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The Fresnel equations (or Fresnel coefficients) describe the reflection and transmission of
light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nan ...

(or
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

in general) when incident on an interface between different optical
media Media may refer to: Physical means Communication * Media (communication), tools used to deliver information or data ** Advertising media, various media, content, buying and placement for advertising ** Broadcast media, communications deliv ...
. They were deduced by
Augustin-Jean Fresnel Augustin-Jean Fresnel ( or ; ; 10 May 1788 – 14 July 1827) was a French civil engineer A civil engineer is a person who practices civil engineering Civil engineering is a Regulation and licensure in engineering, professional engi ...
() who was the first to understand that light is a , even though no one realized that the "vibrations" of the wave were electric and magnetic fields. For the first time,
polarization Polarization or polarisation may refer to: In the physical sciences *Polarization (waves), the ability of waves to oscillate in more than one direction, in particular polarization of light, responsible for example for the glare-reducing effect of ...
could be understood quantitatively, as Fresnel's equations correctly predicted the differing behaviour of waves of the ''s'' and ''p'' polarizations incident upon a material interface.

Overview

When light strikes the interface between a medium with
refractive index In optics Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or ...

''n''1 and a second medium with refractive index ''n''2, both
reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal r ...
and
refraction In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force ...

of the light may occur. The Fresnel equations give the ratio of the ''reflected'' wave's electric field to the incident wave's electric field, and the ratio of the ''transmitted'' wave's electric field to the incident wave's electric field, for each of two components of polarization. (The ''magnetic'' fields can also be related using similar coefficients.) These ratios are generally complex, describing not only the relative amplitudes but also the
phase shift In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...
s at the interface. The equations assume the interface between the media is flat and that the media are homogeneous and
isotropic Isotropy is uniformity in all orientations; it is derived from the Greek ''isos'' (ἴσος, "equal") and ''tropos'' (τρόπος, "way"). Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by ...
. The incident light is assumed to be a
plane wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...

, which is sufficient to solve any problem since any incident light field can be decomposed into plane waves and polarizations.

S and P polarizations

There are two sets of Fresnel coefficients for two different linear
polarization Polarization or polarisation may refer to: In the physical sciences *Polarization (waves), the ability of waves to oscillate in more than one direction, in particular polarization of light, responsible for example for the glare-reducing effect of ...
components of the incident wave. Since any polarization state can be resolved into a combination of two orthogonal linear polarizations, this is sufficient for any problem. Likewise, unpolarized (or "randomly polarized") light has an equal amount of power in each of two linear polarizations. The s polarization refers to polarization of a wave's electric field '''' to the
plane of incidence In describing reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirro ...

(the direction in the derivation below); then the magnetic field is ''in'' the plane of incidence. The p polarization refers to polarization of the electric field ''in'' the plane of incidence (the plane in the derivation below); then the magnetic field is ''normal'' to the plane of incidence. Although the reflection and transmission are dependent on polarization, at normal incidence (''θ'' = 0) there is no distinction between them so all polarization states are governed by a single set of Fresnel coefficients (and another special case is mentioned
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in which that is true).

Power (intensity) reflection and transmission coefficients

In the diagram on the right, an incident
plane wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...

in the direction of the ray IO strikes the interface between two media of refractive indices ''n''1 and ''n''2 at point O. Part of the wave is reflected in the direction OR, and part refracted in the direction OT. The angles that the incident, reflected and refracted rays make to the of the interface are given as ''θ''i, ''θ''r and ''θ''t, respectively. The relationship between these angles is given by the
law of reflection Image:Tso Kiagar Lake Ladakh.jpg, Reflections on still water are an example of specular reflection. Specular reflection, or regular reflection, is the mirror-like reflection (physics), reflection of waves, such as light, from a surface. The la ...
: $\theta_\mathrm = \theta_\mathrm,$ and
Snell's law of light at the interface between two media of different refractive index, refractive indices, with n2 > n1. Since the velocity is lower in the second medium (v2 < v1), the angle of refraction θ2 is less than the angle of in ...

: $n_1 \sin \theta_\mathrm = n_2 \sin \theta_\mathrm.$ The behavior of light striking the interface is solved by considering the electric and magnetic fields that constitute an
electromagnetic wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

, and the laws of
electromagnetism Electromagnetism is a branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in ...
, as shown
below Below may refer to: *Earth *Ground (disambiguation) *Soil *Floor *Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Fred Below (1926–1988), American blues drummer *Fritz von Below (1853 ...
. The ratio of waves' electric field (or magnetic field) amplitudes are obtained, but in practice one is more often interested in formulae which determine ''power'' coefficients, since power (or
irradiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measure ...

) is what can be directly measured at optical frequencies. The power of a wave is generally proportional to the square of the electric (or magnetic) field amplitude. We call the fraction of the incident
power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one joule per second. In older works, p ...
that is reflected from the interface the ''
reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion ...

'' (or reflectivity, or power reflection coefficient) ''R'', and the fraction that is refracted into the second medium is called the ''
transmittance File:Atmosfaerisk spredning.png, Earth's atmospheric transmittance over 1 nautical mile sea level path (infrared region). Because of the natural radiation of the hot atmosphere, the intensity of radiation is different from the transmitted part. ...

