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Refraction in a Perspex (acrylic) block.

Refraction is the change in direction of a wave due to a change in its velocity. This is most commonly observed when a wave passes from one medium to another. 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 waves 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 the angle of incidence θ₁ is related to the angle of refraction θ₂ by

{\frac {\sin \theta _{1}}{\sin \theta _{2}}}={\frac {v_{1}}{v_{2}}}={\frac {n_{2}}{n_{1}}}

where v₁ and v₂ are the wave velocities in the respective media, and n₁ and n₂ the refractive indices. In general, the incident wave is partially refracted and partially reflected; the details of this behavior are described by the Fresnel equations.

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Clinical significance

In medicine, particularly optometry, ophthalmology and orthoptics, refraction (also known as refractometry) is a clinical test in which a phoropter may used by the appropriate eye care professional to determine the eye's refractive error and the best corrective lenses to be prescribed. A series of test lenses in graded optical powers or focal lengths are presented to determine which provide the sharpest, clearest vision.^[1]

Acoustics

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.^[2] 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;^[3] however, beginning in the early 1970s, widespread analysis of this effect came into vogue through the designing of urban highways and noise barriers to address the meteorological effects of bending of sound rays in the lower atmosphere.^[4]

References

^ "Eye Glossary". Retrieved 2006-05-23.
^ Navy Supplement to the DOD Dictionary of Military and Associated Terms (PDF). Department Of The Navy. 2006. NTRP 1-02. {{cite book}}: Unknown parameter |month= ignored (help)
^ Mary Somerville, On the Connexion of the Physical Sciences, J. Murray Publishers, (originally by Harvard University), 499 pages (1840)
^ Hogan, C. Michael (1973). "Analysis of highway noise". Water Air and Soil Pollution. 2: 387. doi:10.1007/BF00159677.

External links

Java explanatory animation
Java illustration of refraction
Java simulation of refraction through a prism
Reflections and Refractions in Ray Tracing, a simple but thorough discussion of the mathematics behind refraction and reflection.
Flash refraction simulation- includes source, Explains refraction and Snell's Law.

[1] "Eye Glossary". Retrieved 2006-05-23.

[2] Navy Supplement to the DOD Dictionary of Military and Associated Terms (PDF). Department Of The Navy. 2006. NTRP 1-02. {{cite book}}: Unknown parameter |month= ignored (help)

[3] Mary Somerville, On the Connexion of the Physical Sciences, J. Murray Publishers, (originally by Harvard University), 499 pages (1840)

[4] Hogan, C. Michael (1973). "Analysis of highway noise". Water Air and Soil Pollution. 2: 387. doi:10.1007/BF00159677.

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 where ''v<sub>1</sub>'' and ''v<sub>2</sub>'' are the wave velocities in the respective media, and ''n<sub>1</sub>'' and ''n<sub>2</sub>'' the [[refractive index|refractive indices]]. 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==
-[[Image:Pencil in a bowl of water.png|left|frame|Refraction of light waves in water. The dark rectangle represents the actual position of a pencil sitting in a bowl of water. The light rectangle represents the apparent position of the pencil. Notice that the end (X) looks like it is at (Y), a position that is considerably shallower than (X).]]
-[[Image:Refraction-with-soda-straw.jpg|thumb|The straw appears to be broken, due to refraction of light as it emerges into the air.]]
-[[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.]]
-[[Image:Refraction-ripple-tank.JPG|thumb|Photograph of refraction of waves in a [[ripple tank]]]]
-[[Image:Refraction in a ripple tank.png|thumb|Diagram of refraction of water waves]]
-In [[optics]], refraction occurs when [[light wave]]s travel from a medium with a given [[refractive index]] to a medium with another. 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 (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]].
-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 1.33. 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.
-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 more slowly in the shallower water, so the wavelength decreases and the wave bends at the boundary. The dotted line represents the [[Surface normal|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>
-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.
-While refraction allows for beautiful 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.
-Recently some [[metamaterial]]s have been created which have a [[negative refractive index]].
-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|pages=213}}</ref>
-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.
-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).
 ==Clinical significance==

Revision as of 00:48, 5 October 2009

Clinical significance

Acoustics

See also

References

External links