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| {{about||the property of metals|refraction (metallurgy)|the magic effect|David Penn (magician)|the refraction in atmosphere|Atmospheric refraction}}
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| [[Image:Fénytörés.jpg|thumb|Light on air–plexi surface in this experiment undergoes refraction (lower ray) and [[Reflection (physics)|reflection]] (upper ray).]]
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| [[Image:GGB reflection in raindrops.jpg|thumb|An image of the [[Golden Gate Bridge]] is refracted and bent by many differing three dimensional drops of water.]]
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| '''Refraction''' is the change in direction of a [[wave]] due to a change in its [[transmission medium]].
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| Refraction is essentially a [[surface phenomenon]]. The phenomenon is mainly in governance to the law of conservation of [[energy]] and [[momentum]]. Due to change of medium, the [[phase velocity]] of the wave is changed but its [[frequency]] remains constant. This is most commonly observed when a wave passes from one [[transmission medium|medium]] to another at any angle other than 90° or 0°. Refraction of [[light]] is the most commonly observed phenomenon, but any type of wave can refract when it interacts with a medium, for example when [[sound wave]]s pass from one medium into another or when water waves move into water of a different depth. Refraction is described by [[Snell's law]], which states that for a given pair of media and a wave with a single frequency, the ratio of the sines of the [[angle of incidence]] ''θ<sub>1</sub>'' and [[angle of refraction]] ''θ<sub>2</sub>'' is equivalent to the ratio of phase velocities (''v<sub>1</sub> / v<sub>2</sub>'') in the two media, or equivalently, to the opposite ratio of the indices of refraction (''n<sub>2</sub> / n<sub>1</sub>''):
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| :<math>\frac{\sin\theta_1}{\sin\theta_2} = \frac{v_1}{v_2} = \frac{n_2}{n_1} .</math>
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| In general, the incident wave is partially refracted and partially [[reflection (physics)|reflected]]; the details of this behavior are described by the [[Fresnel equations]].
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| ==Explanation==
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| [[Image:Snells law.svg|thumb|Refraction of light at the interface between two media of different [[refractive indices]], with n<sub>2</sub> > n<sub>1</sub>. Since the phase 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.]]
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| In [[optics]], refraction is a phenomenon that often occurs when waves travel from a medium with a given [[refractive index]] to a medium with another at an oblique angle. At the boundary between the media, the wave's [[phase velocity]] is altered, usually causing a change in direction. Its [[wavelength]] increases or decreases but its [[frequency]] remains constant. For example, a [[ray (optics)|light ray]] will refract as it enters and leaves [[glass]], assuming there is a change in [[refractive index]]. A ray traveling along the [[Normal (geometry)|normal]] (perpendicular to the boundary) will change speed, but not direction. Refraction still occurs in this case. Understanding of this concept led to the [[invention]] of [[lens (optics)|lens]]es and the [[refracting telescope]].
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| [[File:Pencil in a bowl of water.svg|thumb|left|An object (in this case a pencil) part immersed in water looks bent due to refraction: the light waves from X change direction and so seem to originate at Y. (More accurately, for any angle of view, Y should be vertically above X, and the pencil should appear shorter, not longer as shown.)]]