'' (or transmissivity, or power transmission coefficient) ''T''. Note that these are what would be measured right ''at'' each side of an interface and do not account for attenuation of a wave in an absorbing medium ''following'' transmission or reflection. The reflectance for
s-polarized light Polarization (American and British English spelling differences, also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation ...
is $R_\mathrm = \left, \frac\^2,$ while the reflectance for
p-polarized light Polarization (American and British English spelling differences, also polarisation) is a property applying to transverse waves that specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation ...
is $R_\mathrm = \left, \frac\^2,$ where and are the
wave impedanceThe wave impedance of an electromagnetic wave Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The elect ...
s of media 1 and 2, respectively. We assume that the media are non-magnetic (i.e., ''μ''1 = ''μ''2 = ''μ''0), which is typically a good approximation at optical frequencies (and for transparent media at other frequencies). Then the wave impedances are determined solely by the refractive indices ''n''1 and ''n''2: $Z_i = \frac\,,$ where is the
impedance of free spaceThe impedance of free space, , is a physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant (mathematic ...
and =1,2. Making this substitution, we obtain equations using the refractive indices: $R_\mathrm = \left, \frac\^2 = \left, \frac \^2\!,$ $R_\mathrm = \left, \frac\^2 = \left, \frac \^2\!.$ The second form of each equation is derived from the first by eliminating ''θ''t using
Snell's law of light at the interface between two media of different refractive index, refractive indices, with n2 > n1. Since the velocity is lower in the second medium (v2 < v1), the angle of refraction θ2 is less than the angle of in ...

and
trigonometric identities In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ...
. As a consequence of
conservation of energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...
, one can find the transmitted power (or more correctly,
irradiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measure ...

: power per unit area) simply as the portion of the incident power that isn't reflected: $T_\mathrm = 1 - R_\mathrm$ and $T_\mathrm = 1 - R_\mathrm$ Note that all such intensities are measured in terms of a wave's irradiance in the direction normal to the interface; this is also what is measured in typical experiments. That number could be obtained from irradiances ''in the direction of an incident or reflected wave'' (given by the magnitude of a wave's
Poynting vector In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...
) multiplied by cos''θ'' for a wave at an angle ''θ'' to the normal direction (or equivalently, taking the dot product of the Poynting vector with the unit vector normal to the interface). This complication can be ignored in the case of the reflection coefficient, since cos''θ''i = cos''θ''r, so that the ratio of reflected to incident irradiance in the wave's direction is the same as in the direction normal to the interface. Although these relationships describe the basic physics, in many practical applications one is concerned with "natural light" that can be described as unpolarized. That means that there is an equal amount of power in the ''s'' and ''p'' polarizations, so that the ''effective'' reflectivity of the material is just the average of the two reflectivities: $R_\mathrm = \frac\left(R_\mathrm + R_\mathrm\right).$ For low-precision applications involving unpolarized light, such as
computer graphics Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great dea ...

, rather than rigorously computing the effective reflection coefficient for each angle,
Schlick's approximationIn 3D computer graphics, Schlick's approximation, named after Christophe Schlick, is a formula for approximating the contribution of the Fresnel equation, Fresnel factor in the specular reflection of light from a non-conducting interface (surface) be ...
is often used.

Special cases

Normal incidence

For the case of normal incidence, and there is no distinction between s and p polarization. Thus, the reflectance simplifies to $R = \left, \frac\^2\,.$ For common glass (''n''2 ≈ 1.5) surrounded by air (''n''1=1), the power reflectance at normal incidence can be seen to be about 4%, or 8% accounting for both sides of a glass pane.

Brewster's angle

At a dielectric interface from to , there is a particular angle of incidence at which goes to zero and a p-polarised incident wave is purely refracted, thus all reflected light is s-polarised. This angle is known as
Brewster's angle Brewster's angle (also known as the polarization angle) is an angle of incidence at which light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that can be visual perception, perceived ...
, and is around 56° for ''n''1=1 and ''n''2=1.5 (typical glass).

Total internal reflection

When light travelling in a denser medium strikes the surface of a less dense medium (i.e., ), beyond a particular incidence angle known as the ''critical angle'', all light is reflected and . This phenomenon, known as
total internal reflection Total internal reflection (TIR) is the optical phenomenon Optical phenomena are any observable events that result from the interaction of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic s ...

, occurs at incidence angles for which Snell's law predicts that the sine of the angle of refraction would exceed unity (whereas in fact sin''θ'' ≤ 1 for all real ''θ''). For glass with ''n''=1.5 surrounded by air, the critical angle is approximately 41°.

Complex amplitude reflection and transmission coefficients

The above equations relating powers (which could be measured with a
photometer A photometer is an instrument that measures the strength of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is t ...

for instance) are derived from the Fresnel equations which solve the physical problem in terms of
electromagnetic field An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the in ...
complex amplitude The UCL Faculty of Mathematical and Physical Sciences is one of the University College London#Faculties and departments, 11 constituent faculties of University College London (UCL). The Faculty, the UCL Faculty of Engineering Sciences and the The B ...
s, i.e., considering
phase Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
amplitudes The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelength, spatial period). There are various definitions of amplitude ...
. Those underlying equations supply generally complex-valued ratios of those EM fields and may take several different forms, depending on the formalism used. The complex amplitude coefficients for reflection and transmission are usually represented by lower case ''r'' and ''t'' (whereas the power coefficients are capitalized). As before, we are assuming the magnetic permeability, of both media to be equal to the permeability of free space as is essentially true of all dielectrics at optical frequencies. In the following equations and graphs, we adopt the following conventions. For ''s'' polarization, the reflection coefficient is defined as the ratio of the reflected wave's complex electric field amplitude to that of the incident wave, whereas for ''p'' polarization is the ratio of the waves complex ''magnetic'' field amplitudes (or equivalently, the ''negative'' of the ratio of their electric field amplitudes). The transmission coefficient is the ratio of the transmitted wave's complex electric field amplitude to that of the incident wave, for either polarization. The coefficients and are generally different between the ''s'' and ''p'' polarizations, and even at normal incidence (where the designations ''s'' and ''p'' do not even apply!) the sign of is reversed depending on whether the wave is considered to be ''s'' or ''p'' polarized, an artifact of the adopted sign convention (see graph for an air-glass interface at 0° incidence). The equations consider a plane wave incident on a plane interface at angle of incidence a wave reflected at angle and a wave transmitted at angle In the case of an interface into an absorbing material (where ''n'' is complex) or total internal reflection, the angle of transmission does not generally evaluate to a real number. In that case, however, meaningful results can be obtained using formulations of these relationships in which trigonometric functions and geometric angles are avoided; the inhomogeneous waves launched into the second medium cannot be described using a single propagation angle. Using this convention,Lecture notes by Bo Sernelius
main site
, see especiall
Lecture 12
.
Born & Wolf, 1970, p.40, eqs.(20),(21). $\begin r_\text &= \frac, \\$ t_\text &= \frac , \\ r_\text &= \frac, \\ t_\text &= \frac . \end One can see that and . One can write very similar equations applying to the ratio of the waves' magnetic fields, but comparison of the electric fields is more conventional. Because the reflected and incident waves propagate in the same medium and make the same angle with the normal to the surface, the power reflection coefficient R is just the squared magnitude of ''r'': $R = , r, ^2.$ On the other hand, calculation of the power transmission coefficient is less straightforward, since the light travels in different directions in the two media. What's more, the wave impedances in the two media differ; power (
irradiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measure ...

