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Snell's law
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{{short description|The relation between the angles of incidence and refraction of waves crossing the interface between isotropic media}}Image:Snells law2.svg|thumb|Refraction of light at the interface between two media of different refractive indices, with n2 > n1. Since the velocity is lower in the second medium (v2 < v1), the angle of refraction Î¸2 is less than the angle of incidence Î¸1; that is, the ray in the higher-index medium is closer to the normal.]]Snell's law (also known as Snellâ€“Descartes law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass, or air.In optics, the law is used in 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, which allow light to be bent "backward" at a negative angle of refraction with a negative refractive index.Snell's law states that the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media, or equivalent to the reciprocal of the ratio of the indices of refraction:
frac{sintheta_2}{sintheta_1} = frac{v_2}{v_1} = frac{n_1}{n_2}
with each theta as the angle measured from the normal of the boundary, v as the velocity of light in the respective medium (SI units are meters per second, or m/s), lambda as the wavelength of light in the respective medium and n as the refractive index (which is unitless) of the respective medium.The law follows from Fermat's principle of least time, which in turn follows from the propagation of light as waves.

## History

missing image!
- Ibn Sahl manuscript.jpg -
thumbnail|Reproduction of a page of Ibn Sahl's manuscript showing his discovery of the law of refraction.
Ptolemy, in Alexandria, Egypt,David Michael Harland (2007). "Cassini at Saturn: Huygens results". p.1. {{ISBN|0-387-26129-X}} 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 fudging his data to fit theory (see: confirmation bias).WEB, Ptolemy (ca. 100-ca. 170), Eric Weinstein's World of Scientific Biography,weblink Alhazen, in his Book of Optics (1021), came closer to discovering the law of refraction, though he did not take this step.A. I. Sabra (1981), Theories of Light from Descartes to Newton, Cambridge University Press. (cf. Pavlos Mihas, weblink" title="web.archive.org/web/20120527202345weblink">Use of History in Developing ideas of refraction, lenses and rainbow, p. 5, Demokritus University, Thrace, Greece.)
missing image!
- Snell Law of Sines 1837.png -
An 1837 view of the history of "the Law of the Sines"William Whewell, History of the Inductive Science from the Earliest to the Present Times, London: John H. Parker, 1837.
Although named after Dutch astronomer Willebrord Snellius (1580â€“1626), the law was first accurately described by the Persian scientist Ibn Sahl at the Baghdad court in 984. In the manuscript On Burning Mirrors and Lenses, Sahl used the law to derive lens shapes that focus light with no geometric aberrations.JOURNAL, Rashed, Roshdi, 1990, A pioneer in anaclastics: Ibn Sahl on burning mirrors and lenses, Isis (journal), Isis, 10.1086/355456, 81, 3, 464â€“491, {{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: â€œwhen 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 aberrationâ€}}The law was rediscovered by Thomas Harriot in 1602,JOURNAL, Kwan, A., Dudley, J., Lantz, E., 2002, Who really discovered Snell's law?, Physics World, 15, 4, 64,weblink 10.1088/2058-7058/15/4/44, who however did not publish his results although he had corresponded with Kepler on this very subject. In 1621, Willebrord Snellius (Snell) derived a mathematically equivalent form, that remained unpublished during his lifetime. RenÃ© Descartes independently derived the law using heuristic momentum conservation arguments in terms of sines in his 1637 essay Dioptrics, 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 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.Florian Cajori, A History of Physics in its Elementary Branches: Including the Evolution of Physical Laboratories (1922)Ferdinand Rosenberger, Geschichte der Physik (1882) Part. II, p.114 Fermat's derivation also utilized his invention of adequality, a mathematical procedure equivalent to differential calculus, for finding maxima, minima, and tangents.Carl Benjamin Boyer, The Rainbow: From Myth to Mathematics (1959)Florian Cajori, "Who was the First Inventor of Calculus" The American Mathematical Monthly (1919) Vol.26In his influential mathematics book 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.The Geometry of Rene Descartes (Dover Books on Mathematics) by Rene Descartes, David Eugene Smith and Marcia L. Latham (Jun 1, 1954).According to Dijksterhuis,BOOK, Dijksterhuis, Fokko Jan, 2004, Lenses and Waves: Christiaan Huygens and the Mathematical Science of Optics in the Seventeenth Century, Springer, 1-4020-2697-8,weblink "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, Snell's Law is called "la loi de Descartes" or "loi de Snell-Descartes."
missing image!
- Huygens Refracted Waves.png -
Christiaan Huygens' construction
In his 1678 TraitÃ© de la LumiÃ¨re, 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 Huygensâ€“Fresnel principle.With the development of modern optical and electromagnetic theory, the ancient Snell's law was brought into a new stage. In 1962, Bloembergen showed that at the boundary of nonlinear medium, the Snell's law should be written in a general form.JOURNAL, Bloembergen, N., Pershan, P. S., 1962, Light waves at the boundary of nonlinear media, Physical Review, 1962PhRv..128..606B, 10.1103/PhysRev.128.606, 128, 606,weblink In 2008 and 2011, plasmonic metasurfaces were also demonstrated to change the reflection and refraction directions of light beam.JOURNAL, Xu, T., etal, 2008, Plasmonic deflector, Opt. Express, 16, 4753, 10.1364/oe.16.004753, 2008OExpr..16.4753X, JOURNAL, Yu, Nanfang, Genevet, Patrice, Kats, Mikhail A., Aieta, Francesco, Tetienne, Jean-Philippe, Capasso, Federico, Gaburro, Zeno, October 2011, Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction, Science, 2011Sci...334..333Y, 10.1126/science.1210713, 334, 6054, 333, 21885733, {{clear}}

