the instantaneous physical electric and magnetic fields are given by the real parts of the complex quantities occurring in the following equations. As a function of time t and spatial position z (since for a plane wave in the +z direction the fields have no dependence on x or y) these complex fields can be written as:

${\displaystyle {\vec {E}}(z,t)={\begin{bmatrix}e_{x}\\e_{y}\\0\end{bmatrix}}\;e^{i2\pi \left({\frac {z}{\lambda }}-{\frac {t}{T}}\right)}={\begin{bmatrix}e_{x}\\e_{y}\\0\end{bmatrix}}\;e^{i(kz-\omega t)}}$

and

${\displaystyle {\vec {H}}(z,t)={\begin{bmatrix}h_{x}\\h_{y}\\0\end{bmatrix}}\;e^{i2\pi \left({\frac {z}{\lambda }}-{\frac {t}{T}}\right)}={\begin{bmatrix}h_{x}\\h_{y}\\0\end{bmatrix}}\;e^{i(kz-\omega t)}}$

where λ = λ₀/n is the wavelength in the medium (whose refractive index is n) and T = 1/f is the period of the wave. Here e_x, e_y, h_x, and h_y are complex numbers. In the second more compact form, as these equations are customarily expressed, these factors are described using the wavenumber ${\displaystyle k=2\pi n/\lambda _{0}}$ and angular frequency (or "radian frequency") ${\displaystyle \omega =2\pi f}$. In a more general formulation with propagation not restricted to the +z direction, then the spatial dependence kz is replaced by ${\displaystyle {\vec {k}}\cdot {\vec {r}}}$ where ${\displaystyle {\vec {k}}}$ is called the wave vector, the magnitude of which is the wavenumber.

Thus the leading vectors e and h each contain up to two nonzero (complex) components describing the amplitude and phase of the wave's x and y polarization components (again, there can be no z polarization component for a transverse wave in the +z direction). For a given medium with a characteristic impedance ${\displaystyle \eta }$, h is related to e by:

${\displaystyle h_{y}={\frac {e_{x}}{\eta }}}$

and

${\displaystyle h_{x}=-{\frac {e_{y}}{\eta }}}$.

In a dielectric, η is real and has the value η₀/n, where n is the refractive index and η₀ is the impedance of free space. The impedance will be complex in a conducting medium. Note that given that relationship, the dot product of E and H must be zero:

${\displaystyle {\vec {E}}\left({\vec {r}},t\right)\cdot {\vec {H}}\left({\vec {r}},t\right)=e_{x}h_{x}+e_{y}h_{y}+e_{z}h_{z}=e_{x}\left(-{\frac {e_{y}}{\eta }}\right)+e_{y}\left({\frac {e_{x}}{\eta }}\right)+0\cdot 0=0}$

indicating that these vectors are orthogonal (at right angles to each other), as expected.

So knowing the propagation direction (+z in this case) and η, one can just as well specify the wave in terms of just e_x and e_y describing the electric field. The vector containing e_x and e_y (but without the z component which is necessarily zero for a transverse wave) is known as a Jones vector. In addition to specifying the polarization state of the wave, a general Jones vector also specifies the overall magnitude and phase of that wave. Specifically, the intensity of the light wave is proportional to the sum of the squared magnitudes of the two electric field components:

${\displaystyle I=\left(\left|e_{x}\right|^{2}+\left|e_{y}\right|^{2}\right)\,{\frac {1}{2\eta }}}$

however the wave's state of polarization is only dependent on the (complex) ratio of e_y to e_x. So let us just consider waves whose |e_x|² + |e_y|² = 1; this happens to correspond to an intensity of about .00133 watts per square meter in free space (where ${\displaystyle \eta =}$ ${\displaystyle \eta _{0}}$). And since the absolute phase of a wave is unimportant in discussing its polarization state, let us stipulate that the phase of e_x is zero, in other words e_x is a real number while e_y may be complex. Under these restrictions, e_x and e_y can be represented as follows:

${\displaystyle e_{x}={\sqrt {\frac {1+Q}{2}}}}$

${\displaystyle e_{y}={\sqrt {\frac {1-Q}{2}}}\,e^{i\phi }}$

where the polarization state is now fully parameterized by the value of Q (such that −1 < Q < 1) and the relative phase ${\displaystyle \phi }$.

Non-transverse waves

In addition to transverse waves, there are many wave motions where the oscillation is not limited to directions perpendicular to the direction of propagation. These cases are far beyond the scope of the current article which concentrates on transverse waves (such as most electromagnetic waves in bulk media), however one should be aware of cases where the polarization of a coherent wave cannot be described simply using a Jones vector, as we have just done.

Just considering electromagnetic waves, we note that the preceding discussion strictly applies to plane waves in a homogeneous isotropic non-attenuating medium, whereas in an anisotropic medium (such as birefringent crystals as discussed below) the electric or magnetic field may have longitudinal as well as transverse components. In those cases the electric displacement D and magnetic flux density B still obey the above geometry but due to anisotropy in the electric susceptibility (or in the magnetic permeability), now given by a tensor, the direction of E (or H) may differ from that of D (or B). Even in isotropic media, so-called inhomogeneous waves can be launched into a medium whose refractive index has a significant imaginary part (or "extinction coefficient") such as metals; these fields are also not strictly transverse. Surface waves or waves propagating in a waveguide (such as an optical fiber) are generally not transverse waves, but might be described as an electric or magnetic transverse mode, or a hybrid mode.