) is given by the square of the electric field amplitude ''divided by'' the
characteristic impedance File:Transmission line schematic.svg, Schematic representation of a electrical circuit, circuit where a source is coupled to a electrical load, load with a transmission line having characteristic impedance Z_0. The characteristic impedance or sur ...
of the medium (or by the square of the magnetic field ''multiplied by'' the characteristic impedance). This results in: $T = \frac , t, ^2$ using the above definition of . The introduced factor of is the reciprocal of the ratio of the media's wave impedances. The factors adjust the waves' powers so they are reckoned ''in the direction'' normal to the interface, for both the incident and transmitted waves, so that full power transmission corresponds to ''T''=1. In the case of
total internal reflection Total internal reflection (TIR) is the optical phenomenon Optical phenomena are any observable events that result from the interaction of light Light or visible light is electromagnetic radiation within the portion of the electromagnetic s ...

where the power transmission is zero, nevertheless describes the electric field (including its phase) just beyond the interface. This is an
evanescent field In electromagnetics Electromagnetism is a branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matte ...
which does not propagate as a wave (thus =0) but has nonzero values very close to the interface. The phase shift of the reflected wave on total internal reflection can similarly be obtained from the phase angles of and (whose magnitudes are unity in this case). These phase shifts are different for ''s'' and ''p'' waves, which is the well-known principle by which total internal reflection is used to effect .

Alternative forms

In the above formula for , if we put $n_2=n_1\sin\theta_\text/\sin\theta_\text$ (Snell's law) and multiply the numerator and denominator by , we obtain $r_\text=-\frac.$ If we do likewise with the formula for , the result is easily shown to be equivalent to $r_\text=\frac.$ These formulas are known respectively as ''Fresnel's sine law'' and ''Fresnel's tangent law''. Although at normal incidence these expressions reduce to 0/0, one can see that they yield the correct results in the
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as .

Multiple surfaces

When light makes multiple reflections between two or more parallel surfaces, the multiple beams of light generally interfere with one another, resulting in net transmission and reflection amplitudes that depend on the light's wavelength. The interference, however, is seen only when the surfaces are at distances comparable to or smaller than the light's
coherence length In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Ph ...
, which for ordinary white light is few micrometers; it can be much larger for light from a
laser A laser is a device that emits light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as h ...

. An example of interference between reflections is the
iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear to Gradient, gradually change color as the angle of view or the angle of illumination changes. Examples of iridescence include soap bubbles, feathers, ...
colours seen in a
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or in thin oil films on water. Applications include
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s,
antireflection coating An antireflective or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses and other optical elements to reduce reflection. In typical imaging systems, this improves the efficiency since less light Li ...
s, and
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s. A quantitative analysis of these effects is based on the Fresnel equations, but with additional calculations to account for interference. The
transfer-matrix method In statistical mechanics, the transfer-matrix method is a Mathematical physics, mathematical technique which is used to write the Partition function (mathematics), partition function into a simpler form. It was introduced in 1941 by Hans Kramers an ...
, or the recursive Rouard method can be used to solve multiple-surface problems.

History

In 1808, Étienne-Louis Malus discovered that when a ray of light was reflected off a non-metallic surface at the appropriate angle, it behaved like ''one'' of the two rays emerging from a calcite crystal. He later coined the term ''polarization'' to describe this behavior.  In 1815, the dependence of the polarizing angle on the refractive index was determined experimentally by
David Brewster Sir David Brewster KH PRSE FRS FRS may also refer to: Government and politics * Facility Registry System, a centrally managed Environmental Protection Agency database that identifies places of environmental interest in the United States * F ...