## Explanation

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 n_1, n_2 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 crystals, 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, lambda_1 and lambda_2:
frac{sintheta_1}{sintheta_2} = frac{v_1}{v_2} = frac{lambda_1}{lambda_2}

## Derivations and formula

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- Snells law wavefronts.gif -
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 (though it should be noted that the result does not show light taking the least time path, but rather one that is stationary with respect to small variations as there are cases where light actually takes the greatest time path, as 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 reaches point P finally.)As shown in the figure to the right, assume the refractive index of medium 1 and medium 2 are n_1 and n_2 respectively. Light enters medium 2 from medium 1 via point O.theta_1 is the angle of incidence, theta_2 is the angle of refraction.The traveling velocities of light in medium 1 and medium 2 are
v_1=c/n_1 and v_2=c/n_2 respectively.
c is the speed of light in vacuum.Let T be the time required for the light to travel from point Q to point P.
T=frac{sqrt{x^2 + a^2}}{v_1} + frac{sqrt{b^2 + (l - x)^2}}{v_2}
frac{dT}{dx}=frac{x}{v_1sqrt{x^2 + a^2}} + frac{ - (l - x)}{v_2sqrt{(l-x)^2 + b^2}}=0 (stationary point)
Note thatfrac{x}{sqrt{x^2 + a^2}} =sintheta_1frac{ l - x}{sqrt{(l-x)^2 + b^2}}=sintheta_2
frac{dT}{dx}=frac{sintheta_1}{v_1} - frac{sintheta_2}{v_2}=0
frac{sintheta_1}{v_1}=frac{sintheta_2}{v_2}
frac{n_1sintheta_1}{c}=frac{n_2sintheta_2}{c}
n_1sintheta_1=n_2sintheta_2

### Derivation from Huygens's principle

Alternatively, Snell's law can be derived using interference of all possible paths of light wave from source to observerâ€”it results in destructive interference everywhere except extrema of phase (where interference is constructive)â€”which become actual paths.

### Derivation from Maxwell's Equations

Another way to derive Snell's Law involves an application of the general boundary conditions of Maxwell equations for electromagnetic radiation.

