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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. As the beam enters the medium, it will

1. diverge
2. converge
3. diverge near the axis and converge near the periphery
4. travel as a cylindrical beam

Correct answer: converge

Solution

Since mu = mu0 + mu2*I and I is largest on the axis (decreasing with radius), the refractive index is highest on the axis. The medium acts like a converging (GRIN) lens, bending rays toward the axis, so the beam converges.

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