Chirality (chemistry)

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Chirality_(chemistry) 

Two enantiomers of a generic amino acid that are chiral

 

(S)-Alanine (left) and (R)-alanine (right) in zwitterionic form at neutral pH

In chemistry, a molecule or ion is called chiral (/ˈkaɪrəl/) if it cannot be superposed on its mirror image by any combination of rotations, translations, and some conformational changes. This geometric property is called chirality (/kaɪˈrælɪti/). The terms are derived from Ancient Greek χείρ (cheir) 'hand'; which is the canonical example of an object with this property.

A chiral molecule or ion exists in two stereoisomers that are mirror images of each other, called enantiomers; they are often distinguished as either "right-handed" or "left-handed" by their absolute configuration or some other criterion. The two enantiomers have the same chemical properties, except when reacting with other chiral compounds. They also have the same physical properties, except that they often have opposite optical activities. A homogeneous mixture of the two enantiomers in equal parts is said to be racemic, and it usually differs chemically and physically from the pure enantiomers.

Chiral molecules will usually have a stereogenic element from which chirality arises. The most common type of stereogenic element is a stereogenic center, or stereocenter. In the case of organic compounds, stereocenters most frequently take the form of a carbon atom with four distinct groups attached to it in a tetrahedral geometry. A given stereocenter has two possible configurations, which give rise to stereoisomers (diastereomers and enantiomers) in molecules with one or more stereocenter. For a chiral molecule with one or more stereocenter, the enantiomer corresponds to the stereoisomer in which every stereocenter has the opposite configuration. An organic compound with only one stereogenic carbon is always chiral. On the other hand, an organic compound with multiple stereogenic carbons is typically, but not always, chiral. In particular, if the stereocenters are configured in such a way that the molecule has an internal plane of symmetry, then the molecule is achiral and is known as a meso compound. Less commonly, other atoms like N, P, S, and Si can also serve as stereocenters, provided they have four distinct substituents (including lone pair electrons) attached to them.

Molecules with chirality arising from one or more stereocenters are classified as possessing central chirality. There are two other types of stereogenic elements that can give rise to chirality, a stereogenic axis (axial chirality) and a stereogenic plane (planar chirality). Finally, the inherent curvature of a molecule can also give rise to chirality (inherent chirality). These types of chirality are far less common than central chirality. BINOL is a typical example of an axially chiral molecule, while trans-cyclooctene is a commonly cited example of a planar chiral molecule. Finally, helicene possesses helical chirality, which is one type of inherent chirality.

Chirality is an important concept for stereochemistry and biochemistry. Most substances relevant to biology are chiral, such as carbohydrates (sugars, starch, and cellulose), the amino acids that are the building blocks of proteins, and the nucleic acids. In living organisms, one typically finds only one of the two enantiomers of a chiral compound. For that reason, organisms that consume a chiral compound usually can metabolize only one of its enantiomers. For the same reason, the two enantiomers of a chiral pharmaceutical usually have vastly different potencies or effects.

Definition

The chirality of a molecule is based on the molecular symmetry of its conformations. A conformation of a molecule is chiral if and only if it belongs to the C_n, D_n, T, O, I point groups (the chiral point groups). However, whether the molecule itself is considered to be chiral depends on whether its chiral conformations are persistent isomers that could be isolated as separated enantiomers, at least in principle, or the enantiomeric conformers rapidly interconvert at a given temperature and timescale through low-energy conformational changes (rendering the molecule achiral). For example, despite having chiral gauche conformers that belong to the C₂ point group, butane is considered achiral at room temperature because rotation about the central C–C bond rapidly interconverts the enantiomers (3.4 kcal/mol barrier). Similarly, cis-1,2-dichlorocyclohexane consists of chair conformers that are nonidentical mirror images, but the two can interconvert via the cyclohexane chair flip (~10 kcal/mol barrier). As another example, amines with three distinct substituents (R¹R²R³N:) are also regarded as achiral molecules because their enantiomeric pyramidal conformers rapidly invert and interconvert through a planar transition state (~6 kcal/mol barrier).

However, if the temperature in question is low enough, the process that interconverts the enantiomeric chiral conformations becomes slow compared to a given timescale. The molecule would then be considered to be chiral at that temperature. The relevant timescale is, to some degree, arbitrarily defined: 1000 seconds is sometimes employed, as this is regarded as the lower limit for the amount of time required for chemical or chromatographic separation of enantiomers in a practical sense. Molecules that are chiral at room temperature due to restricted rotation about a single bond (barrier to rotation ≥ ca. 23 kcal/mol) are said to exhibit atropisomerism.

A chiral compound can contain no improper axis of rotation (S_n), which includes planes of symmetry and inversion center. Chiral molecules are always dissymmetric (lacking S_n) but not always asymmetric (lacking all symmetry elements except the trivial identity). Asymmetric molecules are always chiral.^[5]

The following table shows some examples of chiral and achiral molecules, with the Schoenflies notation of the point group of the molecule. In the achiral molecules, X and Y (with no subscript) represent achiral groups, whereas X_R and X_S or Y_R and Y_S represent enantiomers. Note that there is no meaning to the orientation of an S₂ axis, which is just an inversion. Any orientation will do, so long as it passes through the center of inversion. Also note that higher symmetries of chiral and achiral molecules also exist, and symmetries that do not include those in the table, such as the chiral C₃ or the achiral S₄.

Molecular symmetry and chirality

Rotational axis (Cn)	Improper rotational elements (Sn)
	Chiral no Sn	Achiral mirror plane S1 = σ	Achiral inversion center S2 = i
C1	C1	Cs	Ci
C2	C2 (Note: This molecule has only one C2 axis: perpendicular to line of three C, but not in the plane of the figure.)	C2v	C2h Note: This also has a mirror plane.

Stereogenic centers

Main article: Stereogenic center

Many chiral molecules have point chirality, namely a single chiral stereogenic center that coincides with an atom. This stereogenic center usually has four or more bonds to different groups, and may be carbon (as in many biological molecules), phosphorus (as in many organophosphates), silicon, or a metal (as in many chiral coordination compounds). However, a stereogenic center can also be a trivalent atom whose bonds are not in the same plane, such as phosphorus in P-chiral phosphines (PRR′R″) and sulfur in S-chiral sulfoxides (OSRR′), because a lone-pair of electrons is present instead of a fourth bond.

Chirality can also arise from isotopic differences between atoms, such as in the deuterated benzyl alcohol PhCHDOH; which is chiral and optically active ([α]_D = 0.715°), even though the non-deuterated compound PhCH₂OH is not.

If two enantiomers easily interconvert, the pure enantiomers may be practically impossible to separate, and only the racemic mixture is observable. This is the case, for example, of most amines with three different substituents (NRR′R″), because of the low energy barrier for nitrogen inversion.

1,1′-Bi-2-naphthol is an example of a molecule lacking point chirality.

