An analyzer is rotated about an incident ray. The intensity of light is found to increase and decrease, but it does not become zero at any point. The incident light is:
a.partially polarized
b.plane-polarized
c.polarized in two planes
d.unpolarized
They probably expect answer (a), but it could also be (c).
To determine the type of polarization of the incident light, we need to understand the behavior of a polarizer/analyzer system.
A perfect polarizer only allows the transmission of light waves oscillating in a specific plane, known as the polarization plane. When a polarized light wave passes through a polarizer aligned with its polarization plane, the intensity of the transmitted light is at its maximum. If the polarizer is rotated, thereby misaligning with the polarization plane, the intensity of the transmitted light decreases.
Now, let's consider the scenario described in the question. The intensity of light is found to increase and decrease as the analyzer is rotated, but it never becomes zero. This means that the transmitted light is not being completely blocked by the analyzer at any orientation.
If the incident light were plane-polarized, it would only oscillate in a single plane, and rotating the analyzer would completely block the light at certain orientations, resulting in zero intensity. Since this does not occur in the given scenario, we can rule out option b (plane-polarized).
If the incident light were polarized in two planes, rotating the analyzer would cause the intensity to go through a complete cycle, from maximum to minimum and back to maximum, as it aligns and misaligns with the two planes of polarization. However, in the given scenario, the intensity does not reach zero, which eliminates option c (polarized in two planes).
If the incident light were unpolarized, it would consist of a random mixture of light waves oscillating in different planes. When such light passes through a polarizer, it becomes partially polarized, meaning some of the light aligned with the polarizer's polarization plane is transmitted, while the rest is blocked. The intensity varies as the analyzer is rotated because the polarization of the transmitted light changes, aligning or misaligning with the analyzer. However, it never reaches zero intensity since some of the randomly polarized light always manages to get through even at the extreme rotations. Therefore, option d (unpolarized) is the correct answer.
In conclusion, the incident light is unpolarized (option d).