Even in free space, longitudinal field components can be generated in focal regions, where the plane wave approximation breaks down. An extreme example is radially or tangentially polarized light, at the focus of which the electric or magnetic field respectively is entirely longitudinal (along the direction of propagation).

For longitudinal waves such as sound waves in fluids, the direction of oscillation is by definition along the direction of travel, so the issue of polarization is not normally even mentioned. On the other hand, sound waves in a bulk solid can be transverse as well as longitudinal, for a total of three polarization components. In this case, the transverse polarization is associated with the direction of the shear stress and displacement in directions perpendicular to the propagation direction, while the longitudinal polarization describes compression of the solid and vibration along the direction of propagation. The differential propagation of transverse and longitudinal polarizations is important in seismology.

Polarization state

Further information: Linear polarization, Circular polarization, and Elliptical polarization

 

Electric field oscillation

Polarization is best understood by initially considering only pure polarization states, and only a coherent sinusoidal wave at some optical frequency. The vector in the adjacent diagram might describe the oscillation of the electric field emitted by a single-mode laser (whose oscillation frequency would be typically 10¹⁵ times faster). The field oscillates in the x-y plane, along the page, with the wave propagating in the z direction, perpendicular to the page. The first two diagrams below trace the electric field vector over a complete cycle for linear polarization at two different orientations; these are each considered a distinct state of polarization (SOP). Note that the linear polarization at 45° can also be viewed as the addition of a horizontally linearly polarized wave (as in the leftmost figure) and a vertically polarized wave of the same amplitude in the same phase.

Animation showing four different polarization states and three orthogonal projections.

 

A circularly polarized wave as a sum of two linearly polarized components 90° out of phase

Now if one were to introduce a phase shift in between those horizontal and vertical polarization components, one would generally obtain elliptical polarization as is shown in the third figure. When the phase shift is exactly ±90°, then circular polarization is produced (fourth and fifth figures). Thus is circular polarization created in practice, starting with linearly polarized light and employing a quarter-wave plate to introduce such a phase shift. The result of two such phase-shifted components in causing a rotating electric field vector is depicted in the animation on the right. Note that circular or elliptical polarization can involve either a clockwise or counterclockwise rotation of the field. These correspond to distinct polarization states, such as the two circular polarizations shown above.

Of course the orientation of the x and y axes used in this description is arbitrary. The choice of such a coordinate system and viewing the polarization ellipse in terms of the x and y polarization components, corresponds to the definition of the Jones vector (below) in terms of those basis polarizations. One would typically choose axes to suit a particular problem such as x being in the plane of incidence. Since there are separate reflection coefficients for the linear polarizations in and orthogonal to the plane of incidence (p and s polarizations, see below), that choice greatly simplifies the calculation of a wave's reflection from a surface.

Moreover, one can use as basis functions any pair of orthogonal polarization states, not just linear polarizations. For instance, choosing right and left circular polarizations as basis functions simplifies the solution of problems involving circular birefringence (optical activity) or circular dichroism.

Polarization ellipse

Main article: Polarization ellipse

 

Consider a purely polarized monochromatic wave. If one were to plot the electric field vector over one cycle of oscillation, an ellipse would generally be obtained, as is shown in the figure, corresponding to a particular state of elliptical polarization. Note that linear polarization and circular polarization can be seen as special cases of elliptical polarization.

A polarization state can then be described in relation to the geometrical parameters of the ellipse, and its "handedness", that is, whether the rotation around the ellipse is clockwise or counter clockwise. One parameterization of the elliptical figure specifies the orientation angle ψ, defined as the angle between the major axis of the ellipse and the x-axis along with the ellipticity ε = a/b, the ratio of the ellipse's major to minor axis. (also known as the axial ratio). The ellipticity parameter is an alternative parameterization of an ellipse's eccentricity ${\textstyle e={\sqrt {1-b^{2}/a^{2}}},}$ or the ellipticity angle, ${\textstyle \chi =\arctan b/a}$ ${\textstyle =\arctan 1/\varepsilon }$ as is shown in the figure. The angle χ is also significant in that the latitude (angle from the equator) of the polarization state as represented on the Poincaré sphere (see below) is equal to ±2χ. The special cases of linear and circular polarization correspond to an ellipticity ε of infinity and unity (or χ of zero and 45°) respectively.

Jones vector

Main article: Jones vector

Full information on a completely polarized state is also provided by the amplitude and phase of oscillations in two components of the electric field vector in the plane of polarization. This representation was used above to show how different states of polarization are possible. The amplitude and phase information can be conveniently represented as a two-dimensional complex vector (the Jones vector):

${\displaystyle \mathbf {e} ={\begin{bmatrix}a_{1}e^{i\theta _{1}}\\a_{2}e^{i\theta _{2}}\end{bmatrix}}.}$

Here ${\displaystyle a_{1}}$ and ${\displaystyle a_{2}}$ denote the amplitude of the wave in the two components of the electric field vector, while ${\displaystyle \theta _{1}}$ and ${\displaystyle \theta _{2}}$ represent the phases. The product of a Jones vector with a complex number of unit modulus gives a different Jones vector representing the same ellipse, and thus the same state of polarization. The physical electric field, as the real part of the Jones vector, would be altered but the polarization state itself is independent of absolute phase. The basis vectors used to represent the Jones vector need not represent linear polarization states (i.e. be real). In general any two orthogonal states can be used, where an orthogonal vector pair is formally defined as one having a zero inner product. A common choice is left and right circular polarizations, for example to model the different propagation of waves in two such components in circularly birefringent media (see below) or signal paths of coherent detectors sensitive to circular polarization.

