It is the phenomenon by which, when a ray of light is incident obliquely on a surface of separation between two media, part of it goes into the second medium along a path different from the direction of the incident beam obeying the following laws:-
The incident ray, the normal and the refracted ray at a point lie in the same plane.
The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant and is called the refractive index of the medium.
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Illustrations:
(i) A pond looks shallower than it really is because of refraction. When rays start from a denser to a rarer medium, the rays get bent away from the normal. Hence a point at the bottom of the pond appears to be raised.
(ii) It is due to refraction that a stick immersed in water, when looked obliquely appears bent.
(iii) The brilliancy of a diamond, etc., is due to the fact that when light enters a piece of diamond, on account of high refractive index (and consequential small critical angle) and the cut of its face, it gets totally reflected at most of the face. The few faces from which it comes out throw a good deal of light and make them appear brilliantly lit.
(iv) Air bubbles in water and cracks in transparent bodies appear glittering due to total internal reflection.
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(v) The rays of light from a star as they pass through successively denser and denser layers of air go on bending more and more towards the normal. Hence stars appear to be higher than they actually are.
(vi) The sun seems to rise about two minutes earlier and seems to set about two minutes later on account of refraction in the atmosphere.
(vii) Mirage is an optical illusion owing to total internal reflection. In sandy deserts the air in contact with the hot earth is rarer than the upper layers.
As we go towards the earth the density of the air goes on decreasing. Hence the rays of light from a distant object traverse through successively rare layers and hence go on bending away from the normal and ultimately get totally reflected at the rarest layer, and reach the observer’s eye as if they come from a point as far below the reflecting layer as the object is above it.
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Since these layers of air are shaking the inverted image as seen by total internal reflection also is shaking, giving man the illusion of ‘splashing water everywhere’.
(viii) Stars twinkle because the atmospheric air close to the earth is disturbed by convection currents and on account of passing through such disturbed ‘areas’, the light from a star travelling in a given direction sometimes comes through and sometimes is deflected away; so the star is sometimes seen and sometimes not seen hence the twinkling.