.D. Brewster
"On the laws which regulate the polarisation of light by reflexion from transparent bodies"
''Philosophical Transactions of the Royal Society'', vol.105, pp.125–59, read 16 March 1815.
But the ''reason'' for that dependence was such a deep mystery that in late 1817, was moved to write: In 1821, however,
Augustin-Jean Fresnel Augustin-Jean Fresnel ( or ; ; 10 May 1788 – 14 July 1827) was a French civil engineer A civil engineer is a person who practices civil engineering Civil engineering is a Regulation and licensure in engineering, professional engi ...
derived results equivalent to his sine and tangent laws (above), by modeling light waves as transverse elastic waves with vibrations perpendicular to what had previously been called the
plane of polarization The term ''plane of polarization'' refers to the direction of polarization of '' linearly-polarized'' light or other electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), ...
. Fresnel promptly confirmed by experiment that the equations correctly predicted the direction of polarization of the reflected beam when the incident beam was polarized at 45° to the plane of incidence, for light incident from air onto glass or water; in particular, the equations gave the correct polarization at Brewster's angle. The experimental confirmation was reported in a "postscript" to the work in which Fresnel first revealed his theory that light waves, including "unpolarized" waves, were ''purely'' transverse.A. Fresnel, "Note sur le calcul des teintes que la polarisation développe dans les lames cristallisées" et seq., ''Annales de Chimie et de Physique'', vol.17, pp.102–11 (May 1821), 167–96 (June 1821), 312–15 ("Postscript", July 1821); reprinted in Fresnel, 1866, pp.609–48; translated as "On the calculation of the tints that polarization develops in crystalline plates, &postscript", / , 2021. Details of Fresnel's derivation, including the modern forms of the sine law and tangent law, were given later, in a memoir read to the
French Academy of Sciences The French Academy of Sciences (French: ''Académie des sciences'') is a learned society A learned society (; also known as a learned academy, scholarly society, or academic association) is an organization that exists to promote an discipli ...
in January 1823.A. Fresnel, "Mémoire sur la loi des modifications que la réflexion imprime à la lumière polarisée" ("Memoir on the law of the modifications that reflection impresses on polarized light"), read 7 January 1823; reprinted in Fresnel, 1866, pp.767–99 (full text, published 1831), pp.753–62 (extract, published 1823). See especially pp.773 (sine law), 757 (tangent law), 760–61 and 792–6 (angles of total internal reflection for given phase differences). That derivation combined conservation of energy with continuity of the ''tangential'' vibration at the interface, but failed to allow for any condition on the ''normal'' component of vibration. The first derivation from ''electromagnetic'' principles was given by
Hendrik Lorentz Lorentz' theory of electrons. Formulas for the curl of the magnetic field (IV) and the electrical field E (V), ''La théorie electromagnétique de Maxwell et son application aux corps mouvants'', 1892, p. 452. Hendrik Antoon Lorentz (; 18 Ju ...

in 1875. In the same memoir of January 1823, Fresnel found that for angles of incidence greater than the critical angle, his formulas for the reflection coefficients ( and ) gave complex values with unit magnitudes. Noting that the magnitude, as usual, represented the ratio of peak amplitudes, he guessed that the
argument In logic Logic is an interdisciplinary field which studies truth and reasoning Reason is the capacity of consciously making sense of things, applying logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, la ...
represented the phase shift, and verified the hypothesis experimentally. The verification involved * calculating the angle of incidence that would introduce a total phase difference of 90° between the s and p components, for various numbers of total internal reflections at that angle (generally there were two solutions), * subjecting light to that number of total internal reflections at that angle of incidence, with an initial linear polarization at 45° to the plane of incidence, and * checking that the final polarization was
circular Circular may refer to: * The shape of a circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre; equivalently it is ...
. Thus he finally had a quantitative theory for what we now call the ''Fresnel rhomb'' — a device that he had been using in experiments, in one form or another, since 1817 (see ''''). The success of the complex reflection coefficient inspired and
Augustin-Louis Cauchy Baron Baron is a rank of nobility or title of honour, often hereditary, in various European countries, either current or historical. The female equivalent is baroness. Typically, the title denotes an aristocrat who ranks higher than a lord ...

, beginning in 1836, to analyze reflection from metals by using the Fresnel equations with a complex refractive index. Four weeks before he presented his completed theory of total internal reflection and the rhomb, Fresnel submitted a memoirA. Fresnel, "Mémoire sur la double réfraction que les rayons lumineux éprouvent en traversant les aiguilles de cristal de roche suivant les directions parallèles à l'axe" ("Memoir on the double refraction that light rays undergo in traversing the needles of quartz in the directions parallel to the axis"), read 9 December 1822; printed in Fresnel, 1866, pp.731–51 (full text), pp.719–29 (''extrait'', first published in ''Bulletin de la Société philomathique'' for 1822, pp. 191–8). in which he introduced the needed terms ''
linear polarization In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field A magnetic field is a vector field that describes the magnetic influence on movin ...
'', ''
circular polarization vectors of a traveling circularly polarized electromagnetic wave. This wave is right-circularly-polarized, since the direction of rotation of the vector is related by the right-hand rule In mathematics Mathematics (from Ancient Greek, Gre ...
'', and ''
elliptical polarization In electrodynamics Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carri ...
'', and in which he explained
optical rotation Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization (waves), polarization about the optical axis of linear polarization, linearly polarized light as it tr ...
as a species of
birefringence Birefringence is the property of a material having a that depends on the and propagation direction of . These optically materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference b ...

: linearly-polarized light can be resolved into two circularly-polarized components rotating in opposite directions, and if these propagate at different speeds, the phase difference between them — hence the orientation of their linearly-polarized resultant — will vary continuously with distance. Thus Fresnel's interpretation of the complex values of his reflection coefficients marked the confluence of several streams of his research and, arguably, the essential completion of his reconstruction of physical optics on the transverse-wave hypothesis (see ''
Augustin-Jean Fresnel Augustin-Jean Fresnel ( or ; ; 10 May 1788 – 14 July 1827) was a French civil engineer A civil engineer is a person who practices civil engineering Civil engineering is a Regulation and licensure in engineering, professional engi ...
'').

Theory

Here we systematically derive the above relations from electromagnetic premises.