Coordinate frame

Regardless of whether polarization state is represented using geometric parameters or Jones vectors, implicit in the parameterization is the orientation of the coordinate frame. This permits a degree of freedom, namely rotation about the propagation direction. When considering light that is propagating parallel to the surface of the Earth, the terms "horizontal" and "vertical" polarization are often used, with the former being associated with the first component of the Jones vector, or zero azimuth angle. On the other hand, in astronomy the equatorial coordinate system is generally used instead, with the zero azimuth (or position angle, as it is more commonly called in astronomy to avoid confusion with the horizontal coordinate system) corresponding to due north.

s and p designations

See also: Fresnel equations § S and P polarizations

Another coordinate system frequently used relates to the plane of incidence. This is the plane made by the incoming propagation direction and the vector perpendicular to the plane of an interface, in other words, the plane in which the ray travels before and after reflection or refraction. The component of the electric field parallel to this plane is termed p-like (parallel) and the component perpendicular to this plane is termed s-like (from senkrecht, German for perpendicular). Polarized light with its electric field along the plane of incidence is thus denoted p-polarized, while light whose electric field is normal to the plane of incidence is called s-polarized. P polarization is commonly referred to as transverse-magnetic (TM), and has also been termed pi-polarized or tangential plane polarized. S polarization is also called transverse-electric (TE), as well as sigma-polarized or sagittal plane polarized.

Unpolarized and partially polarized light

See also: Photon polarization

Definition

Natural light, like most other common sources of visible light, is incoherent: radiation is produced independently by a large number of atoms or molecules whose emissions are uncorrelated and generally of random polarizations. In this case the light is said to be unpolarized. This term is somewhat inexact, since at any instant of time at one location there is a definite direction to the electric and magnetic fields, however it implies that the polarization changes so quickly in time that it will not be measured or relevant to the outcome of an experiment. A so-called depolarizer acts on a polarized beam to create one which is actually fully polarized at every point, but in which the polarization varies so rapidly across the beam that it may be ignored in the intended applications.

Unpolarized light can be described as a mixture of two independent oppositely polarized streams, each with half the intensity. Light is said to be partially polarized when there is more power in one of these streams than the other. At any particular wavelength, partially polarized light can be statistically described as the superposition of a completely unpolarized component and a completely polarized one. One may then describe the light in terms of the degree of polarization and the parameters of the polarized component. That polarized component can be described in terms of a Jones vector or polarization ellipse, as is detailed above. However, in order to also describe the degree of polarization, one normally employs Stokes parameters (see below) to specify a state of partial polarization.

Motivation

The transmission of plane waves through a homogeneous medium are fully described in terms of Jones vectors and 2×2 Jones matrices. However, in practice there are cases in which all of the light cannot be viewed in such a simple manner due to spatial inhomogeneities or the presence of mutually incoherent waves. So-called depolarization, for instance, cannot be described using Jones matrices. For these cases it is usual instead to use a 4×4 matrix that acts upon the Stokes 4-vector. Such matrices were first used by Paul Soleillet in 1929, although they have come to be known as Mueller matrices. While every Jones matrix has a Mueller matrix, the reverse is not true. Mueller matrices are then used to describe the observed polarization effects of the scattering of waves from complex surfaces or ensembles of particles, as shall now be presented.

Coherency matrix

The Jones vector perfectly describes the state of polarization and phase of a single monochromatic wave, representing a pure state of polarization as described above. However any mixture of waves of different polarizations (or even of different frequencies) do not correspond to a Jones vector. In so-called partially polarized radiation the fields are stochastic, and the variations and correlations between components of the electric field can only be described statistically. One such representation is the coherency matrix:

${\displaystyle {\begin{aligned}\mathbf {\Psi } &=\left\langle \mathbf {e} \mathbf {e} ^{\dagger }\right\rangle \\&=\left\langle {\begin{bmatrix}e_{1}e_{1}^{*}&e_{1}e_{2}^{*}\\e_{2}e_{1}^{*}&e_{2}e_{2}^{*}\end{bmatrix}}\right\rangle \\&=\left\langle {\begin{bmatrix}a_{1}^{2}&a_{1}a_{2}e^{i\left(\theta _{1}-\theta _{2}\right)}\\a_{1}a_{2}e^{-i\left(\theta _{1}-\theta _{2}\right)}&a_{2}^{2}\end{bmatrix}}\right\rangle \end{aligned}}}$

where angular brackets denote averaging over many wave cycles. Several variants of the coherency matrix have been proposed: the Wiener coherency matrix and the spectral coherency matrix of Richard Barakat measure the coherence of a spectral decomposition of the signal, while the Wolf coherency matrix averages over all time/frequencies.

The coherency matrix contains all second order statistical information about the polarization. This matrix can be decomposed into the sum of two idempotent matrices, corresponding to the eigenvectors of the coherency matrix, each representing a polarization state that is orthogonal to the other. An alternative decomposition is into completely polarized (zero determinant) and unpolarized (scaled identity matrix) components. In either case, the operation of summing the components corresponds to the incoherent superposition of waves from the two components. The latter case gives rise to the concept of the "degree of polarization"; i.e., the fraction of the total intensity contributed by the completely polarized component.