While the presence of a stereogenic center describes the great majority of chiral molecules, many variations and exceptions exist. For instance it is not necessary for the chiral substance to have a stereogenic center. Examples include 1-bromo-3-chloro-5-fluoroadamantane, methylethylphenyltetrahedrane, certain calixarenes and fullerenes, which have inherent chirality. The C₂-symmetric species 1,1′-bi-2-naphthol (BINOL), 1,3-dichloroallene have axial chirality. (E)-cyclooctene and many ferrocenes have planar chirality.

When the optical rotation for an enantiomer is too low for practical measurement, the species is said to exhibit cryptochirality.

Chirality is an intrinsic part of the identity of a molecule, so the systematic name includes details of the absolute configuration (R/S, D/L, or other designations).

Manifestations of chirality

• Flavor: the artificial sweetener aspartame has two enantiomers. L-aspartame tastes sweet whereas D-aspartame is tasteless.
• Odor: R-(–)-carvone smells like spearmint whereas S-(+)-carvone smells like caraway.
• Drug effectiveness: the antidepressant drug Citalopram is sold as a racemic mixture. However, studies have shown that only the (S)-(+) enantiomer is responsible for the drug's beneficial effects.
• Drug safety: D‑penicillamine is used in chelation therapy and for the treatment of rheumatoid arthritis whereas L‑penicillamine is toxic as it inhibits the action of pyridoxine, an essential B vitamin.

In biochemistry

Many biologically active molecules are chiral, including the naturally occurring amino acids (the building blocks of proteins) and sugars.

The origin of this homochirality in biology is the subject of much debate. Most scientists believe that Earth life's "choice" of chirality was purely random, and that if carbon-based life forms exist elsewhere in the universe, their chemistry could theoretically have opposite chirality. However, there is some suggestion that early amino acids could have formed in comet dust. In this case, circularly polarised radiation (which makes up 17% of stellar radiation) could have caused the selective destruction of one chirality of amino acids, leading to a selection bias which ultimately resulted in all life on Earth being homochiral.

Enzymes, which are chiral, often distinguish between the two enantiomers of a chiral substrate. One could imagine an enzyme as having a glove-like cavity that binds a substrate. If this glove is right-handed, then one enantiomer will fit inside and be bound, whereas the other enantiomer will have a poor fit and is unlikely to bind.

L-forms of amino acids tend to be tasteless, whereas D-forms tend to taste sweet. Spearmint leaves contain the L-enantiomer of the chemical carvone or R-(−)-carvone and caraway seeds contain the D-enantiomer or S-(+)-carvone. The two smell different to most people because our olfactory receptors are chiral.

Chirality is important in context of ordered phases as well, for example the addition of a small amount of an optically active molecule to a nematic phase (a phase that has long range orientational order of molecules) transforms that phase to a chiral nematic phase (or cholesteric phase). Chirality in context of such phases in polymeric fluids has also been studied in this context.

In inorganic chemistry

Delta-ruthenium-tris(bipyridine) cation

 

Main article: Complex (chemistry): Isomerism

Chirality is a symmetry property, not a property of any part of the periodic table. Thus many inorganic materials, molecules, and ions are chiral. Quartz is an example from the mineral kingdom. Such noncentric materials are of interest for applications in nonlinear optics.

In the areas of coordination chemistry and organometallic chemistry, chirality is pervasive and of practical importance. A famous example is tris(bipyridine)ruthenium(II) complex in which the three bipyridine ligands adopt a chiral propeller-like arrangement. The two enantiomers of complexes such as [Ru(2,2′-bipyridine)₃]²⁺ may be designated as Λ (capital lambda, the Greek version of "L") for a left-handed twist of the propeller described by the ligands, and Δ (capital delta, Greek "D") for a right-handed twist (pictured). Also cf. dextro- and levo- (laevo-).

Chiral ligands confer chirality to a metal complex, as illustrated by metal-amino acid complexes. If the metal exhibits catalytic properties, its combination with a chiral ligand is the basis of asymmetric catalysis.

Methods and practices

The term optical activity is derived from the interaction of chiral materials with polarized light. In a solution, the (−)-form, or levorotatory form, of an optical isomer rotates the plane of a beam of linearly polarized light counterclockwise. The (+)-form, or dextrorotatory form, of an optical isomer does the opposite. The rotation of light is measured using a polarimeter and is expressed as the optical rotation.

Enantiomers can be separated by chiral resolution. This often involves forming crystals of a salt composed of one of the enantiomers and an acid or base from the so-called chiral pool of naturally occurring chiral compounds, such as malic acid or the amine brucine. Some racemic mixtures spontaneously crystallize into right-handed and left-handed crystals that can be separated by hand. Louis Pasteur used this method to separate left-handed and right-handed sodium ammonium tartrate crystals in 1849. Sometimes it is possible to seed a racemic solution with a right-handed and a left-handed crystal so that each will grow into a large crystal.

Miscellaneous nomenclature

• Any non-racemic chiral substance is called scalemic. Scalemic materials can be enantiopure or enantioenriched.
• A chiral substance is enantiopure when only one of two possible enantiomers is present so that all molecules within a sample have the same chirality sense. Use of homochiral as a synonym is strongly discouraged.
• A chiral substance is enantioenriched or heterochiral when its enantiomeric ratio is greater than 50:50 but less than 100:0.
• Enantiomeric excess or e.e. is the difference between how much of one enantiomer is present compared to the other. For example, a sample with 40% e.e. of R contains 70% R and 30% S (70% − 30% = 40%).

History

The rotation of plane polarized light by chiral substances was first observed by Jean-Baptiste Biot in 1812, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals. Louis Pasteur deduced in 1848 that this phenomenon has a molecular basis. The term chirality itself was coined by Lord Kelvin in 1894. Different enantiomers or diastereomers of a compound were formerly called optical isomers due to their different optical properties. At one time, chirality was thought to be restricted to organic chemistry, but this misconception was overthrown by the resolution of a purely inorganic compound, a cobalt complex called hexol, by Alfred Werner in 1911.

In the early 1970s, various groups established that the human olfactory organ is capable of distinguishing chiral compounds.

Transparency and translucency

From Wikipedia, the free encyclopedia

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

Dichroic filters are created using optically transparent materials.

In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions are much larger than the wavelengths of the photons in question), the photons can be said to follow Snell's Law. Translucency (also called translucence or translucidity) allows light to pass through, but does not necessarily (again, on the macroscopic scale) follow Snell's law; the photons can be scattered at either of the two interfaces, or internally, where there is a change in index of refraction. In other words, a translucent material is made up of components with different indices of refraction. A transparent material is made up of components with a uniform index of refraction. Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant spectrum of every color. The opposite property of translucency is opacity.

When light encounters a material, it can interact with it in several different ways. These interactions depend on the wavelength of the light and the nature of the material. Photons interact with an object by some combination of reflection, absorption and transmission. Some materials, such as plate glass and clean water, transmit much of the light that falls on them and reflect little of it; such materials are called optically transparent. Many liquids and aqueous solutions are highly transparent. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are mostly responsible for excellent optical transmission.

Materials which do not transmit light are called opaque. Many such substances have a chemical composition which includes what are referred to as absorption centers. Many substances are selective in their absorption of white light frequencies. They absorb certain portions of the visible spectrum while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected or transmitted for our physical observation. This is what gives rise to color. The attenuation of light of all frequencies and wavelengths is due to the combined mechanisms of absorption and scattering.

