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Unpolarized light of intensity I passes through an ideal polarizer A. Another identical polarizer B is placed behind A. The intensity of light beyond B is found to be I/2. Now another identical polarizer C is placed between A and B. The intensity beyond B is now found to be I/8. The angle between polarizer A and C is:

1. 0°
2. 30°
3. 45°
4. 60°

Correct answer: 45°

Solution

A and B alone give I/2, so they are parallel. With C at angle theta from A (and B), intensity = (I/2)cos^2(theta)cos^2(theta) = (I/2)cos^4(theta) = I/8, so cos^4(theta) = 1/4, cos^2(theta) = 1/2, theta = 45 degrees.

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