Stokes parameters

Main article: Stokes parameters

The coherency matrix is not easy to visualize, and it is therefore common to describe incoherent or partially polarized radiation in terms of its total intensity (I), (fractional) degree of polarization (p), and the shape parameters of the polarization ellipse. An alternative and mathematically convenient description is given by the Stokes parameters, introduced by George Gabriel Stokes in 1852. The relationship of the Stokes parameters to intensity and polarization ellipse parameters is shown in the equations and figure below.

${\displaystyle S_{0}=I\,}$

${\displaystyle S_{1}=Ip\cos 2\psi \cos 2\chi \,}$

${\displaystyle S_{2}=Ip\sin 2\psi \cos 2\chi \,}$

${\displaystyle S_{3}=Ip\sin 2\chi \,}$

Here Ip, 2ψ and 2χ are the spherical coordinates of the polarization state in the three-dimensional space of the last three Stokes parameters. Note the factors of two before ψ and χ corresponding respectively to the facts that any polarization ellipse is indistinguishable from one rotated by 180°, or one with the semi-axis lengths swapped accompanied by a 90° rotation. The Stokes parameters are sometimes denoted I, Q, U and V.

The four Stokes parameters are enough to describe 2D polarization of a paraxial wave, but not the 3D polarization of a general non-paraxial wave or an evanescent field.

Poincaré sphere

See also: Bloch sphere

Neglecting the first Stokes parameter S₀ (or I), the three other Stokes parameters can be plotted directly in three-dimensional Cartesian coordinates. For a given power in the polarized component given by

${\displaystyle P={\sqrt {S_{1}^{2}+S_{2}^{2}+S_{3}^{2}}}}$

the set of all polarization states are then mapped to points on the surface of the so-called Poincaré sphere (but of radius P), as shown in the accompanying diagram.

Poincaré sphere, on or beneath which the three Stokes parameters [S₁, S₂, S₃] (or [Q, U, V]) are plotted in Cartesian coordinates

 

Depiction of the polarization states on Poincaré sphere

Often the total beam power is not of interest, in which case a normalized Stokes vector is used by dividing the Stokes vector by the total intensity S₀:

${\displaystyle \mathbf {S'} ={\frac {1}{S_{0}}}{\begin{bmatrix}S_{0}\\S_{1}\\S_{2}\\S_{3}\end{bmatrix}}.}$

The normalized Stokes vector ${\displaystyle \mathbf {S'} }$ then has unity power (${\displaystyle S'_{0}=1}$) and the three significant Stokes parameters plotted in three dimensions will lie on the unity-radius Poincaré sphere for pure polarization states (where ${\displaystyle P'_{0}=1}$). Partially polarized states will lie inside the Poincaré sphere at a distance of ${\displaystyle P'={\sqrt {S_{1}'^{2}+S_{2}'^{2}+S_{3}'^{2}}}}$ from the origin. When the non-polarized component is not of interest, the Stokes vector can be further normalized to obtain

${\displaystyle \mathbf {S''} ={\frac {1}{P'}}{\begin{bmatrix}1\\S'_{1}\\S'_{2}\\S'_{3}\end{bmatrix}}={\frac {1}{P}}{\begin{bmatrix}S_{0}\\S_{1}\\S_{2}\\S_{3}\end{bmatrix}}.}$

When plotted, that point will lie on the surface of the unity-radius Poincaré sphere and indicate the state of polarization of the polarized component.

Any two antipodal points on the Poincaré sphere refer to orthogonal polarization states. The overlap between any two polarization states is dependent solely on the distance between their locations along the sphere. This property, which can only be true when pure polarization states are mapped onto a sphere, is the motivation for the invention of the Poincaré sphere and the use of Stokes parameters, which are thus plotted on (or beneath) it.

Note that the IEEE defines RHCP and LHCP the opposite as those used by Physicists. The IEEE 1979 Antenna Standard will show RHCP on the South Pole of the Poincare Sphere. The IEEE defines RHCP using the right hand with thumb pointing in the direction of transmit, and the fingers showing the direction of rotation of the E field with time. The rationale for the opposite conventions used by Physicists and Engineers is that Astronomical Observations are always done with the incoming wave traveling toward the observer, where as for most engineers, they are assumed to be standing behind the transmitter watching the wave traveling away from them. This article is not using the IEEE 1979 Antenna Standard and is not using the +t convention typically used in IEEE work.

Implications for reflection and propagation

Polarization in wave propagation

In a vacuum, the components of the electric field propagate at the speed of light, so that the phase of the wave varies in space and time while the polarization state does not. That is, the electric field vector e of a plane wave in the +z direction follows:

${\displaystyle \mathbf {e} (z+\Delta z,t+\Delta t)=\mathbf {e} (z,t)e^{ik(c\Delta t-\Delta z)},}$

where k is the wavenumber. As noted above, the instantaneous electric field is the real part of the product of the Jones vector times the phase factor ${\displaystyle e^{-i\omega t}}$. When an electromagnetic wave interacts with matter, its propagation is altered according to the material's (complex) index of refraction. When the real or imaginary part of that refractive index is dependent on the polarization state of a wave, properties known as birefringence and polarization dichroism (or diattenuation) respectively, then the polarization state of a wave will generally be altered.

In such media, an electromagnetic wave with any given state of polarization may be decomposed into two orthogonally polarized components that encounter different propagation constants. The effect of propagation over a given path on those two components is most easily characterized in the form of a complex 2×2 transformation matrix J known as a Jones matrix:

${\displaystyle \mathbf {e'} =\mathbf {J} \mathbf {e} .}$

The Jones matrix due to passage through a transparent material is dependent on the propagation distance as well as the birefringence. The birefringence (as well as the average refractive index) will generally be dispersive, that is, it will vary as a function of optical frequency (wavelength). In the case of non-birefringent materials, however, the 2×2 Jones matrix is the identity matrix (multiplied by a scalar phase factor and attenuation factor), implying no change in polarization during propagation.