Transparency can provide almost perfect camouflage for animals able to achieve it. This is easier in dimly-lit or turbid seawater than in good illumination. Many marine animals such as jellyfish are highly transparent.

Comparisons of 1. opacity, 2. translucency, and 3. transparency; behind each panel is a star.

Etymology

• late Middle English: from Old French, from medieval Latin transparent- ‘shining through’, from Latin transparere, from trans- ‘through’ + parere ‘be visible’.
• late 16th century (in the Latin sense): from Latin translucent- ‘shining through’, from the verb translucere, from trans- ‘through’ + lucere ‘to shine’.
• late Middle English opake, from Latin opacus ‘darkened’. The current spelling (rare before the 19th century) has been influenced by the French form.

Introduction

With regard to the absorption of light, primary material considerations include:

• At the electronic level, absorption in the ultraviolet and visible (UV-Vis) portions of the spectrum depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific frequency, and does not violate selection rules. For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible light, or if they do, they violate selection rules, meaning there is no appreciable absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.
• At the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds, and on selection rules. Nitrogen and oxygen are not greenhouse gases because there is no molecular dipole moment.

With regard to the scattering of light, the most critical factor is the length scale of any or all of these structural features relative to the wavelength of the light being scattered. Primary material considerations include:

• Crystalline structure: whether the atoms or molecules exhibit the 'long-range order' evidenced in crystalline solids.
• Glassy structure: scattering centers include fluctuations in density or composition.
• Microstructure: scattering centers include internal surfaces such as grain boundaries, crystallographic defects and microscopic pores.
• Organic materials: scattering centers include fiber and cell structures and boundaries.

Main article: Light scattering

 

General mechanism of diffuse reflection

Diffuse reflection - Generally, when light strikes the surface of a (non-metallic and non-glassy) solid material, it bounces off in all directions due to multiple reflections by the microscopic irregularities inside the material (e.g., the grain boundaries of a polycrystalline material, or the cell or fiber boundaries of an organic material), and by its surface, if it is rough. Diffuse reflection is typically characterized by omni-directional reflection angles. Most of the objects visible to the naked eye are identified via diffuse reflection. Another term commonly used for this type of reflection is "light scattering". Light scattering from the surfaces of objects is our primary mechanism of physical observation.

Light scattering in liquids and solids depends on the wavelength of the light being scattered. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension (or spatial scale) of the scattering center. Visible light has a wavelength scale on the order of a half a micrometer. Scattering centers (or particles) as small as one micrometer have been observed directly in the light microscope (e.g., Brownian motion).

Transparent ceramics

Optical transparency in polycrystalline materials is limited by the amount of light which is scattered by their microstructural features. Light scattering depends on the wavelength of the light. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometer, scattering centers will have dimensions on a similar spatial scale. Primary scattering centers in polycrystalline materials include microstructural defects such as pores and grain boundaries. In addition to pores, most of the interfaces in a typical metal or ceramic object are in the form of grain boundaries which separate tiny regions of crystalline order. When the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.

In the formation of polycrystalline materials (metals and ceramics) the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during formation (or pressing) of the object. Moreover, the size of the grain boundaries scales directly with particle size. Thus a reduction of the original particle size well below the wavelength of visible light (about 1/15 of the light wavelength or roughly 600/15 = 40 nanometers) eliminates much of light scattering, resulting in a translucent or even transparent material.

Computer modeling of light transmission through translucent ceramic alumina has shown that microscopic pores trapped near grain boundaries act as primary scattering centers. The volume fraction of porosity had to be reduced below 1% for high-quality optical transmission (99.99 percent of theoretical density). This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of sol-gel chemistry and nanotechnology.

Translucency of a material being used to highlight the structure of a photographic subject

Transparent ceramics have created interest in their applications for high energy lasers, transparent armor windows, nose cones for heat seeking missiles, radiation detectors for non-destructive testing, high energy physics, space exploration, security and medical imaging applications. Large laser elements made from transparent ceramics can be produced at a relatively low cost. These components are free of internal stress or intrinsic birefringence, and allow relatively large doping levels or optimized custom-designed doping profiles. This makes ceramic laser elements particularly important for high-energy lasers.

The development of transparent panel products will have other potential advanced applications including high strength, impact-resistant materials that can be used for domestic windows and skylights. Perhaps more important is that walls and other applications will have improved overall strength, especially for high-shear conditions found in high seismic and wind exposures. If the expected improvements in mechanical properties bear out, the traditional limits seen on glazing areas in today's building codes could quickly become outdated if the window area actually contributes to the shear resistance of the wall.

Currently available infrared transparent materials typically exhibit a trade-off between optical performance, mechanical strength and price. For example, sapphire (crystalline alumina) is very strong, but it is expensive and lacks full transparency throughout the 3–5 micrometer mid-infrared range. Yttria is fully transparent from 3–5 micrometers, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. Not surprisingly, a combination of these two materials in the form of the yttrium aluminium garnet (YAG) is one of the top performers in the field.

Absorption of light in solids

When light strikes an object, it usually has not just a single frequency (or wavelength) but many. Objects have a tendency to selectively absorb, reflect or transmit light of certain frequencies. That is, one object might reflect green light while absorbing all other frequencies of visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light. The manner in which visible light interacts with an object is dependent upon the frequency of the light, the nature of the atoms in the object, and often the nature of the electrons in the atoms of the object.

Some materials allow much of the light that falls on them to be transmitted through the material without being reflected. Materials that allow the transmission of light waves through them are called optically transparent. Chemically pure (undoped) window glass and clean river or spring water are prime examples of this.

Materials which do not allow the transmission of any light wave frequencies are called opaque. Such substances may have a chemical composition which includes what are referred to as absorption centers. Most materials are composed of materials which are selective in their absorption of light frequencies. Thus they absorb only certain portions of the visible spectrum. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. In the visible portion of the spectrum, this is what gives rise to color.

Absorption centers are largely responsible for the appearance of specific wavelengths of visible light all around us. Moving from longer (0.7 micrometer) to shorter (0.4 micrometer) wavelengths: red, orange, yellow, green and blue (ROYGB) can all be identified by our senses in the appearance of color by the selective absorption of specific light wave frequencies (or wavelengths). Mechanisms of selective light wave absorption include:

• Electronic: Transitions in electron energy levels within the atom (e.g., pigments). These transitions are typically in the ultraviolet (UV) and/or visible portions of the spectrum.
• Vibrational: Resonance in atomic/molecular vibrational modes. These transitions are typically in the infrared portion of the spectrum.

UV-Vis: Electronic transitions

In electronic absorption, the frequency of the incoming light wave is at or near the energy levels of the electrons within the atoms which compose the substance. In this case, the electrons will absorb the energy of the light wave and increase their energy state, often moving outward from the nucleus of the atom into an outer shell or orbital.

