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

Explanation

Passage 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.