Material parameters

In order to compute meaningful Fresnel coefficients, we must assume that the medium is (approximately)
linear Linearity is the property of a mathematical relationship (''function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out se ...

and
homogeneous Homogeneity and heterogeneity are concepts often used in the sciences Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and predictions about th ...
. If the medium is also
isotropic Isotropy is uniformity in all orientations; it is derived from the Greek ''isos'' (ἴσος, "equal") and ''tropos'' (τρόπος, "way"). Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by ...
, the four field vectors are
related ''Related'' is an American comedy-drama Comedy-drama, or dramedy, is a genre of dramatic works that combines elements of comedy and Drama (film and television), drama. History The advent of radio drama, film, cinema and in particular, televisi ...

by $\begin \mathbf &= \epsilon \mathbf \\ \mathbf &= \mu \mathbf\,, \end$ where ''ϵ'' and ''μ'' are scalars, known respectively as the (electric) ''
permittivity In electromagnetism Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is car ...
'' and the (magnetic) '' permeability'' of the medium. For a vacuum, these have the values ''ϵ''0 and ''μ''0, respectively. Hence we define the ''relative'' permittivity (or
dielectric constant The dielectric constant (or relative permittivity) is the permittivity, electric permeability of a material expressed as a ratio with the vacuum permittivity, electric permeability of a vacuum. A dielectric is an insulating material, and the diele ...
) , and the ''relative'' permeability . In optics it is common to assume that the medium is non-magnetic, so that ''μ''rel=1. For
ferromagnetic Ferromagnetism is the basic mechanism by which certain materials (such as iron Iron () is a with Fe (from la, ) and 26. It is a that belongs to the and of the . It is, on , right in front of (32.1% and 30.1%, respectively), formi ...
materials at radio/microwave frequencies, larger values of ''μ''rel must be taken into account. But, for optically transparent media, and for all other materials at optical frequencies (except possible
metamaterial A metamaterial (from the Greek word μετά ''meta'', meaning "beyond" and the Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ...

s), ''μ''rel is indeed very close to 1; that is, ''μ''≈''μ''0. In optics, one usually knows the
refractive index In optics Optics is the branch of physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or ...

''n'' of the medium, which is the ratio of the speed of light in a vacuum () to the speed of light in the medium. In the analysis of partial reflection and transmission, one is also interested in the electromagnetic
wave impedanceThe wave impedance of an electromagnetic wave Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The elect ...
, which is the ratio of the amplitude of to the amplitude of . It is therefore desirable to express ''n'' and in terms of ''ϵ'' and ''μ'', and thence to relate to ''n''. The last-mentioned relation, however, will make it convenient to derive the reflection coefficients in terms of the wave ''admittance'' , which is the reciprocal of the wave impedance . In the case of ''uniform
plane Plane or planes may refer to: * Airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft A fixed-wing aircraft is a heavier-than-air flying machine Early flying machines include all forms of aircraft studied ...

sinusoidal A sine wave or sinusoid is any of certain mathematical curves that describe a smooth periodic oscillation Oscillation is the repetitive variation, typically in time Time is the indefinite continued sequence, progress of existence and ev ...

'' waves, the wave impedance or admittance is known as the ''intrinsic'' impedance or admittance of the medium. This case is the one for which the Fresnel coefficients are to be derived.

Electromagnetic plane waves

In a uniform plane sinusoidal
electromagnetic wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...

, the
electric field An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' ' ...

has the form where is the (constant) complex amplitude vector,  is the
imaginary unit The imaginary unit or unit imaginary number () is a solution to the quadratic equation In algebra Algebra (from ar, الجبر, lit=reunion of broken parts, bonesetting, translit=al-jabr) is one of the areas of mathematics, broad are ...
,  is the
wave vector In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...
(whose magnitude is the angular
wavenumber In the physical science Physical science is a branch of natural science that studies non-living systems, in contrast to life science. It in turn has many branches, each referred to as a "physical science", together called the "physical scienc ...
),  is the
position vector In geometry Geometry (from the grc, γεωμετρία; ''wikt:γῆ, geo-'' "earth", ''wikt:μέτρον, -metron'' "measurement") is, with arithmetic, one of the oldest branches of mathematics. It is concerned with properties of space tha ...
,  ''ω'' is the
angular frequency In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular succ ...
,  is time, and it is understood that the ''real part'' of the expression is the physical field.The above form () is typically used by physicists. typically prefer the form that is, they not only use instead of for the imaginary unit, but also change the sign of the exponent, with the result that the whole expression is replaced by its
complex conjugate In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gen ...
, leaving the real part unchanged . The electrical engineers' form and the formulas derived therefrom may be converted to the physicists' convention by substituting for .
The value of the expression is unchanged if the position varies in a direction normal to ; hence ''is normal to the wavefronts''. To advance the
phase Phase or phases may refer to: Science * State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter) In the physical sciences, a phase is a region of space (a thermodynamic system A thermodynamic system is a ...
by the angle ''ϕ'', we replace by (that is, we replace by ), with the result that the (complex) field is multiplied by . So a phase ''advance'' is equivalent to multiplication by a complex constant with a ''negative''
argument In logic Logic is an interdisciplinary field which studies truth and reasoning Reason is the capacity of consciously making sense of things, applying logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, la ...
. This becomes more obvious when the field () is factored as where the last factor contains the time-dependence. That factor also implies that differentiation w.r.t. time corresponds to multiplication by .In the electrical engineering convention, the time-dependent factor is so that a phase advance corresponds to multiplication by a complex constant with a ''positive'' argument, and differentiation w.r.t. time corresponds to multiplication by . This article, however, uses the physics convention, whose time-dependent factor is . Although the imaginary unit does not appear explicitly in the results given here, the time-dependent factor affects the interpretation of any results that turn out to be complex. If ''ℓ'' is the component of in the direction of the field () can be written .  If the argument of is to be constant,  ''ℓ'' must increase at the velocity $\omega/k\,,\,$ known as the ''
phase velocity The phase velocity of a wave In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or ...