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| Refraction can be seen when looking into a bowl of water. Air has a refractive index of about 1.0003, and water has a refractive index of about 1.3330. If a person looks at a straight object, such as a pencil or straw, which is placed at a slant, partially in the water, the object appears to bend at the water's surface. This is due to the bending of light rays as they move from the water to the air. Once the rays reach the eye, the eye traces them back as straight lines (lines of sight). The lines of sight (shown as dashed lines) intersect at a higher position than where the actual rays originated. This causes the pencil to appear higher and the water to appear shallower than it really is. The depth that the water appears to be when viewed from above is known as the ''apparent depth''. This is an important consideration for [[spearfishing]] from the surface because it will make the target fish appear to be in a different place, and the fisher must aim lower to catch the fish. Conversely, an object above the water has a higher ''apparent height'' when viewed from below the water. The opposite correction must be made by an [[archer fish]].<ref>Dill, Lawrence M. "Refraction and the spitting behavior of the archerfish (Toxotes chatareus)." Behavioral Ecology and Sociobiology 2.2 (1977): 169-184.</ref> For small angles of incidence (measured from the normal, when sin θ is approximately the same as tan θ), the ratio of apparent to real depth is the ratio of the refractive indexes of air to that of water. But as the angle of incidence approaches 90<sup>o</sup>, the apparent depth approaches zero, albeit reflection increases, which limits observation at high angles of incidence. Conversely, the apparent height approaches infinity as the angle of incidence (from below) increases, but even earlier, as the angle of [[total internal reflection]] is approached, albeit the image also fades from view as this limit is approached.
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| [[Image:Refraction in a ripple tank.png|thumb|Diagram of refraction of water waves.]]
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| The diagram on the right shows an example of refraction in [[water wave]]s. Ripples travel from the left and pass over a shallower region inclined at an angle to the wavefront. The waves travel slower in the more shallow water, so the wavelength decreases and the wave bends at the boundary. The dotted line represents the normal to the boundary. The dashed line represents the original direction of the waves. This phenomenon explains why waves on a shoreline tend to strike the shore close to a perpendicular angle. As the waves travel from deep water into shallower water near the shore, they are refracted from their original direction of travel to an angle more normal to the shoreline.<ref>{{cite web|url=http://www.coastal.udel.edu/ngs/waves.html |title=Shoaling, Refraction, and Diffraction of Waves |accessdate=2009-07-23 |publisher=University of Delaware Center for Applied Coastal Research}}</ref>
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| Refraction is also responsible for [[rainbow]]s and for the splitting of white light into a rainbow-spectrum as it passes through a glass [[triangular prism (optics)|prism]]. Glass has a higher refractive index than air. When a beam of white light passes from air into a material having an index of refraction that varies with frequency, a phenomenon known as [[dispersion (optics)|dispersion]] occurs, in which different coloured components of the white light are refracted at different angles, i.e., they bend by different amounts at the interface, so that they become separated. The different colors correspond to different frequencies.
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| While refraction allows for phenomena such as rainbows, it may also produce peculiar [[optical phenomenon|optical phenomena]], such as [[mirage]]s and [[Fata Morgana (mirage)|Fata Morgana]]. These are caused by the change of the refractive index of air with temperature.
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| The refractive index of materials can also be nonlinear, as occurs with the [[Kerr effect]] when high intensity light leads to a refractive index proportional to the intensity of the incident light.
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| Recently some [[metamaterial]]s have been created which have a [[negative refractive index]].
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| With metamaterials, we can also obtain [[total refraction]] phenomena when the wave impedances of the two media are matched. There is then no reflected wave.<ref>{{cite journal|doi=10.1088/1367-2630/7/1/213|title=On the physical origins of the negative index of refraction|year=2005|author=Ward, David W|journal=New Journal of Physics|volume=7|page=213|last2=Nelson|first2=Keith A|last3=Webb|first3=Kevin J|arxiv = physics/0409083 |bibcode = 2005NJPh....7..213W }}</ref>
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| Also, since refraction can make objects appear closer than they are, it is responsible for allowing water to magnify objects. First, as light is entering a drop of water, it slows down. If the water's surface is not flat, then the light will be bent into a new path. This round shape will bend the light outwards and as it spreads out, the image you see gets larger.
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| [[Image:RefractionReflextion.svg|thumb|center|650px|Refraction of light at the interface between two media.]]
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| A useful analogy in explaining the refraction of light would be to imagine a marching band as they march at an oblique angle from pavement (a fast medium) into mud (a slower medium). The marchers on the side that runs into the mud first will slow down first. This causes the whole band to pivot slightly toward the normal (make a smaller angle from the normal).
