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An optical prism is a transparent
optical Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
element with flat, polished surfaces that are designed to
refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 terahe ...
. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical prism is the
triangular prism In geometry, a triangular prism is a three-sided prism; it is a polyhedron made of a triangular base, a translated copy, and 3 faces joining corresponding sides. A right triangular prism has rectangular sides, otherwise it is ''oblique''. A un ...
, which has a triangular base and rectangular sides. Not all optical prisms are geometric prisms, and not all geometric prisms would count as an optical prism. Prisms can be made from any material that is transparent to the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s for which they are designed. Typical materials include
glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) o ...
,
acrylic Acrylic may refer to: Chemicals and materials * Acrylic acid, the simplest acrylic compound * Acrylate polymer, a group of polymers (plastics) noted for transparency and elasticity * Acrylic resin, a group of related thermoplastic or thermosett ...
and
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs scale ...
. A
dispersive prism In optics, a dispersive prism is an optical prism that is used to disperse light, that is, to separate light into its spectral components (the colors of the rainbow). Different wavelengths (colors) of light will be deflected by the prism at d ...
can be used to break white light up into its constituent
spectral color A spectral color is a color that is evoked by ''monochromatic light'', i.e. either a single wavelength of light in the visible spectrum, or by a relatively narrow band of wavelengths (e.g. lasers). Every wavelength of visible light is percei ...
s (the colors of the
rainbow A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows ...
) as described in the following section. Other types of prisms noted below can be used to reflect light, or to split light into components with different
polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
s.


Types


Dispersive

''Dispersive prisms'' are used to break up light into its constituent spectral colors because the refractive index depends on
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
; the white light entering the prism is a mixture of different wavelengths, each of which gets bent slightly differently. Blue light is slowed more than red light and will therefore be bent more than red light. *
Triangular prism In geometry, a triangular prism is a three-sided prism; it is a polyhedron made of a triangular base, a translated copy, and 3 faces joining corresponding sides. A right triangular prism has rectangular sides, otherwise it is ''oblique''. A un ...
*
Amici prism An Amici prism, named for the astronomer Giovanni Battista Amici, is a type of compound dispersive prism used in spectrometers. The Amici prism consists of two triangular prisms in contact, with the first typically being made from a medium- ...
and other types of
compound prism A compound prism is a set of multiple triangular prism, triangular prism elements placed in contact, and often cemented together to form a solid assembly. The use of multiple elements gives several advantages to an optical designer:Nathan Hagen and ...
s *
Littrow prism In optics, a Littrow prism or Littrow spectrograph or Littrow mirror is a retro-reflecting dispersing prism arranged in such a way that an incident light beam which enters at the Brewster angle undergoes minimal deviation and hence maximum dispersi ...
with mirror on its rear facet *
Pellin–Broca prism A Pellin–Broca prism is a type of constant-deviation dispersive prism similar to an Abbe prism. The prism is named for its inventors, the French instrument maker Ph. Pellin and professor of physiological optics André Broca. The prism con ...
*
Abbe prism In optics, an Abbe prism, named for its inventor, the German physicist Ernst Abbe, is a type of ''constant deviation dispersive prism'' similar to a Pellin–Broca prism. Structure The prism consists of a block of glass forming a right prism wi ...
*
Grism A grism (also called a grating prism) is a combination of a prism and grating arranged so that light at a chosen central wavelength passes straight through. The advantage of this arrangement is that one and the same camera can be used both for ima ...
, a dispersive prism with a diffraction grating on its surface * Féry prism Spectral
dispersion Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...
is the best known property of optical prisms, although not the most frequent purpose of using optical prisms in practice.


