View source for Snell's law

{{Short description|Formula for refraction angles}}

[[Image:Snells law2.svg|thumb|[[Refraction]] of light at the interface between two media of different [[refractive index | refractive indices]], with n<sub>2</sub> > n<sub>1</sub>. Since the velocity is lower in the second medium (v<sub>2</sub> < v<sub>1</sub>), the angle of refraction θ<sub>2</sub> is less than the angle of incidence θ<sub>1</sub>; that is, the ray in the higher-index medium is closer to the normal.]]

'''Snell's law''' (also known as the '''Snell–Descartes law''',  the '''ibn-Sahl''' '''law''',<ref>*{{Cite book |last1=Boudrioua |first1=Azzedine |url=https://books.google.com/books?id=6_0wDwAAQBAJ&pg=PT207 |page=207 |title=Light-Based Science: Technology and Sustainable Development, The Legacy of Ibn al-Haytham |last2=Rashed |first2=Roshdi |last3=Lakshminarayanan |first3=Vasudevan |date=2017-08-15 |publisher=CRC Press |isbn=978-1-351-65112-7}}<br>*{{Cite book |last1=Yao |first1=X. Steve |url=https://books.google.com/books?id=KV2aEAAAQBAJ&pg=PA4 |title=Polarization Measurement and Control in Optical Fiber Communication and Sensor Systems |last2=Chen |first2=Xiaojun (James) |date=2022 |page=4 |publisher=John Wiley & Sons |isbn=978-1-119-75850-1}}<br>*{{Cite journal |last1=Wolf |first1=K B |last2=Krotzsch |first2=G |date=1995-01-01 |title=Geometry and dynamics in refracting systems |url=https://iopscience.iop.org/article/10.1088/0143-0807/16/1/003 |journal=European Journal of Physics |volume=16 |issue=1 |pages=14–20 |doi=10.1088/0143-0807/16/1/003 |bibcode=1995EJPh...16...14W |issn=0143-0807}}<br>*{{Cite book |last=Wolf |first=Kurt Bernardo |url=https://books.google.com/books?id=KmpCDg-D39gC&dq=%22ibn+sahl+law%22&pg=PA9 |title=Geometric Optics on Phase Space |date=2004-07-21 |publisher=Springer Science & Business Media |isbn=978-3-540-22039-8 |language=en}}</ref> and the '''law of refraction''') is a [[formula]] used to describe the relationship between the [[angle of incidence (optics)|angles of incidence]] and [[refraction]], when referring to [[light]] or other [[wave]]s passing through a boundary between two different [[isotropic]] [[medium (optics)|media]], such as water, glass, or air.
In optics, the law is used in [[Ray tracing (physics)|ray tracing]] to compute the angles of incidence or refraction, and in experimental optics to find the [[refractive index]] of a material. The law is also satisfied in [[Metamaterials#Negative refractive index|meta-materials]], which allow light to be bent "backward" at a negative angle of refraction with a [[Refractive index#Negative refractive index|negative refractive index]].

The law states that, for a given pair of media, the ratio of the sines of [[Angle of incidence (optics)|angle of incidence]] (<math>\theta_1 </math>) and angle of refraction (<math>\theta_2 </math>) is equal to the refractive index of the second medium with regard to the first (<math>n_{21}</math>) which is equal to the ratio of the [[refractive indices]] (<math>\tfrac{n_2}{n_1}</math>) of the two media, or equivalently, to the ratio of the [[phase velocities]] (<math>\tfrac{v_1}{v_2}</math>) in the two media.<ref>{{Cite book |title=[[Principles of Optics]]|last=Born and Wolf|publisher=Pergamon Press INC.|year=1959|location=New York, NY|page=37}}</ref>

:<math>\frac{\sin\theta_1}{\sin\theta_2}= n_{21} = \frac{n_2}{n_1} = \frac{v_1}{v_2} </math>

The law follows from [[Fermat]]'s [[Fermat's principle|principle of least time]], which in turn follows from the propagation of light as waves.

