JEE Main · 2010 · 11 Apr 2010 · Offline · Wave Optics

Passage

An initially parallel cylindrical beam travels in a medium of refractive index

μ (I) = μ_{0} + μ_{2} I

, where

μ_{0}

and

μ_{2}

are positive constants and

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
(D)planar✓ Correct

Answer: (D)

Solution

The beam is described as initially parallel, and a parallel beam is simply a set of rays all travelling in one direction. The wavefront is the surface drawn perpendicular to the rays, so for parallel rays it is a flat plane. Before the medium has had any chance to bend the rays, the initial wavefront is therefore planar.

JEE Main · 2010 · 11 Apr 2010 · Offline · Single correct option

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