The atoms that bind together to make the molecules of any particular substance contain a number of electrons (given by the atomic number Z in the periodic chart). Recall that all light waves are electromagnetic in origin. Thus they are affected strongly when coming into contact with negatively charged electrons in matter. When photons (individual packets of light energy) come in contact with the valence electrons of atom, one of several things can and will occur:

• A molecule absorbs the photon, some of the energy may be lost via luminescence, fluorescence and phosphorescence.
• A molecule absorbs the photon which results in reflection or scattering.
• A molecule cannot absorb the energy of the photon and the photon continues on its path. This results in transmission (provided no other absorption mechanisms are active).

Most of the time, it is a combination of the above that happens to the light that hits an object. The states in different materials vary in the range of energy that they can absorb. Most glasses, for example, block ultraviolet (UV) light. What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum. But there are also existing special glass types, like special types of borosilicate glass or quartz that are UV-permeable and thus allow a high transmission of ultra violet light.

Thus, when a material is illuminated, individual photons of light can make the valence electrons of an atom transition to a higher electronic energy level. The photon is destroyed in the process and the absorbed radiant energy is transformed to electric potential energy. Several things can happen then to the absorbed energy: it may be re-emitted by the electron as radiant energy (in this case the overall effect is in fact a scattering of light), dissipated to the rest of the material (i.e. transformed into heat), or the electron can be freed from the atom (as in the photoelectric and Compton effects).

Infrared: Bond stretching

Normal modes of vibration in a crystalline solid

The primary physical mechanism for storing mechanical energy of motion in condensed matter is through heat, or thermal energy. Thermal energy manifests itself as energy of motion. Thus, heat is motion at the atomic and molecular levels. The primary mode of motion in crystalline substances is vibration. Any given atom will vibrate around some mean or average position within a crystalline structure, surrounded by its nearest neighbors. This vibration in two dimensions is equivalent to the oscillation of a clock’s pendulum. It swings back and forth symmetrically about some mean or average (vertical) position. Atomic and molecular vibrational frequencies may average on the order of 10¹² cycles per second (Terahertz radiation).

When a light wave of a given frequency strikes a material with particles having the same or (resonant) vibrational frequencies, then those particles will absorb the energy of the light wave and transform it into thermal energy of vibrational motion. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared light. Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When infrared light of these frequencies strikes an object, the energy is reflected or transmitted.

If the object is transparent, then the light waves are passed on to neighboring atoms through the bulk of the material and re-emitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted.

Transparency in insulators

An object may be not transparent either because it reflects the incoming light or because it absorbs the incoming light. Almost all solids reflect a part and absorb a part of the incoming light.

When light falls onto a block of metal, it encounters atoms that are tightly packed in a regular lattice and a "sea of electrons" moving randomly between the atoms. In metals, most of these are non-bonding electrons (or free electrons) as opposed to the bonding electrons typically found in covalently bonded or ionically bonded non-metallic (insulating) solids. In a metallic bond, any potential bonding electrons can easily be lost by the atoms in a crystalline structure. The effect of this delocalization is simply to exaggerate the effect of the "sea of electrons". As a result of these electrons, most of the incoming light in metals is reflected back, which is why we see a shiny metal surface.

Most insulators (or dielectric materials) are held together by ionic bonds. Thus, these materials do not have free conduction electrons, and the bonding electrons reflect only a small fraction of the incident wave. The remaining frequencies (or wavelengths) are free to propagate (or be transmitted). This class of materials includes all ceramics and glasses.

If a dielectric material does not include light-absorbent additive molecules (pigments, dyes, colorants), it is usually transparent to the spectrum of visible light. Color centers (or dye molecules, or "dopants") in a dielectric absorb a portion of the incoming light. The remaining frequencies (or wavelengths) are free to be reflected or transmitted. This is how colored glass is produced.

Most liquids and aqueous solutions are highly transparent. For example, water, cooking oil, rubbing alcohol, air, and natural gas are all clear. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are chiefly responsible for their excellent optical transmission. The ability of liquids to "heal" internal defects via viscous flow is one of the reasons why some fibrous materials (e.g., paper or fabric) increase their apparent transparency when wetted. The liquid fills up numerous voids making the material more structurally homogeneous.

Light scattering in an ideal defect-free crystalline (non-metallic) solid which provides no scattering centers for incoming light will be due primarily to any effects of anharmonicity within the ordered lattice. Light transmission will be highly directional due to the typical anisotropy of crystalline substances, which includes their symmetry group and Bravais lattice. For example, the seven different crystalline forms of quartz silica (silicon dioxide, SiO₂) are all clear, transparent materials.

Optical waveguides

Propagation of light through a multi-mode optical fiber

 

A laser beam bouncing down an acrylic rod, illustrating the total internal reflection of light in a multimode optical fiber

Optically transparent materials focus on the response of a material to incoming light waves of a range of wavelengths. Guided light wave transmission via frequency selective waveguides involves the emerging field of fiber optics and the ability of certain glassy compositions to act as a transmission medium for a range of frequencies simultaneously (multi-mode optical fiber) with little or no interference between competing wavelengths or frequencies. This resonant mode of energy and data transmission via electromagnetic (light) wave propagation is relatively lossless.

An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The refractive index is the parameter reflecting the speed of light in a material. (Refractive index is the ratio of the speed of light in vacuum to the speed of light in a given medium. The refractive index of vacuum is therefore 1.) The larger the refractive index, the more slowly light travels in that medium. Typical values for core and cladding of an optical fiber are 1.48 and 1.46, respectively.

When light traveling in a dense medium hits a boundary at a steep angle, the light will be completely reflected. This effect, called total internal reflection, is used in optical fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the boundary. Because the light must strike the boundary with an angle greater than the critical angle, only light that enters the fiber within a certain range of angles will be propagated. This range of angles is called the acceptance cone of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding. Optical waveguides are used as components in integrated optical circuits (e.g. combined with lasers or light-emitting diodes, LEDs) or as the transmission medium in local and long haul optical communication systems.

Mechanisms of attenuation

See also: Light scattering

 

Light attenuation by ZBLAN and silica fibers

Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the very high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a signal across large distances. In optical fibers the main attenuation source is scattering from molecular level irregularities (Rayleigh scattering) due to structural disorder and compositional fluctuations of the glass structure. This same phenomenon is seen as one of the limiting factors in the transparency of infrared missile domes. Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.

As camouflage

Many animals of the open sea, like this Aurelia labiata jellyfish, are largely transparent.

 

Further information: List of camouflage methods

Many marine animals that float near the surface are highly transparent, giving them almost perfect camouflage. However, transparency is difficult for bodies made of materials that have different refractive indices from seawater. Some marine animals such as jellyfish have gelatinous bodies, composed mainly of water; their thick mesogloea is acellular and highly transparent. This conveniently makes them buoyant, but it also makes them large for their muscle mass, so they cannot swim fast, making this form of camouflage a costly trade-off with mobility. Gelatinous planktonic animals are between 50 and 90 percent transparent. A transparency of 50 percent is enough to make an animal invisible to a predator such as cod at a depth of 650 metres (2,130 ft); better transparency is required for invisibility in shallower water, where the light is brighter and predators can see better. For example, a cod can see prey that are 98 percent transparent in optimal lighting in shallow water. Therefore, sufficient transparency for camouflage is more easily achieved in deeper waters. For the same reason, transparency in air is even harder to achieve, but a partial example is found in the glass frogs of the South American rain forest, which have translucent skin and pale greenish limbs. Several Central American species of clearwing (ithomiine) butterflies and many dragonflies and allied insects also have wings which are mostly transparent, a form of crypsis that provides some protection from predators.

