An initially parallel cylindrical beam travels in a medium of refractive index $\mu(I) = \mu_0 + \mu_2I$, where $\mu_0$ and $\mu_2$ are positive constants and $I$ is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius. <br><br> QUESTION : The initial shape of the wave front of the beam is

Question

( A ) convex near the axis and concave near the periphery
( B ) planar
( C ) concave
( D ) convex