'' . This in turn is equal to Solving for gives As usual, we drop the time-dependent factor which is understood to multiply every complex field quantity. The electric field for a uniform plane sine wave will then be represented by the location-dependent ''
phasor In and , a phasor (a of phase vector), is a representing a whose (''A''), (''ω''), and (''θ'') are . It is related to a more general concept called ,Bracewell, Ron. ''The Fourier Transform and Its Applications''. McGraw-Hill, 1965. p2 ...

'' For fields of that form, Faraday's law and the Maxwell-Ampère law respectively reduce toCompare M.V. Berry and M.R. Jeffrey, "Conical diffraction: Hamilton's diabolical point at the heart of crystal optics", in E. Wolf (ed.), ''Progress in Optics'', vol.50, Amsterdam: Elsevier, 2007, pp.13–50, , at p.18, eq.(2.2). $\begin \omega\mathbf &= \mathbf\times\mathbf\\ \omega\mathbf &= -\mathbf\times\mathbf\,. \end$ Putting and as above, we can eliminate and to obtain equations in only and : $\begin \omega\mu\mathbf &= \mathbf\times\mathbf\\ \omega\epsilon\mathbf &= -\mathbf\times\mathbf\,. \end$ If the material parameters ''ϵ'' and ''μ'' are real (as in a lossless dielectric), these equations show that form a ''right-handed orthogonal triad'', so that the same equations apply to the magnitudes of the respective vectors. Taking the magnitude equations and substituting from (), we obtain $\begin \mu cH &= nE\\ \epsilon cE &= nH\,, \end$ where and are the magnitudes of and . Multiplying the last two equations gives Dividing (or cross-multiplying) the same two equations gives where This is the ''intrinsic admittance''. From () we obtain the phase velocity For a vacuum this reduces to Dividing the second result by the first gives $n=\sqrt\,.$ For a ''non-magnetic'' medium (the usual case), this becomes Taking the reciprocal of (), we find that the intrinsic ''impedance'' is In a vacuum this takes the value $Z_0=\sqrt\,\approx 377\,\Omega\,,$ known as the
impedance of free spaceThe impedance of free space, , is a physical constant A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant (mathematic ...
. By division, For a ''non-magnetic'' medium, this becomes $Z=Z_0\big/\!\sqrt=Z_0/n.$

The wave vectors

In Cartesian coordinates , let the region have refractive index intrinsic admittance etc., and let the region have refractive index intrinsic admittance etc. Then the plane is the interface, and the axis is normal to the interface (see diagram). Let and (in bold
roman type In Latin script Latin script, also known as Roman script, is a set of graphic signs (Writing system#General properties, script) based on the letters of the classical Latin alphabet. This is derived from a form of the Cumae alphabet, Cumaean Gre ...
) be the unit vectors in the and directions, respectively. Let the plane of incidence be the plane (the plane of the page), with the angle of incidence measured from towards . Let the angle of refraction, measured in the same sense, be where the subscript stands for ''transmitted'' (reserving for ''reflected''). In the absence of , ''ω'' does not change on reflection or refraction. Hence, by (), the magnitude of the wave vector is proportional to the refractive index. So, for a given ''ω'', if we ''redefine'' as the magnitude of the wave vector in the ''reference'' medium (for which ), then the wave vector has magnitude in the first medium (region in the diagram) and magnitude in the second medium. From the magnitudes and the geometry, we find that the wave vectors are where the last step uses Snell's law. The corresponding dot products in the phasor form () are Hence:

The ''s'' components

For the ''s'' polarization, the field is parallel to the axis and may therefore be described by its component in the  direction. Let the reflection and transmission coefficients be and respectively. Then, if the incident field is taken to have unit amplitude, the phasor form () of its  component is and the reflected and transmitted fields, in the same form, are Under the sign convention used in this article, a positive reflection or transmission coefficient is one that preserves the direction of the ''transverse'' field, meaning (in this context) the field normal to the plane of incidence. For the ''s'' polarization, that means the field. If the incident, reflected, and transmitted fields (in the above equations) are in the  direction ("out of the page"), then the respective fields are in the directions of the red arrows, since form a right-handed orthogonal triad. The fields may therefore be described by their components in the directions of those arrows, denoted by .  Then, since At the interface, by the usual
interface conditions for electromagnetic fields Interface conditions describe the behaviour of electromagnetic fields An electromagnetic field (also EM field) is a classical (i.e. non-quantum) field produced by accelerating electric charge Electric charge is the physical property of matter ...
, the tangential components of the and fields must be continuous; that is, } When we substitute from equations () to () and then from (), the exponential factors cancel out, so that the interface conditions reduce to the simultaneous equations which are easily solved for and yielding and At ''normal incidence'' indicated by an additional subscript 0, these results become and At ''grazing incidence'' , we have hence and .

The ''p'' components

For the ''p'' polarization, the incident, reflected, and transmitted fields are parallel to the red arrows and may therefore be described by their components in the directions of those arrows. Let those components be (redefining the symbols for the new context). Let the reflection and transmission coefficients be and . Then, if the incident field is taken to have unit amplitude, we have If the fields are in the directions of the red arrows, then, in order for to form a right-handed orthogonal triad, the respective fields must be in the  direction ("into the page") and may therefore be described by their components in that direction. This is consistent with the adopted sign convention, namely that a positive reflection or transmission coefficient is one that preserves the direction of the transverse field the field in the case of the ''p'' polarization. The agreement of the ''other'' field with the red arrows reveals an alternative definition of the sign convention: that a positive reflection or transmission coefficient is one for which the field vector in the plane of incidence points towards the same medium before and after reflection or transmission. So, for the incident, reflected, and transmitted fields, let the respective components in the  direction be .  Then, since At the interface, the tangential components of the and fields must be continuous; that is, } When we substitute from equations () and () and then from (), the exponential factors again cancel out, so that the interface conditions reduce to Solving for and we find and At ''normal incidence'' indicated by an additional subscript 0, these results become and At ''grazing incidence'' , we again have hence and . Comparing () and () with () and (), we see that at ''normal'' incidence, under the adopted sign convention, the transmission coefficients for the two polarizations are equal, whereas the reflection coefficients have equal magnitudes but opposite signs. While this clash of signs is a disadvantage of the convention, the attendant advantage is that the signs agree at ''grazing'' incidence.