==History==
[[Image:Ibn Sahl manuscript.jpg|thumbnail|left|Reproduction of a page of [[Ibn Sahl (mathematician)|Ibn Sahl]]'s manuscript showing his discovery of the law of refraction]]

[[Ptolemy]], in [[Alexandria]], Egypt,<ref>David Michael Harland (2007). "''[https://books.google.com/books?id=ScORNbV0E8wC&pg=PA1 Cassini at Saturn: Huygens results]''". p.1. {{ISBN|0-387-26129-X}}</ref> had found a relationship regarding refraction angles, but it was inaccurate for angles that were not small. Ptolemy was confident he had found an accurate empirical law, partially as a result of slightly altering his data to fit theory (see: [[confirmation bias]]).<ref>{{cite web |title=Ptolemy (ca. 100-ca. 170) |work=Eric Weinstein's World of Scientific Biography |url=http://scienceworld.wolfram.com/biography/Ptolemy.html}}</ref> 

[[Image:Snell Law of Sines 1837.png|thumb|right|An 1837 view of the history of "the Law of the Sines"<ref>William Whewell, ''History of the Inductive Science from the Earliest to the Present Times'', London: John H. Parker, 1837.</ref>]]

The law was eventually named after [[Willebrord Snellius |Snell]], although it was first discovered by the Persian scientist [[Ibn Sahl (mathematician)|Ibn Sahl]], at [[Baghdad]] court in 984.<ref>{{Cite web |last=Papadopoulos |first=Athanase |date=2017 |title=Roshdi Rashed, Historian of Greek and Arabic Mathematics |url=https://hal.archives-ouvertes.fr/hal-01653436/document |website=HAL Open Science |id=hal-01653436 |page=12}}</ref><ref>{{cite web |author1=Mourad Zghal |author2=Hamid-Eddine Bouali |author3=Zohra Ben Lakhdar |author4=Habib Hamam |title=The first steps for learning optics: Ibn Sahl's, Al-Haytham's and Young's works on refraction as typical examples |url=http://spie.org/etop/2007/etop07fundamentalsII.pdf |quote=R. Rashed credited Ibn Sahl with discovering the law of refraction [23], usually called Snell's law and also Snell and Descartes' law.}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=Xd5XBQAAQBAJ&q=ibn+sahl+law+of+refraction&pg=PA178 |title=From Sight to Light: The Passage from Ancient to Modern Optics |last=Smith |first=A. Mark |date=2015 |publisher=University of Chicago Press |isbn=978-0-226-17476-1 |page=178}}</ref> In the manuscript ''On Burning Mirrors and Lenses'', Sahl used the law to derive lens shapes that focus light with no [[Optical aberration|geometric aberration]].<ref name="Rashed1990">{{cite journal |last=Rashed |first=Roshdi |year=1990 |title=A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses |journal=[[Isis (journal)|Isis]] |doi=10.1086/355456 |volume=81 |issue=3 |pages=464–491 |s2cid=144361526}}{{Disputed inline|Elaborate on Ibn Sahl's authorship of the law|date=November 2017|reason=Treatise by Roshdi Rashed/Ibn Sahl is without experimental underpinning and lacks foundation in theory: conflict with article by Dr. Gorden Videen.}}{{clarify |date=November 2017 |reason=conflicts with Rashed (1990), p.491: "when attention is paid to the problems raised by the image of an object observed through a lens, the situation becomes quite different; in this case it is impossible to avoid difficulties as astigmatism and aberration"}}</ref>

[[Alhazen]], in his ''[[Book of Optics]]'' (1021), came close to rediscovering the law of refraction, but he did not take this step.<ref>[[A. I. Sabra]] (1981), ''Theories of Light from Descartes to Newton'', [[Cambridge University Press]]. ([[cf.]] Pavlos Mihas, [https://web.archive.org/web/20120527202345/http://www.ihpst2005.leeds.ac.uk/papers/Mihas.pdf Use of History in Developing ideas of refraction, lenses and rainbow], p. 5, Demokritus University, [[Thrace]], [[Greece]].)</ref>

