An initially parallel cylindrical 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 $\mathrm{I}$ is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius. The initial shape of the wave front of the beam is

• A

  convex
• B

  concave
• C

  convex near the axis and concave near the periphery
• Correct answer
  D

  planar

Hence, the correct answer is option: 'D'.