Power ratios (reflectivity and transmissivity)

The ''
Poynting vector In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...
'' for a wave is a vector whose component in any direction is the ''
irradiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measure ...

'' (power per unit area) of that wave on a surface perpendicular to that direction. For a plane sinusoidal wave the Poynting vector is where and are due ''only'' to the wave in question, and the asterisk denotes complex conjugation. Inside a lossless dielectric (the usual case), and are in phase, and at right angles to each other and to the wave vector ; so, for s polarization, using the and components of and respectively (or for p polarization, using the and components of and ), the
irradiance In radiometry Radiometry is a set of techniques for measuring ' Measurement is the numerical quantification of the attributes of an object or event, which can be used to compare with other objects or events. The scope and application of measure ...

in the direction of is given simply by which is in a medium of intrinsic impedance . To compute the irradiance in the direction normal to the interface, as we shall require in the definition of the power transmission coefficient, we could use only the component (rather than the full component) of or or, equivalently, simply multiply by the proper geometric factor, obtaining . From equations () and (), taking squared magnitudes, we find that the ''
reflectivity The reflectance of the surface of a material is its effectiveness in reflecting radiant energy In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion ...
'' (ratio of reflected power to incident power) is for the s polarization, and for the p polarization. Note that when comparing the powers of two such waves in the same medium and with the same cos''θ'', the impedance and geometric factors mentioned above are identical and cancel out. But in computing the power ''transmission'' (below), these factors must be taken into account. The simplest way to obtain the power transmission coefficient ('''', the ratio of transmitted power to incident power ''in the direction normal to the interface'', i.e. the direction) is to use (conservation of energy). In this way we find for the s polarization, and for the p polarization. In the case of an interface between two lossless media (for which ϵ and μ are ''real'' and positive), one can obtain these results directly using the squared magnitudes of the amplitude transmission coefficients that we found earlier in equations () and (). But, for given amplitude (as noted above), the component of the Poynting vector in the direction is proportional to the geometric factor and inversely proportional to the wave impedance . Applying these corrections to each wave, we obtain two ratios multiplying the square of the amplitude transmission coefficient: for the s polarization, and for the p polarization. The last two equations apply only to lossless dielectrics, and only at incidence angles smaller than the critical angle (beyond which, of course, ).

Equal refractive indices

From equations () and (), we see that two dissimilar media will have the same refractive index, but different admittances, if the ratio of their permeabilities is the inverse of the ratio of their permittivities. In that unusual situation we have (that is, the transmitted ray is undeviated), so that the cosines in equations (), (), (), (), and () to () cancel out, and all the reflection and transmission ratios become independent of the angle of incidence; in other words, the ratios for normal incidence become applicable to all angles of incidence. When extended to spherical reflection or scattering, this results in the Kerker effect for
Mie scattering The Mie solution to Maxwell's equations Maxwell's equations are a set of coupled partial differential equations that, together with the Lorentz force law, form the foundation of classical electromagnetism, classical optics, and electric cir ...

.

Non-magnetic media

Since the Fresnel equations were developed for optics, they are usually given for non-magnetic materials. Dividing () by ()) yields $Y=\frac\,.$ For non-magnetic media we can substitute the
vacuum permeability Vacuum permeability is the magnetic permeability in a classical vacuum. ''Vacuum permeability'' is derived from production of a magnetic field by an electric current or by a moving electric charge and in all other formulas for magnetic-field prod ...
''μ''0 for ''μ'', so that $Y_1=\frac ~~;~~~ Y_2=\frac\,;$ that is, the admittances are simply proportional to the corresponding refractive indices. When we make these substitutions in equations () to () and equations () to (), the factor ''cμ''0 cancels out. For the amplitude coefficients we obtain: For the case of normal incidence these reduce to: The power reflection coefficients become: The power transmissions can then be found from .

Brewster's angle

For equal permeabilities (e.g., non-magnetic media), if and are ''
complementary A complement is often something that completes something else, or at least adds to it in some useful way. Thus it may be: * Complement (linguistics), a word or phrase having a particular syntactic role ** Subject complement, a word or phrase addi ...

'', we can substitute for and for so that the numerator in equation () becomes which is zero (by Snell's law). Hence and only the s-polarized component is reflected. This is what happens at the
Brewster angle Brewster's angle (also known as the polarization angle) is an angle of incidence (optics), angle of incidence at which light with a particular Polarization (waves), polarization is perfectly transmitted through a transparent dielectric surface, wi ...

. Substituting for in Snell's law, we readily obtain for Brewster's angle.