Racial profiling in the United States

From Wikipedia, the free encyclopedia

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

Racial profiling is referred to regarding its use by law enforcement at the local, state, and federal levels, which leads to discrimination against people in the African American, Native American, Asian, Pacific Islander, Latino, Arab, and Muslim communities of the United States. Examples of racial profiling are the use of race to determine which drivers to stop for minor traffic violations (commonly referred to as 'driving while black, Asian, Native American, Middle Eastern, Hispanic, or brown'), or the use of race to determine which pedestrians to search for illegal contraband. Besides such disproportionate searching of African Americans, and members of other minority groups, other examples of racial profiling by law enforcement in the U.S. include the targeting of Hispanic and Latino Americans in the investigation of illegal immigration; and the focus on Middle Eastern and South Asians present in the country in screenings for ties to Islamic terrorism. These suspicions may be held on the basis of belief that members of a target racial group commit crimes at a higher rate than that of other racial groups.

Definitions

According to the American Civil Liberties Union (ACLU):

'Racial profiling refers to the practice by law enforcement officials of targeting individuals for suspicion of crime based on the individual's race, ethnicity, religion or national origin.

According to Minnesota House of Representatives analyst Jim Cleary:

"There appears to be at least two clearly distinguishable definitions of the term 'racial profiling': a narrow definition and a broad definition... Under the narrow definition, racial profiling occurs when a police officer stops and/or searches someone solely on the basis of the person's race or ethnicity... Under the broader definition, racial profiling occurs whenever police routinely use race as a factor that, along with an accumulation of other factors, causes an officer to react with suspicion and take action."

History

Sociologist Robert Staples emphasizes that racial profiling in the U.S. is "not merely a collection of individual offenses" but, rather, a systemic phenomenon across American society, dating back to the era of slavery, and, until the 1950s, was, in some instances, "codified into law". Enshrinement of racial profiling ideals in United States law can be exemplified by several major periods in U.S. history.

In 1693, Philadelphia's court officials gave police legal authority to stop and detain any Black person (freed or enslaved) seen wandering about. Starting around the mid 18th century, slave patrols were used to stop slaves at any location in order to ensure they were being lawful. In the mid 19th century, the Black Codes, a set of statutes, laws and rules, were enacted in the South in order to regain control over freed and former slaves and relegate African Americans to a lower social status. Similar discriminatory practices continued through the Jim Crow era.

Prior to U.S. immigration restrictions following the September 11 attacks, Japanese immigrants were rejected U.S. citizenship during World War II, for fear of disloyalty following the attacks on Pearl Harbor. What resulted was the government's preemptive internment of more than 100,000 Japanese immigrants and Japanese American citizens during World War II, as a measure against potential Japanese espionage, constituting a form of racial profiling.

In the late 1990s racial profiling became politicized when police and other law enforcement fell under scrutiny for the disproportionate traffic stops of minority motorists. Researchers from the American Civil Liberties Union (ACLU) provided evidence of widespread racial profiling, one study showed that while blacks only made up 42 percent of New Jersey's driving population, they accounted for 79 percent of motorists stopped in the state.

Supreme Court cases

Terry v. Ohio was the first challenge to racial profiling in the United States in 1968. This case was about African American people who were suspected to be stealing. The police officer arrested the three men and searched them and found a gun on two of the three men, and John W. Terry (one of the three men searched) was convicted and sentenced to jail. Terry challenged the arrest on the grounds that it violated the search and seizure clause of the Fourth Amendment; however, in an 8–1 ruling, the Supreme Court decided that the police officer acted in a reasonable manner, and with reasonable suspicion, under the Fourth Amendment. The decision in this case allowed for greater police discretion in identifying suspicious or illegal activities. This case resulted in the creation of the Terry stop, in which the police may stop a person based on reasonable suspicion of involvement in criminal activity.

In 1975, United States v. Brignoni-Ponce was decided. Felix Humberto Brignoni-Ponce was traveling in his vehicle and was stopped by border patrol agents because he appeared to be Mexican. The agents questioned Brignoni-Ponce and the other passengers in the car and discovered that the passengers were illegal immigrants, and the border agents subsequently arrested all occupants of the vehicle. The Supreme Court determined that the testimonies that led to the arrests, in this case, were not valid, as they were obtained in the absence of reasonable suspicion and the vehicle was stopped without probable cause, as required under the Fourth Amendment.

In 1996, the U.S. Supreme Court ruled in United States v. Armstrong that disparity in conviction rates is not unconstitutional in the absence of data that "similarly situated" defendants of another race were disparately prosecuted, overturning a 9th Circuit Court ruling that was based on "the presumption that people of all races commit all types of crimes – not with the premise that any type of crime is the exclusive province of any particular racial or ethnic group", waving away challenges based on the Fourth Amendment of the U.S. Constitution which guarantees the right to be safe from search and seizure without a warrant (which is to be issued "upon probable cause"), and the Fourteenth Amendment which requires that all citizens be treated equally under the law. To date, there have been no known cases in which any U.S. court dismissed a criminal prosecution because the defendant was targeted based on race. This Supreme Court decision doesn't prohibit government agencies from enacting policies prohibiting it in the field by agents and employees.

The Supreme Court also decided the case of Whren v. United States in 1996. Michael Whren was arrested on felony drug charges after police officers observed his truck sitting at an intersection for a long period of time before he failed to use his turn signal to drive away, and the officers stopped his vehicle for the traffic violation. Upon approaching the vehicle the officers observed that Whren was in possession of crack cocaine. The Court determined the officers did not violate the Fourth Amendment through an unreasonable search and seizure and that the officers were permitted to stop the vehicle after it committed a traffic violation and the subsequent search of the vehicle was permitted regardless of the pretext of the officers.

In June 2001 the Bureau of Justice Assistance, a component of the Office of Justice Programs of the United States Department of Justice, awarded a Northeastern research team a grant to create the web-based Racial Profiling Data Collection Resource Center. It now maintains a website designed to be a central clearinghouse for police agencies, legislators, community leaders, social scientists, legal researchers, and journalists to access information about current data collection efforts, legislation and model policies, police-community initiatives, and methodological tools that can be used to collect and analyze racial profiling data. The website contains information on the background of data collection, jurisdictions currently collecting data, community groups, legislation that is pending and enacted in states across the country, and has information on planning and implementing data collection procedures, training officers in to implement these systems, and analyzing and reporting the data and results.