The law was rediscovered by [[Thomas Harriot]] in 1602,<ref>{{cite journal |last1=Kwan |first1=A. |last2=Dudley |first2=J. |last3=Lantz |first3=E. |year=2002 |title=Who really discovered Snell's law? |journal=[[Physics World]] |volume=15 |issue=4 |page=64 |doi=10.1088/2058-7058/15/4/44}}</ref> who however did not publish his results although he had corresponded with [[Kepler]] on this very subject. In 1621, the Dutch astronomer [[Willebrord Snellius]] (1580–1626)—Snell—derived a mathematically equivalent form, that remained unpublished during his lifetime. [[René Descartes]] independently derived the law using heuristic momentum conservation arguments in terms of sines in his 1637 essay ''[[Dioptrique]]'', and used it to solve a range of optical problems. Rejecting Descartes' solution, [[Pierre de Fermat]] arrived at the same solution based solely on his [[Fermat's principle|principle of least time]]. Descartes assumed the [[speed of light]] was infinite, yet in his derivation of Snell's law he also assumed the denser the medium, the greater the speed of light. Fermat supported the opposing assumptions, i.e., the speed of light is finite, and his derivation depended upon the speed of light being slower in a denser medium.<ref>[[Florian Cajori]], [https://books.google.com/books?id=XNtUAAAAYAAJ ''A History of Physics in its Elementary Branches: Including the Evolution of Physical Laboratories''] (1922)</ref><ref>Ferdinand Rosenberger, [https://books.google.com/books?id=hxsAAAAAQAAJ ''Geschichte der Physik''] (1882) Part. II, p.114</ref> Fermat's derivation also utilized his invention of [[adequality]], a mathematical procedure equivalent to differential calculus, for finding maxima, minima, and tangents.<ref>[[Carl Benjamin Boyer]], ''The Rainbow: From Myth to Mathematics'' (1959)</ref><ref>[[Florian Cajori]], "Who was the First Inventor of Calculus" ''The American Mathematical Monthly'' (1919) [https://books.google.com/books?id=5wxLAAAAYAAJ Vol.26]</ref>

In his influential mathematics book [[La Géométrie|''Geometry'']], Descartes solves a problem that was worked on by [[Apollonius of Perga]] and [[Pappus of Alexandria]]. Given n lines L and a point P(L) on each line, find the locus of points Q such that the lengths of the line segments QP(L) satisfy certain conditions.  For example, when n = 4, given the lines a, b, c, and d and a point A on a, B on b, and so on, find the locus of points Q such that the product QA*QB equals the product QC*QD. When the lines are not all parallel, Pappus showed that the loci are conics, but when Descartes considered larger n, he obtained cubic and higher degree curves. To show that the cubic curves were interesting, he showed that they arose naturally in optics from Snell's law.<ref>The Geometry of Rene Descartes (Dover Books on Mathematics) by Rene Descartes, David Eugene Smith and Marcia L. Latham (Jun 1, 1954).</ref>

According to Dijksterhuis,<ref>{{cite book |last=Dijksterhuis |first=Fokko Jan |year=2004 |title=Lenses and Waves: Christiaan Huygens and the Mathematical Science of Optics in the Seventeenth Century |publisher=Springer |isbn=1-4020-2697-8 |url=https://books.google.com/books?id=cPFevyomPUIC&q=Descartes-had-seen-Snel%27s+intitle:Lenses+intitle:and+intitle:Waves+intitle:Christiaan&pg=PA135}}</ref> "In ''De natura lucis et proprietate'' (1662) [[Isaac Vossius]] said that Descartes had seen Snell's paper and concocted his own proof.  We now know this charge to be undeserved but it has been adopted many times since."  Both Fermat and Huygens repeated this accusation that Descartes had copied Snell. In [[French language|French]], Snell's Law is sometimes called  "la loi de Descartes" or more frequently "''[[:fr:loi de Snell-Descartes|loi de Snell-Descartes]]''".

[[Image:Huygens Refracted Waves.png|left|thumb|[[Christiaan Huygens]]' construction]]

In his 1678 ''[[Traité de la Lumière]]'', [[Christiaan Huygens]] showed how Snell's law of sines could be explained by, or derived from, the wave nature of light, using what we have come to call the [[Huygens–Fresnel principle]].