For propagation effects in two orthogonal modes, the Jones matrix can be written as

${\displaystyle \mathbf {J} =\mathbf {T} {\begin{bmatrix}g_{1}&0\\0&g_{2}\end{bmatrix}}\mathbf {T} ^{-1},}$

where g₁ and g₂ are complex numbers describing the phase delay and possibly the amplitude attenuation due to propagation in each of the two polarization eigenmodes. T is a unitary matrix representing a change of basis from these propagation modes to the linear system used for the Jones vectors; in the case of linear birefringence or diattenuation the modes are themselves linear polarization states so T and T⁻¹ can be omitted if the coordinate axes have been chosen appropriately.

Birefringence

Main article: Birefringence

In media termed birefringent, in which the amplitudes are unchanged but a differential phase delay occurs, the Jones matrix is a unitary matrix: |g₁| = |g₂| = 1. Media termed diattenuating (or dichroic in the sense of polarization), in which only the amplitudes of the two polarizations are affected differentially, may be described using a Hermitian matrix (generally multiplied by a common phase factor). In fact, since any matrix may be written as the product of unitary and positive Hermitian matrices, light propagation through any sequence of polarization-dependent optical components can be written as the product of these two basic types of transformations.

Color pattern of a plastic box showing stress-induced birefringence when placed in between two crossed polarizers.

In birefringent media there is no attenuation, but two modes accrue a differential phase delay. Well known manifestations of linear birefringence (that is, in which the basis polarizations are orthogonal linear polarizations) appear in optical wave plates/retarders and many crystals. If linearly polarized light passes through a birefringent material, its state of polarization will generally change, unless its polarization direction is identical to one of those basis polarizations. Since the phase shift, and thus the change in polarization state, is usually wavelength-dependent, such objects viewed under white light in between two polarizers may give rise to colorful effects, as seen in the accompanying photograph.

Circular birefringence is also termed optical activity, especially in chiral fluids, or Faraday rotation, when due to the presence of a magnetic field along the direction of propagation. When linearly polarized light is passed through such an object, it will exit still linearly polarized, but with the axis of polarization rotated. A combination of linear and circular birefringence will have as basis polarizations two orthogonal elliptical polarizations; however, the term "elliptical birefringence" is rarely used.

Paths taken by vectors in the Poincaré sphere under birefringence. The propagation modes (rotation axes) are shown with red, blue, and yellow lines, the initial vectors by thick black lines, and the paths they take by colored ellipses (which represent circles in three dimensions).

One can visualize the case of linear birefringence (with two orthogonal linear propagation modes) with an incoming wave linearly polarized at a 45° angle to those modes. As a differential phase starts to accrue, the polarization becomes elliptical, eventually changing to purely circular polarization (90° phase difference), then to elliptical and eventually linear polarization (180° phase) perpendicular to the original polarization, then through circular again (270° phase), then elliptical with the original azimuth angle, and finally back to the original linearly polarized state (360° phase) where the cycle begins anew. In general the situation is more complicated and can be characterized as a rotation in the Poincaré sphere about the axis defined by the propagation modes. Examples for linear (blue), circular (red), and elliptical (yellow) birefringence are shown in the figure on the left. The total intensity and degree of polarization are unaffected. If the path length in the birefringent medium is sufficient, the two polarization components of a collimated beam (or ray) can exit the material with a positional offset, even though their final propagation directions will be the same (assuming the entrance face and exit face are parallel). This is commonly viewed using calcite crystals, which present the viewer with two slightly offset images, in opposite polarizations, of an object behind the crystal. It was this effect that provided the first discovery of polarization, by Erasmus Bartholinus in 1669.

Dichroism

Media in which transmission of one polarization mode is preferentially reduced are called dichroic or diattenuating. Like birefringence, diattenuation can be with respect to linear polarization modes (in a crystal) or circular polarization modes (usually in a liquid).

Devices that block nearly all of the radiation in one mode are known as polarizing filters or simply "polarizers". This corresponds to g₂=0 in the above representation of the Jones matrix. The output of an ideal polarizer is a specific polarization state (usually linear polarization) with an amplitude equal to the input wave's original amplitude in that polarization mode. Power in the other polarization mode is eliminated. Thus if unpolarized light is passed through an ideal polarizer (where g₁=1 and g₂=0) exactly half of its initial power is retained. Practical polarizers, especially inexpensive sheet polarizers, have additional loss so that g₁ < 1. However, in many instances the more relevant figure of merit is the polarizer's degree of polarization or extinction ratio, which involve a comparison of g₁ to g₂. Since Jones vectors refer to waves' amplitudes (rather than intensity), when illuminated by unpolarized light the remaining power in the unwanted polarization will be (g₂/g₁)² of the power in the intended polarization.

Specular reflection

In addition to birefringence and dichroism in extended media, polarization effects describable using Jones matrices can also occur at (reflective) interface between two materials of different refractive index. These effects are treated by the Fresnel equations. Part of the wave is transmitted and part is reflected; for a given material those proportions (and also the phase of reflection) are dependent on the angle of incidence and are different for the s and p polarizations. Therefore, the polarization state of reflected light (even if initially unpolarized) is generally changed.