Statutory law

In April 2010, Arizona enacted SB 1070, a law that would require law-enforcement officers to verify the citizenship of individuals they stop if they have reasonable suspicion that they may be in the United States illegally. The law states that "Any person who is arrested shall have the person's immigration status determined before the person is released". United States federal law requires that all immigrants who remain in the United States for more than 30 days register with the U.S. government. In addition, all immigrants age 18 and over are required to have their registration documents with them at all times. Arizona made it a misdemeanor crime for an illegal immigrant 14 years of age and older to be found without carrying these documents at all times.

According to SB 1070, law-enforcement officials may not consider "race, color, or national origin" in the enforcement of the law, except under the circumstances allowed under the United States and Arizona constitutions. In June 2012, the majority of SB 1070 was struck down by the United States Supreme Court, while the provision allowing for an immigration check on detained persons was upheld.

Some states contain "stop and identify" laws that allow officers to detain suspected persons and ask for identification, and if there is a failure to provide identification punitive measures can be taken by the officer. As of 2017, there are 24 states that have "stop and identify" statues; however, the criminal punishments and requirements to produce identification vary from state to state. Utah HB 497 requires residents to carry relevant identification at all times in order to prove resident status or immigration status; even so, police may still dismiss provided documents under suspicion of falsification and arrest or detain suspects.

In early 2001, a bill was introduced to Congress named "End Racial Profiling Act of 2001" but lost support in the wake of the September 11 attacks. The bill was re-introduced to Congress in 2010 but also failed to gain the support it needed. Several U.S. states now have reporting requirements for incidents of racial profiling. Texas, for example, requires all agencies to provide annual reports to its Law Enforcement Commission. The requirement began on September 1, 2001, when the State of Texas passed a law to require all law enforcement agencies in the state to begin collecting certain data in connection to traffic or pedestrian stops beginning on January 1, 2002. Based on that data, the law mandated law enforcement agencies to submit a report to the law enforcement agencies' governing body beginning March 1, 2003, and each year thereafter no later than March 1. The law is found in the Texas Code of Criminal Procedure beginning with Article 2.131.

Additionally, on January 1, 2011, all Texas law enforcement agencies began submitting annual reports to the Texas State Law Enforcement Officers Standards and Education Commission. The submitted reports can be accessed on the commission's website for public review.

In June 2003, the Department of Justice issued its Guidance Regarding the Use of Race by Federal Law Enforcement Agencies forbidding racial profiling by federal law enforcement officials.

Support

Supporters defend the practice of racial profiling by emphasizing the crime control model. They claim that the practice is both efficient and ideal due to utilizing the laws of probability in order to determine one's criminality. This system focuses on controlling crime with swift judgment, bestowing full discretion on police to handle what they perceive as a threat to society.

The use and support of racial profiling has surged in recent years, namely in North America due to heightened tension and awareness following the events of 9/11. As a result, the issue of profiling has created a debate that centers on the values of equality and self-defense. Supporters uphold the stance that sacrifices must be made in order to maintain national safety, even if it warrants differential treatment. According to a 2011 survey by Rasmussen Reports, a majority of Americans support profiling as necessary "in today's society".

In December 2010, Fernando Mateo, then president of the New York State Federation of Taxi Drivers, made pro-racial profiling remarks in the case of gun-shot taxi-cab driver: "You know sometimes it's good that we are racially profiled because the God's-honest truth is that 99 percent of the people that are robbing, stealing, killing these drivers are blacks and Hispanics." "Clearly everyone knows I'm not racist. I'm Hispanic and my father is black. ... My father is blacker than Al Sharpton." When confronted with accusations of racial profiling the police claim that they do not participate in it. They emphasize that numerous factors (such as race, interactions, and dress) are used to determine if a person is involved in criminal activity and that race is not a sole factor in the decision to detain or question an individual. They further claim that the job of policing is far more imperative than to concerns of minorities or interest groups claiming unfair targeting.

Proponents of racial profiling believe that inner city residents of Hispanic communities are subjected to racial profiling because of theories such as the "gang suppression model". The "gang suppression model" is believed by some to be the basis for increased policing, the theory being based on the idea that Latinos are violent and out of control and are therefore "in need of suppression". Based on research, the criminalization of a people can lead to abuses of power on behalf of law enforcement.

Criticism

Critics of racial profiling argue that the individual rights of a suspect are violated if race is used as a factor in that suspicion. Notably, civil liberties organizations such as the American Civil Liberties Union (ACLU) have labeled racial profiling as a form of discrimination, stating, "Discrimination based on race, ethnicity, religion, nationality or on any other particular identity undermines the basic human rights and freedoms to which every person is entitled."

Conversely, those in opposition of the police tactic employ the teachings of the due process model, arguing that minorities are not granted equal rights and are thus subject to unjust treatment. In addition, some argue that the singling out of individuals based on their ethnicity comes in violation of the Rule of Law, having voided all instance of neutrality. Those in opposition also make note of the role that the news media plays within the conflict. The general public internalizes much of its knowledge from the media, relying on sources to convey information of events that transpire outside of their immediate domain. In conjunction with this power, media outlets are aware of the public's intrigue with controversy and have been known to construct headlines that entail moral panic and negativity.

In the case of racial profiling drivers, the ethnic backgrounds of drivers stopped by traffic police in the U.S. suggests the possibility of biased policing against non-white drivers. Black drivers felt that they were being pulled over by law enforcement officers simply because of their skin color. However, some argue in favor of the "veil of darkness" hypothesis, which states that police are less likely to know the race of a driver before they make a stop at nighttime as opposed to in the daytime. Referring to the veil of darkness hypothesis, it is suggested that if the race distribution of drivers stopped during the day differs from that of drivers stopped at night, officers are engaging in racial profiling. For example, in one study done by Jeffrey Grogger and Greg Ridgeway, the veil of darkness hypothesis was used to determine whether or not racial profiling in traffic stops occurs in Oakland, California. The conductors found that there was little evidence of racial profiling in traffic stops made in Oakland.

Research through random sampling in the South Tucson, Arizona area has established that immigration authorities sometimes target the residents of barrios with the use of possibly discriminatory policing based on racial profiling. Author Mary Romero writes that immigration raids are often carried out at places of gathering and cultural expression such as grocery stores based on the fluency of language of a person (e.g. being bilingual especially in Spanish) and skin color of a person. She goes on to state that immigration raids are often conducted with a disregard for due process, and that these raids lead people from these communities to distrust law enforcement.

In a recent journal comparing the 1990s to the present, studies have established that when the community criticized police for targeting the black community during traffic stops it received more media coverage and toned down racial profiling. However, whenever there was a significant lack of media coverage or concern with racial profiling, the amount of arrests and traffic stops for the African-American community would significantly rise again.

