Ans. CRITICAL ANGLE
The angle of incidence in the denser medium for which corresponding angel of refraction is 90 ° in the rare medium is called the critical angle.
When a ray of light passes from a denser medium (glass) to a rare medium (air), the refracted ray bends away from the normal and the angle of refraction is greater than the angle of incidence.
It is clear from Fig. 12.6 that as the angle of incidence i increases, the angle of refraction r also increases in such a way that r is always greater than i, till for a particular value of angle of incidence, the corresponding angle of refraction becomes 90° and the refracted ray grazes along the surface AB (Fig. .12.6).
If the refractive index of air with respect to glass is n, then in this case the ray OD is travelling from glass to air and is refracted in air therefore, by applying Snell's
law, we will find the value of refractive index.
Now for ray OD
Angle of incidence i = C Angle of refraction r = 90° According to Snell's law
sigg _ sin C _ 1 sin^^in 90° ~ n 1 ' or sin C - —