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The Nature of Refraction
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 reflection (angle of reflection equals angle of incidence). This phenomenon is known as total internal reflection.
Applications of Refraction
refraction(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:
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.