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Directions: Questions number 10-12 are based on the following paragraph. An initially parallel cylindrical beam travels in a medium of refractive index μ(I) = μ0 + μ2 I, where μ0 and μ2 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 wavefront of the beam is

1. convex
2. concave
3. convex near the axis and concave near the periphery
4. planar

Correct answer: convex

Solution

The initial shape of the wavefront is convex because, in a parallel beam, the wavefronts are typically straight lines that curve outward, resembling a convex shape. This is consistent with the behavior of light traveling in a uniform medium before any refraction occurs.

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