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COLOURS IN STILL WATER 61 ‘‘refracted,” or bent downwards, in the direction OC. Fig. 21 is similar to Fig. 20, and shows the passage of light in the opposite direction. In this case the ray is supposed to start from below the surface of the water at C; on reaching the surface at QO, part of it is reflected downwards along OD, and the rest emerges, not in a straight line to c, but bent in the direction OA.' The ray thus follows the same path as that starting from A in Fig. 20, but in the opposite direction. This bending or refraction of light in passing from : The angle of refraction, CON (Fig. 20), is measured, like the angles of incidence and reflexion, from the normal at O, MN. The laws of refraction of light are as follows: (1) Zhe refracted ray lies in the plane containing the incident ray and the normal, and on the opposite side of the normal. (2) The sines of the angles of incidence and refraction always bear a constant ratio to one another, called the index of refraction. sin angle of incidence _ 4 ight ing fr ir into water, — E Oe Eee EM Fram Ase a6 " sin angle of refraction 3 or in other words, the refractive index M of air to water is + or 1°33. 3 If the ray CO is more inclined : than in Fig. 21, it is evident that it | will reach a position at which the : refracted ray OA will lie along the surface of the water. In this case, the angle MOA becoming go’, sin MOA=1; therefore sin CON=# . and CON=about 482. This is known as the critical angle (KON, Fig. 22); and if the incident ray is still further inclined, as, for instance, to the position CO in Fig. 22, there will be no refracted ray, but the whole of the light will be reflected downwards in the direction OD. This is called /ofa/ internal reflexion.