New York Police Department

Suspicionless surveillance of Muslims

Main article: New York City Police Department Intelligence Bureau § Demographics Unit

Between 2003 and 2014, the New York City Police Department (NYPD) operated the "Demographics Unit" (later renamed "Zone Assessment Unit") which mapped communities of 28 "ancestries of interest", including those of Muslims, Arabs, and Albanians. Plain-clothed detectives were sent to public places such as coffee shops, mosques and parks to observe and record the public sentiment, as well as map locations where potential terrorists could "blend in". In its 11 years of operation, however, the unit did not generate any information leading to a criminal charge. A series of publications by the Associated Press during 2011–12 gave rise to public pressure to close the unit, and it was finally disbanded in 2014. Racial profiling not only occurs on the streets but also in many institutions. Much like the book Famous all over Town where the author Danny Santiago mentions this type of racism throughout the novel. According to Jesper Ryberg's 2011 article "Racial Profiling And Criminal Justice" in the Journal of Ethics, "It is argued that, given the assumption that criminals are currently being punished too severely in Western countries, the apprehension of more criminals may not constitute a reason in favor of racial profiling at all." It has been stated in a scholarly journal that for over 30 years the use of racial and/or demographic profiling by local authorities and higher level law enforcement's continue to proceed. NYPD Street cops use racial profiling more often, due to the widespread patterns. They first frisk them to check whether they have enough evidence to be even arrested for the relevant crime. "As a practical matter, the stops display a measurable racial disparity: black and Hispanic people generally represent more than 85 percent of those stopped by the police, though their combined populations make up a small share of the city's racial composition." (Baker)

Stop-and-frisk

Main article: Stop-and-frisk in New York City

The NYPD has been subject to much criticism for its "stop-and-frisk" tactics. According to statistics on the NYPD's stop and frisk policies, collected by the Center for Constitutional Rights, 51% of the people stopped by the police were Black, 33% were Latino, and 9% were White, and only 2% of all stops resulted in contraband findings. Starting in 2013, use of racial profiling by the NYPD was drastically curtailed, as New York Mayor Bill de Blasio was campaigning for the office, and this policy has continued into his term. In June 2019, the independent Office of the Inspector General for the NYPD (OIG-NYPD), under New York City's Department of Investigation (DOI), released a report which found deficiencies in how NYPD tracked and investigated allegations of racial profiling and other types of biased policing against NYPD officers. The report concluded that NYPD had never substantiated any complaints of biased policing since it began tracking them in 2014.

Dealing with terrorism

See also: Islamophobia

The September 11, 2001 attacks on the World Trade Center and the Pentagon have led to targeting of some Muslims and Middle Easterners as potential terrorists and, according to some, are targeted by the national government through preventive measures similar to those practiced by local law enforcement. The national government has passed laws, such as the Patriot Act of 2001, to increase surveillance of potential threats to national security as a result of the events that occurred during 9/11. It is argued that the passage of these laws and provisions by the national government leads to justification of preventative methods, such as racial profiling, that has been controversial for racial profiling and leads to further minority distrust in the national government. One of the techniques used by the FBI to target Muslims was monitoring 100 mosques and business in Washington DC and threatened to deport Muslims who did not agree to serve as informers. The FBI denied to be taking part in blanket profiling and argued that they were trying to build trust within the Muslim community.

On September 14, 2001, three days after the September 11 attacks, an Indian American motorist and three family members were pulled over and ticketed by a Maryland state trooper because their car had broken taillights. The trooper interrogated the family, questioned them about their nationality, and asked for proof of citizenship. When the motorist said that their passports were at home, the officer allegedly stated, "You are lying. You are Arabs involved in terrorism." He ordered them out of the car, had them put their hands on the hood, and searched the car. When he discovered a knife in a toolbox, the officer handcuffed the driver and later reported that the driver "wore and carried a butcher knife, a dangerous, deadly weapon, concealed upon and about his person." The driver was detained for several hours but eventually released.

In December 2001, an American citizen of Middle Eastern descent named Assem Bayaa cleared all the security checks at Los Angeles airport and attempted to board a flight to New York City. Upon boarding, he was told that he made the passengers uncomfortable by being on board the plane and was asked to leave. Once off the plane, he wasn't searched or questioned any further and the only consolation he was given was a boarding pass for the next flight. He filed a lawsuit on the basis of discrimination against United Airlines. United Airlines filed a counter motion which was dismissed by a district judge on October 11, 2002. In June 2005, the ACLU announced a settlement between Bayaa and United Airlines who still disputed Bayaa's allegations, but noted that the settlement "was in the best interest of all".

The events of 9/11 also led to restrictions in immigration laws. The U.S. government imposed stricter immigration quotas to maintain national security at their national borders. In 2002, men over sixteen years old who entered the country from twenty-five Middle Eastern countries and North Korea were required to be photographed, fingerprinted, interviewed and have their financial information copied, and had to register again before leaving the country under the National Security Entry-Exit Registration System. No charges of terrorism resulted from the program, and it was deactivated in April 2011.

In 2006, 18 young men from the Greater Toronto Area were charged with conspiring to carry out a series of bombings and beheadings, resulting in a swell of media coverage. Two media narratives stood out with the former claiming that a militant subculture was forming within the Islamic community while the latter attributed the case to a bunch of deviant youth who had too much testosterone brewing. Eventually, it was shown that government officials had been tracking the group for some time, having supplied the youth with the necessary compounds to create explosives, prompting critics to discern whether the whole situation was a set-up. Throughout the case, many factors were put into question but none more than the Muslim community who faced much scrutiny and vitriol due to the build-up of negative headlines stemming from the media.

Studies

Statistical data demonstrates that although policing practices and policies vary widely across the United States, a large disparity between racial groups in regards to traffic stops and searches exists. Based on academic search, various studies have been conducted regarding the existence of racial profiling in traffic and pedestrian stops. For motor vehicle searches, academic research showed that the probability of a successful search is somewhat similar across races. Similar evidence has been found for pedestrian stops, with identical ratios of stops to arrests for different races.

One study concluded that the ratio of different races stopped by New York cops is about the same for all races tested, though other studies have found the rates of stops to be starkly racially differentiated.

In a study conducted in Cincinnati, Ohio, it was concluded that "Blacks were between three and five times more likely to (a) be asked if they were carrying drugs or weapons, (b) be asked to leave the vehicle, (c) be searched, (d) have a passenger searched, and (e) have the vehicle physically searched in a study conducted. This conclusion was based on the analysis of 313 randomly selected, traffic stop police tapes gathered from 2003 to 2004."

A 2001 study analyzing data from the Richmond, Virginia Police Department found that African Americans were disproportionately stopped compared to their proportion in the general population, but that they were not searched more often than Whites. The same study found that Whites were more likely than African Americans to be "the subjects of consent searches", and that Whites were more likely to be ticked or arrested than minorities, while minorities were more likely to be warned. A 2002 study found that African Americans were more likely to be watched and stopped by police when driving through white areas, despite the fact that African Americans' "hit rates" were lower in such areas. A 2004 study analyzing traffic stop data from suburban police department found that although minorities were disproportionately stopped, there is only a "very weak" relationship between race and police decisions to stop. Other studies have found that young black and Hispanic men were more likely to be issued citations, arrested, and to have force used against them by police, even after controlling for numerous other factors.