A stack of plates at Brewster's angle to a beam reflects off a fraction of the s-polarized light at each surface, leaving (after many such plates) a mainly p-polarized beam.

Any light striking a surface at a special angle of incidence known as Brewster's angle, where the reflection coefficient for p polarization is zero, will be reflected with only the s-polarization remaining. This principle is employed in the so-called "pile of plates polarizer" (see figure) in which part of the s polarization is removed by reflection at each Brewster angle surface, leaving only the p polarization after transmission through many such surfaces. The generally smaller reflection coefficient of the p polarization is also the basis of polarized sunglasses; by blocking the s (horizontal) polarization, most of the glare due to reflection from a wet street, for instance, is removed.

In the important special case of reflection at normal incidence (not involving anisotropic materials) there is no particular s or p polarization. Both the x and y polarization components are reflected identically, and therefore the polarization of the reflected wave is identical to that of the incident wave. However, in the case of circular (or elliptical) polarization, the handedness of the polarization state is thereby reversed, since by convention this is specified relative to the direction of propagation. The circular rotation of the electric field around the x-y axes called "right-handed" for a wave in the +z direction is "left-handed" for a wave in the -z direction. But in the general case of reflection at a nonzero angle of incidence, no such generalization can be made. For instance, right-circularly polarized light reflected from a dielectric surface at a grazing angle, will still be right-handed (but elliptically) polarized. Linear polarized light reflected from a metal at non-normal incidence will generally become elliptically polarized. These cases are handled using Jones vectors acted upon by the different Fresnel coefficients for the s and p polarization components.

Measurement techniques involving polarization

Some optical measurement techniques are based on polarization. In many other optical techniques polarization is crucial or at least must be taken into account and controlled; such examples are too numerous to mention.

Measurement of stress

Stress in plastic glasses

In engineering, the phenomenon of stress induced birefringence allows for stresses in transparent materials to be readily observed. As noted above and seen in the accompanying photograph, the chromaticity of birefringence typically creates colored patterns when viewed in between two polarizers. As external forces are applied, internal stress induced in the material is thereby observed. Additionally, birefringence is frequently observed due to stresses "frozen in" at the time of manufacture. This is famously observed in cellophane tape whose birefringence is due to the stretching of the material during the manufacturing process.

Ellipsometry

Main article: Ellipsometry

Ellipsometry is a powerful technique for the measurement of the optical properties of a uniform surface. It involves measuring the polarization state of light following specular reflection from such a surface. This is typically done as a function of incidence angle or wavelength (or both). Since ellipsometry relies on reflection, it is not required for the sample to be transparent to light or for its back side to be accessible.

Ellipsometry can be used to model the (complex) refractive index of a surface of a bulk material. It is also very useful in determining parameters of one or more thin film layers deposited on a substrate. Due to their reflection properties, not only are the predicted magnitude of the p and s polarization components, but their relative phase shifts upon reflection, compared to measurements using an ellipsometer. A normal ellipsometer does not measure the actual reflection coefficient (which requires careful photometric calibration of the illuminating beam) but the ratio of the p and s reflections, as well as change of polarization ellipticity (hence the name) induced upon reflection by the surface being studied. In addition to use in science and research, ellipsometers are used in situ to control production processes for instance.

Geology

Photomicrograph of a volcanic sand grain; upper picture is plane-polarized light, bottom picture is cross-polarized light, scale box at left-center is 0.25 millimeter.

The property of (linear) birefringence is widespread in crystalline minerals, and indeed was pivotal in the initial discovery of polarization. In mineralogy, this property is frequently exploited using polarization microscopes, for the purpose of identifying minerals. See optical mineralogy for more details.

Sound waves in solid materials exhibit polarization. Differential propagation of the three polarizations through the earth is a crucial in the field of seismology. Horizontally and vertically polarized seismic waves (shear waves) are termed SH and SV, while waves with longitudinal polarization (compressional waves) are termed P-waves.

Chemistry

We have seen (above) that the birefringence of a type of crystal is useful in identifying it, and thus detection of linear birefringence is especially useful in geology and mineralogy. Linearly polarized light generally has its polarization state altered upon transmission through such a crystal, making it stand out when viewed in between two crossed polarizers, as seen in the photograph, above. Likewise, in chemistry, rotation of polarization axes in a liquid solution can be a useful measurement. In a liquid, linear birefringence is impossible, however there may be circular birefringence when a chiral molecule is in solution. When the right and left handed enantiomers of such a molecule are present in equal numbers (a so-called racemic mixture) then their effects cancel out. However, when there is only one (or a preponderance of one), as is more often the case for organic molecules, a net circular birefringence (or optical activity) is observed, revealing the magnitude of that imbalance (or the concentration of the molecule itself, when it can be assumed that only one enantiomer is present). This is measured using a polarimeter in which polarized light is passed through a tube of the liquid, at the end of which is another polarizer which is rotated in order to null the transmission of light through it.

Astronomy

Main article: Polarization in astronomy

In many areas of astronomy, the study of polarized electromagnetic radiation from outer space is of great importance. Although not usually a factor in the thermal radiation of stars, polarization is also present in radiation from coherent astronomical sources (e.g. hydroxyl or methanol masers), and incoherent sources such as the large radio lobes in active galaxies, and pulsar radio radiation (which may, it is speculated, sometimes be coherent), and is also imposed upon starlight by scattering from interstellar dust. Apart from providing information on sources of radiation and scattering, polarization also probes the interstellar magnetic field via Faraday rotation. The polarization of the cosmic microwave background is being used to study the physics of the very early universe. Synchrotron radiation is inherently polarized. It has been suggested that astronomical sources caused the chirality of biological molecules on Earth.