Reflective

''Reflective prisms'' are used to reflect light, in order to flip, invert, rotate, deviate or displace the light beam. They are typically used to erect the image in
binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes ( binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
or
single-lens reflex camera A single-lens reflex camera (SLR) is a camera that typically uses a mirror and prism system (hence "reflex" from the mirror's reflection) that permits the photographer to view through the lens and see exactly what will be captured. With twin l ...
s – without the prisms the image would be upside down for the user. Reflective prisms use
total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
to achieve near-perfect reflection of light that strikes the facets at a sufficiently oblique angle. Prisms are usually made of
optical glass Glass is a non-crystalline, often transparent, amorphous solid that has widespread practical, technological, and decorative use in, for example, window panes, tableware, and optics. Glass is most often formed by rapid cooling (quenching) o ...
which, combined with
anti-reflective coating An antireflective, antiglare or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses, other optical elements, and photovoltaic cells to reduce reflection. In typical imaging systems, this improves the effic ...
of input and output facets, leads to significantly lower light loss than metallic mirrors. *Odd number of reflections, image projects as flipped (mirrored) **triangular prism reflector, projects image sideways (chromatic dispersion is zero in case of perpendicular input and output incidence) ** Roof pentaprism projects image sideways flipped along the other axis **
Dove prism A Dove prism is a type of reflective prism which is used to invert an image. Dove prisms are shaped from a truncated right-angle prism. The Dove prism is named for its inventor, Heinrich Wilhelm Dove. Although the shape of this prism is similar ...
projects image forward ** Corner-cube retroreflector projects image backwards *Even number of reflections, image projects upright (without change in handedness; may or may not be rotated) **
Porro prism In optics, a Porro prism, named for its inventor Ignazio Porro, is a type of ''reflection prism'' used in optical instruments to alter the orientation of an image. Description It consists of a block of material shaped like a right geometri ...
projects image backwards and displaced **
Porro–Abbe prism A Porro–Abbe prism (sometimes called a Abbe–Porro prism), named for Ignazio Porro and Ernst Abbe, is a type of reflection prism used in some optical instruments to alter the orientation of an image. It is a variant of the more common do ...
projects image forward, rotated by 180° and displaced **
Perger prism A Perger prism or Perger–Porro prism system is a prism, that is used to invert (rotate by 180°) an image. The special feature of this prism is that, like a traditional double Porro prism system, it manages this with only four beam deflection ...
a development based on the Porro–Abbe prism, projects image forward, rotated by 180° and displaced ** Abbe–Koenig prism projects image forward, rotated by 180° and collinear (4
internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected ...
s reflections are on roof plains ** Bauernfeind prism projects image sideways (inclined by 45°) **
Amici roof prism An Amici roof prism, named for its inventor, the Italian astronomer Giovanni Battista Amici, is a type of reflecting prism used to deviate a beam of light by 90° while simultaneously inverting the image. It is commonly used in the eyepieces o ...
projects image sideways **
Pentaprism A pentaprism is a five-sided reflecting prism used to deviate a beam of light by a constant 90°, even if the entry beam is not at 90° to the prism. The beam reflects inside the prism ''twice'', allowing the transmission of an image through a ...
projects image sideways ** Schmidt–Pechan prism projects image forward, rotated by 180° (6 reflections reflections are on roof plains composed of Bauernfeind part and Schmidt part) ** Uppendahl prism projects image forward, rotated by 180° and collinear (6 reflections reflections are on roof plains; composed of 3 prisms cemented together)


Beam-splitting

Various thin-film optical layers can be deposited on the hypotenuse of one right-angled prism, and cemented to another prism to form a beam-splitter cube. Overall optical performance of such a cube is determined by the thin layer. In comparison with a usual glass substrate, the glass cube provides protection of the thin-film layer from both sides and better mechanical stability. The cube can also eliminate etalon effects, back-side reflection and slight beam deflection. * dichroic color filters form a
dichroic prism A dichroic prism is a prism that splits light into two beams of differing wavelength (colour). A trichroic prism assembly combines two dichroic prisms to split an image into 3 colours, typically as red, green and blue of the RGB colour model. ...
*Polarizing cube beamsplitters have lower extinction ratio than birefringent ones, but less expensive *Partially-metallized mirrors provide non-polarizing beamsplitters *Air gap − When hypotenuses of two triangular prisms are stacked very close to each other with air gap,
frustrated total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
in one prism makes it possible to couple part of the radiation into a propagating wave in the second prism. The transmitted power drops exponentially with the gap width, so it can be tuned over many orders of magnitude by a micrometric screw.