With the development of modern optical and electromagnetic theory, the ancient Snell's law was brought into a new stage.  In 1962, [[Nicolaas Bloembergen]] showed that at the boundary of nonlinear medium, the Snell's law should be written in a general form.<ref>{{cite journal |last1=Bloembergen |first1=N. |last2=Pershan |first2=P. S. |date=1962 |title=Light waves at the boundary of nonlinear media |journal=Physical Review  |bibcode=1962PhRv..128..606B |doi=10.1103/PhysRev.128.606 |volume=128 |issue=2 |page=606|hdl=1874/7432 |url=https://dspace.library.uu.nl/bitstream/1874/7432/1/1962-bloembergen-ligtwaves.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://dspace.library.uu.nl/bitstream/1874/7432/1/1962-bloembergen-ligtwaves.pdf |archive-date=2022-10-09 |url-status=live }}</ref> In 2008 and 2011, [[Electromagnetic metasurface|plasmonic metasurfaces]] were also demonstrated to change the reflection and refraction directions of light beam.<ref>{{cite journal |last1=Xu |first1=T. |display-authors=etal |date=2008 |title=Plasmonic deflector |journal=Opt. Express |volume=16 |issue=7 |pages=4753–9 |doi=10.1364/oe.16.004753|pmid=18542573 |bibcode=2008OExpr..16.4753X |doi-access=free }}</ref><ref name="capasso">{{cite journal |last1=Yu |first1=Nanfang |last2=Genevet |first2=Patrice |last3=Kats |first3=Mikhail A. |last4=Aieta |first4=Francesco |last5=Tetienne |first5=Jean-Philippe |last6=Capasso |first6=Federico |last7=Gaburro |first7=Zeno |date=October 2011 |title=Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction |journal=Science |bibcode=2011Sci...334..333Y |doi=10.1126/science.1210713 |volume=334 |issue=6054 |pages=333–7 |pmid=21885733|s2cid=10156200 |url=http://metaconferences.org/ocs/index.php/META12/META12/paper/view/808 |doi-access=free }}</ref>

{{clear}}

== Explanation ==
[[File:SnelliusLeiden1.jpg|thumb|Snell's law on a wall in Leiden]]
Snell's law is used to determine the direction of light rays through refractive media with varying indices of refraction. The indices of refraction of the media, labeled <math>n_1</math>, <math>n_2</math> and so on, are used to represent the factor by which a light ray's speed decreases when traveling through a refractive medium, such as glass or water, as opposed to its velocity in a vacuum.

As light passes the border between media, depending upon the relative refractive indices of the two media, the light will either be refracted to a lesser angle, or a greater one. These angles are measured with respect to the ''normal line'', represented perpendicular to the boundary. In the case of light traveling from air into water, light would be refracted towards the normal line, because the light is slowed down in water; light traveling from water to air would refract away from the normal line.

Refraction between two surfaces is also referred to as ''reversible'' because if all conditions were identical, the angles would be the same for light propagating in the opposite direction.

Snell's law is generally true only for isotropic or specular media (such as [[glass]]). In [[anisotropic]] media such as some [[crystal]]s, [[birefringence]] may split the refracted ray into two rays, the ''ordinary'' or ''o''-ray which follows Snell's law, and the other ''extraordinary'' or ''e''-ray which may not be co-planar with the incident ray.

When the light or other wave involved is monochromatic, that is, of a single frequency, Snell's law can also be expressed in terms of a ratio of wavelengths in the two media, <math>\lambda_1</math> and <math>\lambda_2</math>:

:<math>\frac{\sin\theta_1}{\sin\theta_2} = \frac{v_1}{v_2} = \frac{\lambda_1}{\lambda_2}</math>

==Derivations and formula==
[[Image:Snells law wavefronts.gif|right|frame|[[Wavefronts]] from a [[point source]] in the context of Snell's law. The region below the grey line has a higher [[index of refraction]], and proportionally lower [[speed of light]], than the region above it.]]
Snell's law can be derived in various ways.
===Derivation from Fermat's principle===
Snell's law can be derived from [[Fermat's principle]], which states that the light travels the path which takes the least time. By taking the [[derivative]] of the [[optical path length]], the [[stationary point]] is found giving the path taken by the light. (There are situations of light violating Fermat's principle by not taking the least time path, as in reflection in a (spherical) mirror.) In a classic analogy, the area of lower [[refractive index]] is replaced by a beach, the area of higher [[refractive index]] by the sea, and the fastest way for a rescuer on the beach to get to a drowning person in the sea is to run along a path that follows Snell's law.

[[File:Snells law Diagram B vector.svg|right|thumb|250px|Light from medium 1, point Q, enters medium 2, refraction occurs, and finally the light reaches point P.]]
As shown in the figure to the right, assume the refractive index of medium 1 and medium 2 are <math>n_1</math> and <math>n_2</math> respectively. Light  enters medium 2 from medium 1 via point O.