A 2005 study found that the percent of speeding drivers who were black (as identified by other drivers) on the New Jersey Turnpike was very similar to the percent of people pulled over for speeding who were black. A 2004 study looking at motor vehicle searches in Missouri found that unbiased policing did not explain the racial disparity in such searches. In contrast, a 2006 study examining data from Kansas concluded that its results were "consistent with the notion that police in Wichita choose their search strategies to maximize successful searches," and a 2009 study found that racial disparities in people being searched by the Washington state patrol was "likely not the result of intentional or purposeful discrimination." Another 2009 study found that police in Boston were more likely to search if their race was different from that of the suspect, in contrast to what would be expected if preference based discrimination was not occurring (which would be that police search decisions are independent of officer race).

A 2010 study found that black drivers were more likely to be searched at traffic stops in white neighborhoods, whereas white drivers were more likely to be searched by white officers at stops in black neighborhoods. A 2013 study found that police were more likely to issue warnings and citations, but not arrests, to young black men. A 2014 study analyzing data from Rhode Island found that blacks were more likely than whites to be frisked and, to a lesser extent, searched while driving; the study concluded that "Biased policing is largely the product of implicit stereotypes that are activated in contexts in which Black drivers appear out of place and in police actions that require quick decisions providing little time to monitor cognitions."

As far as the consequences, studies have shown a myriad of effects on people who have experienced racially discriminatory police stops. For example, one study found that perception of police discrimination is significantly related to adopting what Elijah Anderson called the "code of the street", and that this relationship is conditioned by neighborhood-level violence.

As a response to the shooting of Michael Brown in Ferguson on August 9, 2014, the Department of Justice recruited in September a team of criminal justice researchers to study racial bias in law enforcement in five cities and to subsequently devise strategic recommendations. In its March 2015 report on the Ferguson Police Department, the Department of Justice found that although only 67% of the population of Ferguson was black, 85% of people pulled over by police in Ferguson were black, as were 93 percent of those arrested and 90 percent of those given citations by the police.

A 2020 study in the journal Nature that analyzed 100 million traffic stops found that "black drivers were less likely to be stopped after sunset, when a ‘veil of darkness’ masks one's race, suggesting bias in stop decisions", "the bar for searching black and Hispanic drivers was lower than that for searching white drivers", and "legalization of recreational marijuana reduced the number of searches of white, black and Hispanic drivers—but the bar for searching black and Hispanic drivers was still lower than that for white drivers post-legalization". The authors concluded that "police stops and search decisions suffer from persistent racial bias and point to the value of policy interventions to mitigate these disparities".

A 2016 study found that shootings of police officers by black suspects increased racial profiling of black civilians (but no similar effect for white or Hispanic shooters on profiling white or Hispanic civilians) for a brief period in the immediate aftermath of the shooting.

Racial profiling in retail

Shopping forms one major avenue for racial profiling. General discrimination devalues the experience of shopping, arguably raising the costs and reducing the rewards derived from consumption for the individual. When a store's sales staff appears hesitant to serve black shoppers or suspects that they are prospective shoplifters, the act of shopping no longer becomes a form of leisure.

Racial profiling in retail was prominent enough in 2001 that psychology researchers such as Jerome D. Williams coined the term "shopping while black", which describes the experience of being denied service or given poor service because one is black. Commonly, "shopping while black" involves, but is not limited to, a black or non-white customer being followed around and/or closely monitored by a clerk or guard who suspects he or she may steal, based on the color of their skin. It can also involve being denied store access, being refused service, use of ethnic slurs, being searched, being asked for extra forms of identification, having purchases limited, being required to have a higher credit limit than other customers, being charged a higher price, or being asked more or more rigorous questions on applications. These negative shopping experiences can directly contribute to the decline of shopping in stores as individuals will come to prefer to shop online, avoiding interactions that are deemed degrading, embarrassing, and highly offensive.

Public opinion

Perceptions of race and safety

In a particular study, Higgins, Gabbidon, and Vito studied the relationship between public opinion on racial profiling in conjunction with their viewpoint of race relations and their perceived awareness of safety. It was found that race relations had a statistical correlation with the legitimacy of racial profiling. Specifically, results showed that those who believed that racial profiling was widespread and that racial tension would never be fixed were more likely to be opposed to racial profiling than those who did not believe racial profiling was as widespread or that racial tensions would be fixed eventually. On the other hand, in reference to the perception of safety, the research concluded that one's perception of safety had no influence on public opinion of racial profiling. Higgins, Gabbidon, and Vito acknowledge that this may not have been the case immediately after 9/11, but state that any support of racial profiling based on safety was "short-lived".

Influence of religious affiliation

One particular study focused on individuals who self-identified as religiously affiliated and their relationship with racial profiling. By using national survey data from October 2001, researcher Phillip H. Kim studied which individuals were more likely to support racial profiling. The research concludes that individuals that identified themselves as either Jewish, Catholic, or Protestant showed higher statistical numbers that illustrated support for racial profiling in comparison to individuals who identified themselves as non-religious.

Contexts of terrorism and crime

After the September 11, 2001 terrorist attacks on the United States, according to Johnson, a new debate concerning the appropriateness of racial profiling in the context of terrorism took place. According to Johnson, prior to the September 11, 2001 attacks the debate on racial profiling within the public targeted primarily African-Americans and Latino Americans with enforced policing on crime and drugs. The attacks on the World Trade Center and the Pentagon changed the focus of the racial profiling debate from street crime to terrorism. According to a June 4–5, 2002 FOX News/Opinion Dynamics Poll, 54% of Americans approved of using "racial profiling to screen Arab male airline passengers." A 2002 survey by Public Agenda tracked the attitudes toward the racial profiling of Blacks and people of Middle Eastern descent. In this survey, 52% of Americans said there was "no excuse" for law enforcement to look at African Americans with greater suspicion and scrutiny because they believe they are more likely to commit crimes, but only 21% said there was "no excuse" for extra scrutiny of Middle Eastern people.

However, using data from an internet survey based experiment performed in 2006 on a random sample of 574 adult university students, a study was conducted that examined public approval for the use of racial profiling to prevent crime and terrorism. It was found that approximately one third of students approved the use of racial profiling in general. Furthermore, it was found that students were equally likely to approve of the use of racial profiling to prevent crime as to prevent terrorism-33% and 35.8% respectively. The survey also asked respondents whether they would approve of racial profiling across different investigative contexts.

The data showed that 23.8% of people approved of law enforcement using racial profiling as a means to stop and question someone in a terrorism context while 29.9% of people approved of racial profiling in a crime context for the same situation. It was found that 25.3% of people approved of law enforcement using racial profiling as a means to search someone's bags or packages in a terrorism context while 33.5% of people approved of racial profiling in a crime context for the same situation. It was also found that 16.3% of people approved of law enforcement wire tapping a person's phone based upon racial profiling in the context of terrorism while 21.4% of people approved of racial profiling in a crime context for the same situation. It was also found that 14.6% of people approved of law enforcement searching someone's home based upon racial profiling in a terrorism context while 18.2% of people approved of racial profiling in a crime context for the same situation.

The study also found that white students were more likely to approve of racial profiling to prevent terrorism than nonwhite students. However, it was found that white students and nonwhite students held the same views about racial profiling in the context of crime. It was also found that foreign born students were less likely to approve of racial profiling to prevent terrorism than non-foreign born students while both groups shared similar views on racial profiling in the context of crime.