With a neat labelled diagram, explain the terms total internal reflection and critical angle.

Explanation

The following figure shows propagation of light from water (denser medium) to air (rarer medium).

The ray of light incident at the boundary separating the two media bends away from the normal on refraction. Here, the angle of refraction r, is greater than the angle of incidence i. Now `""_(a)n_(w)=(sin i)/(sin r) lt 1.` Here, `""_(a)n_(w)` is the refractive index of air with respect to water. As `""_(a)n_(w)` is constant, r increases as i increases. For `r=90^(@)`, the ray travels along the boundary. If i is increased further, as r cannot be greater than `90^(@)`, light does not enter air. There is no refraction of light and all the light enters water on reflection. This is called total internal reflection.
For `r=90^(@), ""_(a)n_(w)=(sin i)/(sin 90^(@))= sin i.` This angle i is called the critical angle.