<math>\theta_1</math> is the angle of incidence, <math>\theta_2</math> is the angle of refraction with respect to the normal.

The phase velocities of light in medium 1 and medium 2 are
:<math>v_1=c/n_1</math> and
:<math>v_2=c/n_2</math> respectively.

<math>c</math>  is the speed of light in vacuum.

Let T be the time required for the light to travel from point Q through point O to point P. 
:<math>T=\frac{\sqrt{x^2 + a^2}}{v_1} + \frac{\sqrt{b^2 + (l - x)^2}}{v_2} = \frac{\sqrt{x^2 + a^2}}{v_1} + \frac{\sqrt{b^2 + l^2 -2lx + x^2}}{v_2}</math>
where a, b, l and x are as denoted in the right-hand figure, x being the varying parameter.

To minimize it, one can differentiate :
:<math>\frac{dT}{dx}=\frac{x}{v_1\sqrt{x^2 + a^2}} + \frac{ - (l - x)}{v_2\sqrt{(l-x)^2 + b^2}}=0</math> (stationary point)

Note that
<math>\frac{x}{\sqrt{x^2 + a^2}} =\sin\theta_1</math>

and <math>\frac{  l - x}{\sqrt{(l-x)^2 + b^2}}=\sin\theta_2</math>

Therefore, 

:<math>\frac{dT}{dx}=\frac{\sin\theta_1}{v_1} - \frac{\sin\theta_2}{v_2}=0</math>

:<math>\frac{\sin\theta_1}{v_1}=\frac{\sin\theta_2}{v_2}</math>

:<math>\frac{n_1\sin\theta_1}{c}=\frac{n_2\sin\theta_2}{c}</math>

:<math>n_1\sin\theta_1=n_2\sin\theta_2</math>

===Derivation from Huygens's principle===
{{further|Huygens–Fresnel principle}}
Alternatively, Snell's law can be derived using interference of all possible paths of light wave from source to observer—it results in destructive interference everywhere except extrema of phase (where interference is constructive)—which become actual paths.

===Derivation from Maxwell's equations===
{{further|Fresnel equations}}
Another way to derive Snell's Law involves an application of the general [[boundary conditions]] of [[Maxwell equations]] for [[electromagnetic radiation]] and [[Electromagnetic induction|induction]].

===Derivation from conservation of energy and momentum===
Yet another way to derive Snell's law is based on translation symmetry considerations.<ref>{{cite book |last1=Joannopoulos |first1=John D |url=http://ab-initio.mit.edu/book/ |title=Photonic Crystals: Molding the Flow of Light |last2=Johnson |first2=SG |last3=Winn |first3=JN |last4=Meade |first4=RD |publisher=Princeton University Press |year=2008 |isbn=978-0-691-12456-8 |edition=2nd |location=Princeton NJ |pages=31}}</ref> For example, a homogeneous surface perpendicular to the z direction cannot change the transverse momentum. Since the [[Wave vector|propagation vector]] <math>\vec{k}</math> is proportional to the photon's momentum, the transverse propagation direction <math>(k_x,k_y,0)</math> must remain the same in both regions. Assume without loss of generality a plane of incidence in the <math>z,x</math> plane <math>k_{x\text{Region}_1} = k_{x\text{Region}_2}</math>. Using the well known dependence of the [[wavenumber]] on the [[refractive index]] of the medium, we derive Snell's law immediately.

:<math>k_{x\text{Region}_1} = k_{x\text{Region}_2} \, </math>

:<math> n_1 k_0\sin\theta_1 = n_2 k_0\sin\theta_2 \, </math>

:<math> n_1\sin\theta_1 = n_2\sin\theta_2 \, </math>

where <math>k_0=\frac{2\pi}{\lambda_0}=\frac{\omega}{c}</math> is the wavenumber in vacuum. Although no surface is truly homogeneous at the atomic scale, full translational symmetry is an excellent approximation whenever the region is homogeneous on the scale of the light wavelength.

