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Plane polarised light is passed through a polaroid. As the polaroid is rotated through one complete turn about the direction of propagation, the transmitted intensity:

1. gradually decreases to zero and stays zero
2. gradually increases to a maximum and stays maximum
3. shows no change
4. varies, passing through a maximum twice and zero twice

Correct answer: varies, passing through a maximum twice and zero twice

Solution

By Malus's law, I = I0*cos²(theta). Over a complete 360 deg rotation, cos²(theta) = 1 at theta = 0 and 180 deg (two maxima) and cos²(theta) = 0 at theta = 90 and 270 deg (two minima/zeros). Hence the intensity passes through a maximum twice and zero twice.

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