#### Question is based on the following paragraph.An initially parallel cylindrical beam travels in a medium of refractive index  are positive constants and is the intensity of the light beam. The intensity of the beam is decreasing with increasing radius.Question: The initial shape of the wavefront of the beam is  Option 1) planar Option 2) convex Option 3) concave Option 4) convex near the axis and concave near the periphery

As we learnt in

Refractive index -

- wherein

speed of light in vacuum.

speed of light in medium.

For a parallel cylindrical beam, wavelength will be planar.

Correct option is 1.

Option 1)

planar

This is the correct option.

Option 2)

convex

This is an incorrect option.

Option 3)

concave

This is an incorrect option.

Option 4)

convex near the axis and concave near the periphery

This is an incorrect option.