Applications and examples

Polarized sunglasses

Effect of a polarizer on reflection from mud flats. In the picture on the left, the horizontally oriented polarizer preferentially transmits those reflections; rotating the polarizer by 90° (right) as one would view using polarized sunglasses blocks almost all specularly reflected sunlight.

 

One can test whether sunglasses are polarized by looking through two pairs, with one perpendicular to the other. If both are polarized, all light will be blocked.

Unpolarized light, after being reflected by a specular (shiny) surface, generally obtains a degree of polarization. This phenomenon was observed in 1808 by the mathematician Étienne-Louis Malus, after whom Malus's law is named. Polarizing sunglasses exploit this effect to reduce glare from reflections by horizontal surfaces, notably the road ahead viewed at a grazing angle.

Wearers of polarized sunglasses will occasionally observe inadvertent polarization effects such as color-dependent birefringent effects, for example in toughened glass (e.g., car windows) or items made from transparent plastics, in conjunction with natural polarization by reflection or scattering. The polarized light from LCD monitors (see below) is very conspicuous when these are worn.

Sky polarization and photography

Further information: Polarizing filter (Photography)

 

The effects of a polarizing filter (right image) on the sky in a photograph

Polarization is observed in the light of the sky, as this is due to sunlight scattered by aerosols as it passes through Earth's atmosphere. The scattered light produces the brightness and color in clear skies. This partial polarization of scattered light can be used to darken the sky in photographs, increasing the contrast. This effect is most strongly observed at points on the sky making a 90° angle to the Sun. Polarizing filters use these effects to optimize the results of photographing scenes in which reflection or scattering by the sky is involved.

Colored fringes in the Embassy Gardens Sky Pool when viewed through a polarizer, due to stress-induced birefringence in the skylight

Sky polarization has been used for orientation in navigation. The Pfund sky compass was used in the 1950s when navigating near the poles of the Earth's magnetic field when neither the sun nor stars were visible (e.g., under daytime cloud or twilight). It has been suggested, controversially, that the Vikings exploited a similar device (the "sunstone") in their extensive expeditions across the North Atlantic in the 9th–11th centuries, before the arrival of the magnetic compass from Asia to Europe in the 12th century. Related to the sky compass is the "polar clock", invented by Charles Wheatstone in the late 19th century.

Display technologies

The principle of liquid-crystal display (LCD) technology relies on the rotation of the axis of linear polarization by the liquid crystal array. Light from the backlight (or the back reflective layer, in devices not including or requiring a backlight) first passes through a linear polarizing sheet. That polarized light passes through the actual liquid crystal layer which may be organized in pixels (for a TV or computer monitor) or in another format such as a seven-segment display or one with custom symbols for a particular product. The liquid crystal layer is produced with a consistent right (or left) handed chirality, essentially consisting of tiny helices. This causes circular birefringence, and is engineered so that there is a 90 degree rotation of the linear polarization state. However, when a voltage is applied across a cell, the molecules straighten out, lessening or totally losing the circular birefringence. On the viewing side of the display is another linear polarizing sheet, usually oriented at 90 degrees from the one behind the active layer. Therefore, when the circular birefringence is removed by the application of a sufficient voltage, the polarization of the transmitted light remains at right angles to the front polarizer, and the pixel appears dark. With no voltage, however, the 90 degree rotation of the polarization causes it to exactly match the axis of the front polarizer, allowing the light through. Intermediate voltages create intermediate rotation of the polarization axis and the pixel has an intermediate intensity. Displays based on this principle are widespread, and now are used in the vast majority of televisions, computer monitors and video projectors, rendering the previous CRT technology essentially obsolete. The use of polarization in the operation of LCD displays is immediately apparent to someone wearing polarized sunglasses, often making the display unreadable.

In a totally different sense, polarization encoding has become the leading (but not sole) method for delivering separate images to the left and right eye in stereoscopic displays used for 3D movies. This involves separate images intended for each eye either projected from two different projectors with orthogonally oriented polarizing filters or, more typically, from a single projector with time multiplexed polarization (a fast alternating polarization device for successive frames). Polarized 3D glasses with suitable polarizing filters ensure that each eye receives only the intended image. Historically such systems used linear polarization encoding because it was inexpensive and offered good separation. However circular polarization makes separation of the two images insensitive to tilting of the head, and is widely used in 3-D movie exhibition today, such as the system from RealD. Projecting such images requires screens that maintain the polarization of the projected light when viewed in reflection (such as silver screens); a normal diffuse white projection screen causes depolarization of the projected images, making it unsuitable for this application.

Although now obsolete, CRT computer displays suffered from reflection by the glass envelope, causing glare from room lights and consequently poor contrast. Several anti-reflection solutions were employed to ameliorate this problem. One solution utilized the principle of reflection of circularly polarized light. A circular polarizing filter in front of the screen allows for the transmission of (say) only right circularly polarized room light. Now, right circularly polarized light (depending on the convention used) has its electric (and magnetic) field direction rotating clockwise while propagating in the +z direction. Upon reflection, the field still has the same direction of rotation, but now propagation is in the −z direction making the reflected wave left circularly polarized. With the right circular polarization filter placed in front of the reflecting glass, the unwanted light reflected from the glass will thus be in very polarization state that is blocked by that filter, eliminating the reflection problem. The reversal of circular polarization on reflection and elimination of reflections in this manner can be easily observed by looking in a mirror while wearing 3-D movie glasses which employ left- and right-handed circular polarization in the two lenses. Closing one eye, the other eye will see a reflection in which it cannot see itself; that lens appears black. However the other lens (of the closed eye) will have the correct circular polarization allowing the closed eye to be easily seen by the open one.

