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in physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass. Other forms of electromagnetic radiationelectromagnetic radiation,
energy radiated in the form of a wave as a result of the motion of electric charges. A moving charge gives rise to a magnetic field, and if the motion is changing (accelerated), then the magnetic field varies and in turn produces an electric field.
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, in addition to light waves, can be refracted, as can sound waves.

The Nature of Refraction

Refraction is commonly explained in terms of the wave theory of light and is based on the fact that light travels with greater velocity in some media than it does in others. When, for example, a ray of light traveling through air strikes the surface of a piece of glass at an oblique angle, one side of the wave front enters the glass before the other and is retarded (since light travels more slowly in glass than in air), while the other side continues to move at its original speed until it too reaches the glass. As a result, the ray bends inside the glass, i.e., the refracted ray lies in a direction closer to the normal (the perpendicular to the boundary of the media) than does the incident ray. A light ray entering a different medium is called the incident ray; after bending, the ray is called the refracted ray. The speed at which a given transparent medium transmits light waves is related to its optical density (not to be confused with mass or weight densitydensity,
ratio of the mass of a substance to its volume, expressed, for example, in units of grams per cubic centimeter or pounds per cubic foot. The density of a pure substance varies little from sample to sample and is often considered a characteristic property of the
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). In general, a ray is refracted toward the normal when it passes into a denser medium and away from the normal when it passes into a less dense medium.

The Law of Refraction

The law of refraction relates the angle of incidence (angle between the incident ray and the normal) to the angle of refraction (angle between the refracted ray and the normal). This law, credited to Willebrord Snell, states that the ratio of the sine of the angle of incidence, i, to the sine of the angle of refraction, r, is equal to the ratio of the speed of light in the original medium, vi, to the speed of light in the refracting medium, vr, or sin i/sin r=vi/vr. Snell's law is often stated in terms of the indexes of refraction of the two media rather than the speeds of light in the media. The index of refraction, n, of a transparent medium is a direct measure of its optical density and is equal to the ratio of the speed of light in a vacuum, c, to the speed of light in the medium: n=c/v.

Indexes of refraction are always equal to or greater than 1; for air, n=1.00029; for water, n=1.33. Using indexes of refraction, Snell's law takes the form sin i/sin r=nr /ni, or ni sin i=nr sin r. If the original medium is denser than the refracting medium (ni greater than nr), sin r will be greater than sin i. Thus, there will be some acute angle less than 90° for the incident ray corresponding to an angle of refraction of 90°. This angle of incidence is known as the critical angle. For angles of incidence greater than the critical angle, refraction cannot take place and the incident ray is instead reflected back into the original medium according to the law of reflectionreflection,
return of a wave from a surface that it strikes into the medium through which it has traveled. The general principles governing the reflection of light and sound are similar, for both normally travel in straight lines and both are wave phenomena.
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 (angle of reflection equals angle of incidence). This phenomenon is known as total internal reflection.

Applications of Refraction

Refraction has many applications in optics and technology. A lenslens,
device for forming an image of an object by the refraction of light. In its simplest form it is a disk of transparent substance, commonly glass, with its two surfaces curved or with one surface plane and the other curved.
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 uses refraction to form an imageimage,
in optics, likeness or counterpart of an object produced when rays of light coming from that object are reflected from a mirror or are refracted by a lens. An image of an object is also formed when this light passes through a very small opening like that of a pinhole
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 of an object for many different purposes, such as magnification. A prismprism,
in optics, a piece of translucent glass or crystal used to form a spectrum of light separated according to colors. Its cross section is usually triangular. The light becomes separated because different wavelengths or frequencies are refracted (bent) by different amounts
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 uses refraction to form a spectrumspectrum,
arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer (see mass spectrograph).
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 of colors from an incident beam of light. Refraction also plays an important role in the formation of a miragemirage
, atmospheric optical illusion in which an observer sees in the distance a nonexistent body of water or an image, sometimes distorted, of some object or of a complete scene.
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 and other optical illusions.



(ri-frak -shŏn) A phenomenon occurring when a beam of light or other wave motion crosses a boundary between two different media, such as air and glass. On passing into the second medium, the direction of motion of the wave is ‘bent’ toward or away from the normal (the line perpendicular to the surface at the point of incidence). The incident and refracted rays and the normal all lie in the same plane. The direction of propagation is changed in accordance with Snell's law:
n 1 sin i = n 2 sin r
i and r are the angles made by the incident and refracted ray to the normal; n 1 and n 2 are the refractive indices of the two media. The change in direction of motion results from a change of wave velocity as the wave passes from the first to the second medium.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006



The Russian words refraktsiia and prelomlenie may both be translated as “refraction.” When used with respect to light, refraktsiia in the broad sense has the same meaning as prelomlenie—that is, the change in the direction of light rays when the index of refraction of the medium through which the rays pass changes (seeREFRACTION OF LIGHT). For historical reasons, refraktsiia is more often used when characterizing the propagation of optical radiation in media whose index of refraction varies continuously from point to point; the paths of light rays in such media are smooth curves. The term prelomlenie is more often applied to the case where there is an abrupt change in the direction of light rays at the interface of two homogeneous media with different indexes of refraction. The term refraktsiia is by tradition used in a number of branches of optics, including atmospheric optics, eyeglass optics, and the optics of the eye.

The eye is an optical system that refracts light. A commonly used measure of the power of the eye as a refracting system is the eye’s power under suspension of accommodation. The principal refracting elements of the eye are the cornea and the lens. The power of these elements varies from 52.59 to 71.30 diopters; the average value is 59.92 diopters. In the normal, or emmetropic, eye, the power of the eye is matched to its dimensions. This means that parallel rays of light that enter the eye are focused at the center of the retina in the region of the macula lutea. A clear image of the object being viewed is then obtained on the retina—a situation that is a necessary condition for good vision.

Errors in refraction result in myopia or hyperopia. The power of accommodation of the eye changes with age: it is less than normal in infants and may again decrease in old age. This change in accommodation power with advancing age is called presbyopia. Anomalies in refraction cannot be treated through medication. Special systems of optical lenses (eyeglasses) are used to correct vision when errors of refraction exist.



The change in direction of lines of force of an electric or magnetic field at a boundary between media with different permittivities or permeabilities.
The change of direction of propagation of any wave, such as an electromagnetic or sound wave, when it passes from one medium to another in which the wave velocity is different, or when there is a spatial variation in a medium's wave velocity.


The change in direction of a light ray or a sound ray in passing from one medium to another.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


1. Physics the change in direction of a propagating wave, such as light or sound, in passing from one medium to another in which it has a different velocity
2. the amount by which a wave is refracted
3. the ability of the eye to refract light
4. the determination of the refractive condition of the eye
5. Astronomy the apparent elevation in position of a celestial body resulting from the refraction of light by the earth's atmosphere
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005


The bending of electromagnetic waves as they pass between materials with different refractive indices. Refraction is an important characteristic of optical systems. As light rays travel at a more perpendicular angle to the edge of a medium, they are refracted outside the medium rather than being reflected inside. See refractive index, total internal reflection and diffraction.