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See the diagram.

Refraction of visible light: When a light ray travels from an optically rarer medium (vacuum or air) to optically denser medium (glass or water ), then it is found that the light ray bends towards the normal drawn at the point of incidence.

In the diagram, the light ray SP from medium 1 (air) is incident on a glass surface. we are seeing the surface LL' from the side. Let the ray b incident at point P with an angle i with the normal NN' drawn at P. The light ray travels along the path PR instead of going along PQ. This effect is called refraction.

1. The straight lines SP, PR, and NN' all lie in the same plane.

2. Snell has found that - for various angles of i and r ,

          \frac{Sin\ i}{ Sin\ r} = constant = Mu_{21} \\ \\

  The constant Mu21 is called refractive index of medium 2 with respect to medium 1. It is found that it is equal to ratio of speed of light in medium 1 to that in the medium 2. The value of Mu is, for many materials, between 1 and 2.  The angle i is more than the angle r.

  The reason for speed of light changing in medium 2 is due to the density of medium and its resistance to passage of light energy. It depends on the chemical properties of the medium.

3.   See diagram 2.

   When light wave passes from a denser medium to rarer medium, then the light ray bends away from the normal at the point of incidence. So i is less than r.  In this case, if we increase the angle i, then r also increases. and at one point the angle of refraction becomes 90 deg. Then the ray goes along the interface LL'.  This angle i for which this happens, is called CRITICAL Angle of refraction.

        Sin\ i = \frac{1 }{Mu_{glass}}, \\ \\ Critical\ angle\ in\ Glass\ =\ Sin^{-1} \frac{1}{Mu_{glass}}