===Vector form===
{{see also|Specular reflection#Direction of reflection}}

Given a normalized light vector <math>\vec{l}</math> (pointing from the light source toward the surface) and a normalized plane normal vector <math>\vec{n}</math>, one can work out the normalized reflected and refracted rays, via the cosines of the angle of incidence <math>\theta_1</math> and angle of refraction <math>\theta_2</math>, without explicitly using the sine values or any trigonometric functions or angles:<ref>{{cite book |last=Glassner |first=Andrew S. |year=1989 |title=An Introduction to Ray Tracing |publisher=Morgan Kaufmann |isbn=0-12-286160-4 |url=https://books.google.com/books?id=YPblYyLqBM4C}}</ref>

:<math>\cos\theta_1 = -\vec{n}\cdot \vec{l}</math>

Note: <math>\cos\theta_1</math> must be positive, which it will be if <math>\vec{n}</math> is the normal vector that points from the surface toward the side where the light is coming from, the region with index <math>n_1</math>. If <math>\cos\theta_1</math> is negative, then <math>\vec{n}</math> points to the side without the light, so start over with <math>\vec{n}</math> replaced by its negative.

:<math>\vec{v}_{\mathrm{reflect}}=\vec{l} + 2\cos\theta_1 \vec{n}</math>
This reflected direction vector points back toward the side of the surface where the light came from.

Now apply Snell's law to the ratio of sines to derive the formula for the refracted ray's direction vector:
:<math>\sin\theta_2 = \left(\frac{n_1}{n_2}\right) \sin\theta_1 = \left( \frac{n_1}{n_2} \right) \sqrt{ 1 - \left(\cos\theta_1 \right)^2 }</math>
:<math>\cos\theta_2 = \sqrt{1-(\sin\theta_2)^2} = \sqrt{1 - \left( \frac{n_1}{n_2} \right)^2 \left( 1 - \left( \cos\theta_1 \right)^2 \right)}</math>
:<math>\vec{v}_{\mathrm{refract}} = \left( \frac{n_1}{n_2} \right) \vec{l} + \left( \frac{n_1}{n_2} \cos\theta_1 - \cos\theta_2 \right) \vec{n}</math>

The formula may appear simpler in terms of renamed simple values <math> r = n_1 / n_2 </math> and <math> c = -\vec{n}\cdot \vec{l}</math>, avoiding any appearance of trig function  names or angle names:
:<math>\vec{v}_{\mathrm{refract}} = r \vec{l} + \left( r c - \sqrt{1 - r^2 \left( 1 - c^2 \right)} \right) \vec{n}</math>

Example:
:<math>\vec{l} = \{0.707107, -0.707107\}, ~ \vec{n} = \{0,1\}, ~ r = \frac{n_1}{n_2} = 0.9</math>
:<math>c = \cos\theta_1=0.707107, ~ \sqrt{1 - r^2 \left( 1 - c^2 \right)} = \cos\theta_2 = 0.771362</math>
:<math>\vec{v}_{\mathrm{reflect}}=\{0.707107, 0.707107\}
,~\vec{v}_{\mathrm{refract}}=\{0.636396, -0.771362\}</math>

The cosine values may be saved and used in the [[Fresnel equations]] for working out the intensity of the resulting rays.

[[Total internal reflection]] is indicated by a negative [[radicand]] in the equation for <math>\cos\theta_2</math>, which can only happen for rays crossing into a less-dense medium (<math> n_2 < n_1</math>).

==Total internal reflection and critical angle==
[[File:Refraction internal reflection diagram.svg|thumb|250px|right|Demonstration of no refraction at angles greater than the critical angle]]
{{Main|Total internal reflection}}
When light travels from a medium with a higher refractive index to one with a lower refractive index, Snell's law seems to require in some cases (whenever the angle of incidence is large enough) that the sine of the angle of refraction be greater than one. This of course is impossible, and the light in such cases is completely reflected by the boundary, a phenomenon known as [[total internal reflection]]. The largest possible angle of incidence which still results in a refracted ray is called the '''critical angle'''; in this case the refracted ray travels along the boundary between the two media.