Radio transmission and reception

See also: Antenna (radio) § Polarization

All radio (and microwave) antennas used for transmitting or receiving are intrinsically polarized. They transmit in (or receive signals from) a particular polarization, being totally insensitive to the opposite polarization; in certain cases that polarization is a function of direction. Most antennas are nominally linearly polarized, but elliptical and circular polarization is a possibility. As is the convention in optics, the "polarization" of a radio wave is understood to refer to the polarization of its electric field, with the magnetic field being at a 90 degree rotation with respect to it for a linearly polarized wave.

The vast majority of antennas are linearly polarized. In fact it can be shown from considerations of symmetry that an antenna that lies entirely in a plane which also includes the observer, can only have its polarization in the direction of that plane. This applies to many cases, allowing one to easily infer such an antenna's polarization at an intended direction of propagation. So a typical rooftop Yagi or log-periodic antenna with horizontal conductors, as viewed from a second station toward the horizon, is necessarily horizontally polarized. But a vertical "whip antenna" or AM broadcast tower used as an antenna element (again, for observers horizontally displaced from it) will transmit in the vertical polarization. A turnstile antenna with its four arms in the horizontal plane, likewise transmits horizontally polarized radiation toward the horizon. However, when that same turnstile antenna is used in the "axial mode" (upwards, for the same horizontally-oriented structure) its radiation is circularly polarized. At intermediate elevations it is elliptically polarized.

Polarization is important in radio communications because, for instance, if one attempts to use a horizontally polarized antenna to receive a vertically polarized transmission, the signal strength will be substantially reduced (or under very controlled conditions, reduced to nothing). This principle is used in satellite television in order to double the channel capacity over a fixed frequency band. The same frequency channel can be used for two signals broadcast in opposite polarizations. By adjusting the receiving antenna for one or the other polarization, either signal can be selected without interference from the other.

Especially due to the presence of the ground, there are some differences in propagation (and also in reflections responsible for TV ghosting) between horizontal and vertical polarizations. AM and FM broadcast radio usually use vertical polarization, while television uses horizontal polarization. At low frequencies especially, horizontal polarization is avoided. That is because the phase of a horizontally polarized wave is reversed upon reflection by the ground. A distant station in the horizontal direction will receive both the direct and reflected wave, which thus tend to cancel each other. This problem is avoided with vertical polarization. Polarization is also important in the transmission of radar pulses and reception of radar reflections by the same or a different antenna. For instance, back scattering of radar pulses by rain drops can be avoided by using circular polarization. Just as specular reflection of circularly polarized light reverses the handedness of the polarization, as discussed above, the same principle applies to scattering by objects much smaller than a wavelength such as rain drops. On the other hand, reflection of that wave by an irregular metal object (such as an airplane) will typically introduce a change in polarization and (partial) reception of the return wave by the same antenna.

The effect of free electrons in the ionosphere, in conjunction with the earth's magnetic field, causes Faraday rotation, a sort of circular birefringence. This is the same mechanism which can rotate the axis of linear polarization by electrons in interstellar space as mentioned below. The magnitude of Faraday rotation caused by such a plasma is greatly exaggerated at lower frequencies, so at the higher microwave frequencies used by satellites the effect is minimal. However medium or short wave transmissions received following refraction by the ionosphere are strongly affected. Since a wave's path through the ionosphere and the earth's magnetic field vector along such a path are rather unpredictable, a wave transmitted with vertical (or horizontal) polarization will generally have a resulting polarization in an arbitrary orientation at the receiver.

Circular polarization through an airplane plastic window, 1989

Polarization and vision

Many animals are capable of perceiving some of the components of the polarization of light, e.g., linear horizontally polarized light. This is generally used for navigational purposes, since the linear polarization of sky light is always perpendicular to the direction of the sun. This ability is very common among the insects, including bees, which use this information to orient their communicative dances. Polarization sensitivity has also been observed in species of octopus, squid, cuttlefish, and mantis shrimp. In the latter case, one species measures all six orthogonal components of polarization, and is believed to have optimal polarization vision. The rapidly changing, vividly colored skin patterns of cuttlefish, used for communication, also incorporate polarization patterns, and mantis shrimp are known to have polarization selective reflective tissue. Sky polarization was thought to be perceived by pigeons, which was assumed to be one of their aids in homing, but research indicates this is a popular myth.

The naked human eye is weakly sensitive to polarization, without the need for intervening filters. Polarized light creates a very faint pattern near the center of the visual field, called Haidinger's brush. This pattern is very difficult to see, but with practice one can learn to detect polarized light with the naked eye.

Angular momentum using circular polarization

It is well known that electromagnetic radiation carries a certain linear momentum in the direction of propagation. In addition, however, light carries a certain angular momentum if it is circularly polarized (or partially so). In comparison with lower frequencies such as microwaves, the amount of angular momentum in light, even of pure circular polarization, compared to the same wave's linear momentum (or radiation pressure) is very small and difficult to even measure. However it was utilized in an experiment to achieve speeds of up to 600 million revolutions per minute.