### Derivation from conservation of energy and momentum

Yet another way to derive Snell's law is based on translation symmetry considerations.BOOK, Joannopoulos, John D, Johnson, SG, Winn, JN, Meade, RD, 2008, Photonic Crystals: Molding the Flow of Light, 2nd, Princeton University Press, Princeton NJ, 978-0-691-12456-8,weblink For example, a homogeneous surface perpendicular to the z direction cannot change the transverse momentum. Since the propagation vector vec{k} is proportional to the photon's momentum, the transverse propagation direction (k_x,k_y,0) must remain the same in both regions. Assume without loss of generality a plane of incidence in the z,x plane k_{xtext{Region}_1} = k_{xtext{Region}_2}. Using the well known dependence of the wavenumber on the refractive index of the medium, we derive Snell's law immediately.
k_{xtext{Region}_1} = k_{xtext{Region}_2} ,
n_1 k_0sintheta_1 = n_2 k_0sintheta_2 ,
n_1sintheta_1 = n_2sintheta_2 ,
where k_0=frac{2pi}{lambda_0}=frac{omega}{c} 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 l (pointing from the light source toward the surface) and a normalized plane normal vector n, one can work out the normalized reflected and refracted rays, via the cosines of the angle of incidence theta_1 and angle of refraction theta_2, without explicitly using the sine values or any trigonometric functions or angles:BOOK, Glassner, Andrew S., 1989, An Introduction to Ray Tracing, Morgan Kaufmann, 0-12-286160-4,weblink
costheta_1 = -mathbf{n}cdot mathbf{l}
Note: costheta_1 must be positive, which it will be if n is the normal vector that points from the surface toward the side where the light is coming from, the region with index n_1. If costheta_1 is negative, then n points to the side without the light, so start over with n replaced by its negative.
mathbf{v}_{mathrm{reflect}}=mathbf{l} + 2costheta_1 mathbf{n}
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:
sintheta_2 = left(frac{n_1}{n_2}right) sintheta_1 = left( frac{n_1}{n_2} right) sqrt{ 1 - left(costheta_1 right)^2 } costheta_2 = sqrt{1-(sintheta_2)^2} = sqrt{1 - left( frac{n_1}{n_2} right)^2 left( 1 - left( costheta_1 right)^2 right)} mathbf{v}_{mathrm{refract}} = left( frac{n_1}{n_2} right) mathbf{l} + left( frac{n_1}{n_2} costheta_1 - costheta_2 right) mathbf{n}
The formula may appear simpler in terms of renamed simple values r = n_1 / n_2 and c = -mathbf{n}cdot mathbf{l}, avoiding any appearance of trig function names or angle names:
mathbf{v}_{mathrm{refract}} = r mathbf{l} + left( r c - sqrt{1 - r^2 left( 1 - c^2 right)} right) mathbf{n}
Example:
mathbf{l} = {0.707107, -0.707107}, ~ mathbf{n} = {0,1}, ~ r = frac{n_1}{n_2} = 0.9 c = costheta_1=0.707107, ~ sqrt{1 - r^2 left( 1 - c^2 right)} = costheta_2 = 0.771362 mathbf{v}_{mathrm{reflect}}={0.707107, 0.707107}
,~mathbf{v}_{mathrm{refract}}={0.636396, -0.771362}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 costheta_2, which can only happen for rays crossing into a less-dense medium ( n_2 < n_1).

## 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.)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.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 50Â°. The refractive indices of water and air are approximately 1.333 and 1, respectively, so Snell's law gives us the relation
sintheta_2 = frac{n_1}{n_2}sintheta_1 = frac{1.333}{1}cdotsinleft(50^circright) = 1.333cdot 0.766 = 1.021,
which is impossible to satisfy. The critical angle Î¸crit is the value of Î¸1 for which Î¸2 equals 90Â°:
theta_text{crit} = arcsinleft(frac{n_2}{n_1}sintheta_2right) = arcsinfrac{n_2}{n_1} = 48.6^circ.

## Dispersion

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 rainbows 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 telescopes, before the invention of 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.Born and Wolf, sec.13.2, "Refraction and reflection at a metal surface" The refracted wave is exponentially attenuated, with exponent proportional to the imaginary component of the index of refraction.Hecht, Optics, sec. 4.8, Optical properties of metals.S. J. Orfanidis, Electromagnetic Waves & Antennas, sec. 7.9, Oblique Incidence on a Lossy Medium, weblink

## References

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{{Science of underwater diving}}

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