Polarizing

Another class is formed by ''polarizing prisms'' which use
birefringence Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringe ...
to split a beam of light into components of varying
polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...
. In the visible and UV regions, they have very low losses and their
extinction ratio In telecommunications, extinction ratio (''r''e) is the ratio of two optical power levels of a digital signal generated by an optical source, e.g., a laser diode. The extinction ratio may be expressed as a fraction, in dB, or as a percentage. It ...
typically exceeds 10^5:1, which is superior to other types of
polarizer A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well- ...
s. They may or may not employ total internal reflection; *One polarization is separated by total internal reflection: **
Nicol prism A Nicol prism is a type of polarizer, an optical device made from calcite crystal used to produce and analyse plane polarized light. It is made in such a way that it eliminates one of the rays by total internal reflection, i.e. the ordinary ra ...
** Glan–Foucault prism ** Glan–Taylor prism, a high-power variant of which is also denoted as Glan–laser prism **
Glan–Thompson prism A Glan–Thompson prism is a type of polarizing prism similar to the Nicol and Glan–Foucault prisms. Design A Glan–Thompson prism consists of two right-angled calcite prisms that are cemented together by their long faces. The optical axe ...
* One polarization is deviated by different refraction only: **
Rochon prism A Rochon prism is a type of polariser. It is made from two prisms of a birefringent material such as calcite, which are cemented together. The Rochon prism was invented by and is named after Abbé Alexis Marie Rochon. It is in many ways simil ...
** Sénarmont prism * Both polarizations are deviated by refraction: **
Wollaston prism A Wollaston prism is an optical device, invented by William Hyde Wollaston, that manipulates polarized light. It separates light into two separate linearly polarized outgoing beams with orthogonal polarization. The two beams will be polarized a ...
**
Nomarski prism A Nomarski prism is a modification of the Wollaston prism that is used in differential interference contrast microscopy. It is named after its inventor, Polish and naturalized-French physicist Georges Nomarski. Like the Wollaston prism, the No ...
– a variant of the Wollaston prism where ''p-'' and ''s-''components emerge displaced and converging towards each other; important for
differential interference contrast microscopy Differential interference contrast (DIC) microscopy, also known as Nomarski interference contrast (NIC) or Nomarski microscopy, is an optical microscopy technique used to enhance the contrast in unstained, transparent samples. DIC works on the p ...
* Both polarizations stay parallel, but are spatially separated: **polarisation beam displacers, typically made of thick anisotropic crystal with plan-parallel facets These are typically made of a
birefringent Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...
crystalline material like
calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
, but other materials like
quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical fo ...
and α-BBO may be necessary for UV applications, and others ( , and ) will extend transmission farther into the
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
spectral range.


Depolarizer

Birefringent crystals can be also assembled in a way that leads to apparent depolarization of the light. * Cornu depolarizer * Lyot depolarizer Note that depolarization would not be observed for an ideal monochromatic
plane wave In physics, a plane wave is a special case of wave or field: a physical quantity whose value, at any moment, is constant through any plane that is perpendicular to a fixed direction in space. For any position \vec x in space and any time t, th ...
, as actually both devices turn reduced temporal coherence or spatial coherence, respectively, of the beam into decoherence of its polarization components.


