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Two beams, A and B, of plane polarized light with mutually perpendicular planes of polarization are seen through a polaroid. From the position when the beam A has maximum intensity (and beam B has zero intensity), a rotation of polaroid through 30° makes the two beams appear equally bright. If the initial intensities of the two beams are I_A and I_B respectively, then I_A/I_B equals:

1. 3
2. 3/2
3. 1
4. 1/3

Correct answer: 1/3

Solution

When the polaroid is rotated, the intensity of beam A decreases while the intensity of beam B increases. The condition for equal brightness after a 30° rotation indicates that the ratio of their initial intensities must be such that the contributions from both beams balance out, leading to the conclusion that I_A is one-third of I_B.

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