An intially parallel cyclindrical beam travels in a medium of refractive index `mu (I) = mu_(0) + mu_(2) I`, where `mu_(0)` and `mu_(2)` are positive constants and `I` is intensity of light beam. The intensity of the beam is decreasing with increasing radius.
Answer the following questions :
As the beam enters the medium, it will

Options

A. travel as a cylindrical beam
B. diverge
C. converge
D. diverge near the axis and converge near the periphery

Answer

C

Explanation

Converge when it enter in the medium.
When light is moving and as it enters the medium than along the axis velocity is decreasing so as we move way from the centre (tant is x in figure) the wave covers less distance and hence shape is convex.