Others

However, prisms made of isotropic materials like glass will also alter polarization of light, as partial reflection under oblique angles does not maintain the amplitude ratio (nor phase) of the s- and p-polarized components of the light, leading to general
elliptical polarization In electrodynamics, elliptical polarization is the polarization of electromagnetic radiation such that the tip of the electric field vector describes an ellipse in any fixed plane intersecting, and normal to, the direction of propagation. An ellip ...
. This is generally an unwanted effect of dispersive prisms. In some cases this can be avoided by choosing prism geometry which light enters and exits under perpendicular angle, by compensation through non-planar light trajectory, or by use of p-polarized light.
Total internal reflection Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflected b ...
alters only the mutual phase between s- and p-polarized light. Under well chosen angle of incidence, this phase is close to \pi/4. *
Fresnel rhomb A Fresnel rhomb is an optical prism that introduces a 90° phase difference between two perpendicular components of polarization, by means of two total internal reflections. If the incident beam is linearly polarized at 45° to the plane of in ...
uses this effect to achieve conversion between circular and linear polarisation. This phase difference is not explicitly dependent on wavelength, but only on refractive index, so Fresnel rhombs made of low-dispersion glasses achieve much broader spectral range than
quarter-wave plate A waveplate or retarder is an optical device that alters the polarization state of a light wave travelling through it. Two common types of waveplates are the ''half-wave plate'', which shifts the polarization direction of linearly polarized ligh ...
s. They displace the beam, however. *Doubled Fresnel rhomb, with quadruple reflection and zero beam displacement, substitutes a half-wave plate. *Similar effect can also be used to make a polarization-maintaining optics.


Other uses

Total internal reflection in prisms finds numerous uses through optics, plasmonics and microscopy. In particular: *Prisms are used to couple propagating light to
surface plasmon Surface plasmons (SPs) are coherent delocalized electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface (e.g. a metal-dielectric interface, such ...
s. Either the hypotenuse of a triangular prism is metallized (Kretschmann configuration), or evanescent wave is coupled to very close metallic surface (Otto configuration). *Some
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
active media can be formed as a prism where the low-quality
pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they ...
beam enters the front facet, while the amplified beam undergoes total internal reflection under
grazing incidence The angle of incidence, in geometric optics, is the angle between a ray incident on a surface and the line perpendicular (at 90 degree angle) to the surface at the point of incidence, called the normal. The ray can be formed by any waves, such as ...
from it. Such a design suffers less from thermal stress and is easy to be pumped by high-power laser diodes. Other uses of prisms are based on their beam-deviating refraction: *
Wedge prism The wedge prism is a prism with a shallow angle between its input and output surfaces. This angle is usually 3 degrees or less. Refraction at the surfaces causes the prism to deflect light by a fixed angle. When viewing a scene through such a prism ...
s are used to deflect a beam of monochromatic light by a fixed angle. A pair of such prisms can be used for
beam steering Beam steering is a technique for changing the direction of the main lobe of a radiation pattern. In radio and radar systems, beam steering may be accomplished by switching the antenna elements or by changing the relative phases of the RF sign ...
; by rotating the prisms the beam can be deflected into any desired angle within a conical "field of regard". The most commonly found implementation is a Risley prism pair. *Transparent windows of, e.g., vacuum chambers or cuvettes can also be slightly wedged (10' − 1°). While this does not reduce reflection, it suppresses Fabry-Pérot interferences that would otherwise modulate their transmission spectrum. *''Anamorphic pair'' of similar, but asymmetrically placed prisms can also change the profile of a beam. This is often used to make a round beam from the elliptical output of a
laser diode The laser diode chip removed and placed on the eye of a needle for scale A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with e ...
. With its monochromatic light, slight chromatic dispersion arising from different wedge inclination is not a problem. *
Deck prism A deck prism is a prism inserted into the deck of a ship to provide light down below. For centuries, sailing ships used deck prisms to provide a safe source of natural sunlight to illuminate areas below decks. Before electricity, light below a ...
s were used on sailing ships to bring daylight below deck, since candles and
kerosene lamp A kerosene lamp (also known as a paraffin lamp in some countries) is a type of lighting device that uses kerosene as a fuel. Kerosene lamps have a wick or mantle as light source, protected by a glass chimney or globe; lamps may be used on a ...
s are a fire hazard on wooden ships.