Equal permittivities

Although it is not encountered in practice, the equations can also apply to the case of two media with a common permittivity but different refractive indices due to different permeabilities. From equations () and (), if ''ϵ'' is fixed instead of ''μ'', then becomes ''inversely'' proportional to , with the result that the subscripts 1 and 2 in equations () to () are interchanged (due to the additional step of multiplying the numerator and denominator by ). Hence, in () and (), the expressions for and in terms of refractive indices will be interchanged, so that Brewster's angle () will give instead of and any beam reflected at that angle will be p-polarized instead of s-polarized. Similarly, Fresnel's sine law will apply to the p polarization instead of the s polarization, and his tangent law to the s polarization instead of the p polarization. This switch of polarizations has an analog in the old mechanical theory of light waves (see '' §History'', above). One could predict reflection coefficients that agreed with observation by supposing (like Fresnel) that different refractive indices were due to different ''densities'' and that the vibrations were ''normal'' to what was then called the
plane of polarization The term ''plane of polarization'' refers to the direction of polarization of '' linearly-polarized'' light or other electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), ...
, or by supposing (like and
Neumann Neumann is German for "new man", and one of the 20 most common German surnames. People * Von Neumann family, a Jewish Hungarian noble family A–G * Adam Neumann (born 1979), Israeli-born entrepreneur and founder of WeWork * Alfred Neumann ( ...
) that different refractive indices were due to different ''elasticities'' and that the vibrations were ''parallel'' to that plane.Whittaker, 1910, pp.133,148–9; Darrigol, 2012, pp.212,229–31. Thus the condition of equal permittivities and unequal permeabilities, although not realistic, is of some historical interest.

*
Jones calculus In optics Optics is the branch of that studies the behaviour and properties of , including its interactions with and the construction of that use or it. Optics usually describes the behaviour of , , and light. Because light is an , other ...
* Polarization mixing *
Index-matching materialIn optics, an index-matching material is a substance, usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of another object (such as a lens, material, fiber-optic, etc.). When two substances w ...
* Field and power quantities *
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 reflection Total internal reflection (TIR) is the optical phenomenon in which (for ...

, Fresnel's apparatus to produce circularly polarised light *
Specular reflection Reflections on still water are an example of specular reflection. Specular reflection, or regular reflection, is the mirror-like reflection of waves, such as light Light or visible light is electromagnetic radiation within the portion of ...
*
Schlick's approximationIn 3D computer graphics, Schlick's approximation, named after Christophe Schlick, is a formula for approximating the contribution of the Fresnel equation, Fresnel factor in the specular reflection of light from a non-conducting interface (surface) be ...
*
Snell's window Snell's window (also called Snell's circle or optical man-hole) is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees. This phenomenon is caused by refraction of light ...

*
X-ray reflectivityX-ray reflectivity (sometimes known as X-ray specular reflectivity, X-ray reflectometry, or XRR) is a surface-sensitive analytical technique used in chemistry Chemistry is the scientific discipline involved with Chemical element, elements and c ...
*
Plane of incidence In describing reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirro ...

*
Reflections of signals on conducting lines A signal travelling along an electrical transmission line . In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the c ...

Sources

* M. Born and E. Wolf, 1970, ''
Principles of Optics ''Principles of Optics'', colloquially known as ''Born and Wolf'', is an optics textbook written by Max Born and Emil Wolf that was initially published in 1959 by Pergamon Press. After going through six editions with Pergamon Press, the book was ...
'', 4th Ed., Oxford: Pergamon Press. * J.Z. Buchwald, 1989, ''The Rise of the Wave Theory of Light: Optical Theory and Experiment in the Early Nineteenth Century'', University of Chicago Press, . * R.E. Collin, 1966, ''Foundations for Microwave Engineering'', Tokyo: McGraw-Hill. * O. Darrigol, 2012, ''A History of Optics: From Greek Antiquity to the Nineteenth Century'', Oxford, . * A. Fresnel, 1866 (ed. H. de Senarmont, E. Verdet, and L. Fresnel), ''Oeuvres complètes d'Augustin Fresnel'', Paris: Imprimerie Impériale (3 vols., 1866–70)
vol.1 (1866)
* E. Hecht, 1987, ''Optics'', 2nd Ed., Addison Wesley, . * E. Hecht, 2002, ''Optics'', 4th Ed., Addison Wesley, . * F.A. Jenkins and H.E. White, 1976, ''Fundamentals of Optics'', 4th Ed., New York: McGraw-Hill, . * H. Lloyd, 1834
"Report on the progress and present state of physical optics"
''Report of the Fourth Meeting of the British Association for the Advancement of Science'' (held at Edinburgh in 1834), London: J. Murray, 1835, pp.295–413. * W. Whewell, 1857, ''History of the Inductive Sciences: From the Earliest to the Present Time'', 3rd Ed., London: J.W. Parker & Son
vol.2
* E. T. Whittaker, 1910, ''A History of the Theories of Aether and Electricity: From the Age of Descartes to the Close of the Nineteenth Century'', London: Longmans, Green, & Co.

* * * * *''Encyclopaedia of Physics (2nd Edition)'', Rita G. Lerner, R.G. Lerner, G.L. Trigg, VHC publishers, 1991, ISBN (Verlagsgesellschaft) 3-527-26954-1, ISBN (VHC Inc.) 0-89573-752-3 *''McGraw Hill Encyclopaedia of Physics (2nd Edition)'', C.B. Parker, 1994,

Fresnel Equations
– Wolfram.
Fresnel equations calculatorFreeSnell
– Free software computes the optical properties of multilayer materials.
Thinfilm
– Web interface for calculating optical properties of thin films and multilayer materials (reflection & transmission coefficients, ellipsometric parameters Psi & Delta).
Simple web interface for calculating single-interface reflection and refraction angles and strengths

Reflection and transmittance for two dielectrics
– Mathematica interactive webpage that shows the relations between index of refraction and reflection.
A self-contained first-principles derivation
of the transmission and reflection probabilities from a multilayer with complex indices of refraction. {{Portal bar, Physics Light Geometrical optics Physical optics Polarization (waves) Equations of physics History of physics