[[Image:RefractionReflextion.svg|thumb|center|650px|Refraction of light at the interface between two media]]

For example, consider a ray of light moving from water to air with an angle of incidence of 50°. The refractive indices of water and air are approximately 1.333 and 1, respectively, so Snell's law gives us the relation

:<math>\sin\theta_2 = \frac{n_1}{n_2}\sin\theta_1 = \frac{1.333}{1}\cdot\sin\left(50^\circ\right) = 1.333\cdot 0.766 = 1.021,</math>

which is impossible to satisfy. The critical angle θ<sub>crit</sub> is the value of θ<sub>1</sub> for which θ<sub>2</sub> equals 90°:

:<math>\theta_\text{crit} = \arcsin\left(\frac{n_2}{n_1}\sin\theta_2\right) = \arcsin\frac{n_2}{n_1} = 48.6^\circ.</math>

==Dispersion==
{{Main|Dispersion (optics)}}

In many wave-propagation media, wave velocity changes with frequency or wavelength of the waves; this is true of light propagation in most transparent substances other than a vacuum.  These media are called dispersive.  The result is that the angles determined by Snell's law also depend on frequency or wavelength, so that a ray of mixed wavelengths, such as white light, will spread or disperse.  Such dispersion of light in glass or water underlies the origin of [[rainbow]]s and other [[optical phenomena]], in which different wavelengths appear as different colors.

In optical instruments, dispersion leads to [[chromatic aberration]]; a color-dependent blurring that sometimes is the resolution-limiting effect.  This was especially true in [[refracting telescope]]s, before the invention of [[Achromatic lens|achromatic]] objective lenses.

==Lossy, absorbing, or conducting media==
{{see also|Mathematical descriptions of opacity}}
In a conducting medium, permittivity and index of refraction are complex-valued. Consequently, so are the angle of refraction and the wave-vector. This implies that, while the surfaces of constant real phase are planes whose normals make an angle equal to the angle of refraction with the interface normal, the surfaces of constant amplitude, in contrast, are planes parallel to the interface itself. Since these two planes do not in general coincide with each other, the wave is said to be inhomogeneous.<ref>Born and Wolf, sec.13.2, "Refraction and reflection at a metal surface"</ref> The refracted wave is exponentially attenuated, with exponent proportional to the imaginary component of the index of refraction.<ref>Hecht, ''Optics'', sec. 4.8, Optical properties of metals.</ref><ref>S. J. Orfanidis, ''Electromagnetic Waves & Antennas'', sec. 7.9, Oblique Incidence on a Lossy Medium, [http://www.ece.rutgers.edu/~orfanidi/ewa/ch07.pdf] {{Webarchive|url=https://web.archive.org/web/20200730090501/https://www.ece.rutgers.edu/~orfanidi/ewa/ch07.pdf |date=2020-07-30 }}</ref>

==See also==
*{{annotated link|Brachistochrone curve}} for a simple proof by Jacob Bernoulli
*{{annotated link|Calculus of variations#Snell's law|Calculus of variations}}
*{{annotated link|Computation of radiowave attenuation in the atmosphere}}
*{{annotated link|Evanescent wave}}
*{{annotated link|Hamiltonian optics}}
*{{annotated link|List of refractive indices}}
*{{annotated link|N-slit interferometric equation}}
*{{annotated link|Reflection (physics)}}
*{{annotated link|Snell's window}}
*{{annotated link|Sellmeier equation|The refractive index vs wavelength of light}}

==References==
{{reflist|30em}}

==External links==
*[http://materiaislamica.com/index.php/History_of_Islamic_Physics_(Snell%27s_Law) Ibn Sahl and Snell's Law] {{Webarchive|url=https://web.archive.org/web/20161023051637/http://materiaislamica.com/index.php/History_of_Islamic_Physics_(Snell%27s_Law) |date=2016-10-23 }}
* [http://www-rohan.sdsu.edu/~aty/explain/optics/discovery.html Discovery of the law of refraction]
* [http://demonstrations.wolfram.com/SnellsLawOfRefractionWaveFronts/ Snell's Law of Refraction (Wave Fronts)] by Todd Rowland, [[Wolfram Demonstrations Project]]
* [http://ilorentz.org/history/wallformulas/images/pages/page_4.html Snell's law on a wall in downtown Leiden] {{Webarchive|url=https://web.archive.org/web/20180427184259/http://ilorentz.org/history/wallformulas/images/pages/page_4.html |date=2018-04-27 }}
*[http://www.boldmethod.com/learn-to-fly/navigation/shoreline-effect/ Shore line effect]

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{{DEFAULTSORT:Snell's Law}}
[[Category:Eponymous laws of physics]]
[[Category:Geometrical optics]]