In optometry

By shifting
corrective lens A corrective lens is a lens (i.e. a transmissive optical device) that is typically worn in front of the eye to improve daily vision. The most common use is to treat refractive errors: myopia, hypermetropia, astigmatism, and presbyopia. Glasse ...
es off
axis An axis (plural ''axes'') is an imaginary line around which an object rotates or is symmetrical. Axis may also refer to: Mathematics * Axis of rotation: see rotation around a fixed axis *Axis (mathematics), a designator for a Cartesian-coordinate ...
, images seen through them can be displaced in the same way that a prism displaces images.
Eye care professional An eye care professional (ECP) is an individual who provides a service related to the eyes or vision. It is any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level ...
s use prisms, as well as lenses off axis, to treat various
orthoptics Orthoptics is a profession allied to the eye care profession. Orthoptists are the experts in diagnosing and treating defects in eye movements and problems with how the eyes work together, called binocular vision. These can be caused by issues with ...
problems: *
Diplopia Diplopia is the simultaneous perception of two images of a single object that may be displaced horizontally or vertically in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often v ...
(double vision) *Positive and negative fusion problems Prism spectacles with a single prism perform a relative displacement of the two eyes, thereby correcting eso-, exo, hyper- or hypotropia. In contrast, spectacles with prisms of equal power for both eyes, called yoked prisms (also: ''conjugate prisms'', ''ambient lenses'' or ''performance glasses'') shift the visual field of both eyes to the same extent.


See also

*
Minimum deviation In a prism, the angle of deviation () decreases with increase in the angle of incidence () up to a particular angle. This angle of incidence where the angle of deviation in a prism is minimum is called the minimum deviation position of the prism ...
*
Multiple-prism dispersion theory The first description of multiple-prism arrays, and multiple-prism dispersion, was given by Newton in his book ''Opticks''. Prism pair expanders were introduced by Brewster in 1813. A modern mathematical description of the single-prism dispersio ...
*
Prism compressor A prism compressor is an optical device used to shorten the duration of a positively chirped ultrashort laser pulse by giving different wavelength components a different time delay. It typically consists of two prisms and a mirror. Figure 1 show ...
*
Prism dioptre Eye care professionals use prism correction as a component of some eyeglass prescriptions. A lens which includes some amount of prism correction will displace the viewed image horizontally, vertically, or a combination of both directions. The mo ...
*
Prism spectrometer A prism spectrometer is an optical spectrometer which uses a dispersive prism as its dispersive element. The prism refracts light into its different colors (wavelengths). The dispersion occurs because the angle of refraction is dependent on the ...
*
Prism (geometry) In geometry, a prism is a polyhedron comprising an polygon base, a second base which is a translated copy (rigidly moved without rotation) of the first, and other faces, necessarily all parallelograms, joining corresponding sides of the two b ...
*
Theory of Colours ''Theory of Colours'' (german: Zur Farbenlehre, links=no) is a book by Johann Wolfgang von Goethe about the poet's views on the nature of colours and how these are perceived by humans. It was published in German in 1810 and in English in 1840 ...
*
Triangular prism In geometry, a triangular prism is a three-sided prism; it is a polyhedron made of a triangular base, a translated copy, and 3 faces joining corresponding sides. A right triangular prism has rectangular sides, otherwise it is ''oblique''. A un ...
(geometry) * Superprism *
Eyeglass prescription An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patie ...
*
Prism lighting Prism lighting is the use of prisms to improve the distribution of light in a space. It is usually used to distribute daylight, and is a form of anidolic lighting. Prism lighting was popular from its introduction in the 1890s through to the 1 ...


References


Further reading

*


External links

*
Java applet of refraction through a prism
{{Authority control Optical components