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| {{clear}}
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| ==Clinical significance==
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| In [[medicine]], particularly [[optometry]], [[ophthalmology]] and [[orthoptics]], ''refraction'' (also known as ''refractometry'') is a clinical test in which a [[phoropter]] may be used by the appropriate [[eye care professional]] to determine the eye's [[refractive error]] and the best [[corrective lens]]es to be prescribed. A series of test lenses in graded [[optical power]]s or [[focal length]]s are presented to determine which provides the sharpest, clearest vision.<ref>{{cite web|url=http://www.eyeglossary.net/#R|title=Eye Glossary|accessdate=2006-05-23}}</ref>
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| ==Acoustics==
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| In [[underwater acoustics]], refraction is the bending or curving of a sound ray that results when the ray passes through a [[sound speed gradient]] from a region of one sound speed to a region of a different speed. The amount of ray bending is dependent upon the amount of difference between sound speeds, that is, the variation in temperature, salinity, and pressure of the water.<ref>{{cite book
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| |title=Navy Supplement to the DOD Dictionary of Military and Associated Terms
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| |date=August 2006
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| |publisher=[[United States Navy|Department Of The Navy]]
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| |url=https://www.nwdc.navy.mil/Documents/NTRP_1-02.pdf
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| |id=NTRP 1-02}}</ref>
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| Similar [[acoustics]] effects are also found in the [[Earth's atmosphere]]. The phenomenon of refraction of sound in the atmosphere has been known for centuries;<ref>Mary Somerville (1840), ''On the Connexion of the Physical Sciences'', J. Murray Publishers, (originally by Harvard University)</ref> however, beginning in the early 1970s, widespread analysis of this effect came into vogue through the designing of urban [[highway]]s and [[noise barrier]]s to address the [[meteorological]] effects of bending of sound rays in the lower atmosphere.<ref>{{cite journal|doi=10.1007/BF00159677|title=Analysis of highway noise|year=1973|author=Hogan, C. Michael|journal=Water Air and Soil Pollution|volume=2|pages=387|issue=3}}</ref>
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| ==Gallery==
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| {{Gallery
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| |align=center
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| |lines = 3
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| |width = 250
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| |File:Refraction.jpg|Refraction in a Perspex ([[acrylic glass|acrylic]]) block.
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| |File:Refraction-with-soda-straw.jpg|The straw appears to be broken, due to refraction of light as it emerges into the air.
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| |File:Refraction-ripple-tank.JPG|Photograph of refraction of waves in a [[ripple tank]].
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| |File:Angle of incidence Refraction example.jpg|Refraction at a steep angle of incidence
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| |File:Aorta duplication artifact 131206105958250c.jpg|Double aort artifact in sonography due to difference in velocity of sound in muscle and fat.
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| }}
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| ==See also==
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| *[[Birefringence]] (double refraction)
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| *[[Huygens–Fresnel principle]]
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| *[[List of indices of refraction]]
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| *[[Metamaterials]]
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| *[[Negative refraction]]
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| *[[Parallax]], a visually similar principle caused by angle of perspective
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| *[[Reflection (physics)|Reflection]]
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| *[[Snell's law]]
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| *[[Total internal reflection]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| {{commons category|Refraction}}
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| *[http://www.falstad.com/ripple/ex-refraction.html Java illustration of refraction]
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| *[http://www.phy.hk/wiki/englishhtm/RefractionByPrism.htm Java simulation of refraction through a prism]
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| *[http://www.flipcode.com/archives/reflection_transmission.pdf Reflections and Refractions in Ray Tracing], a simple but thorough discussion of the mathematics behind refraction and reflection.
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| *[http://www.interactagram.com/physics/optics/refraction/ Flash refraction simulation- includes source], Explains refraction and Snell's Law.
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| *[http://qed.wikina.org/refraction/ Animations demonstrating optical refraction] by QED
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| [[Category:Geometrical optics]]
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| [[Category:Physical optics]]
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| [[Category:Concepts in physics]]
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| [[ml:അപവര്ത്തനം]]
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