Optics - Polarization

A steady of light is normally incident on a piece of Polaroid. As the Polaroid is rotated around the beam axis, the transmitted intensity varies as A + B cos 2θ where θ is the angle of rotation, and A and B are constants with . Which of the following may be correctly concluded about the incident light? 

A. The light is completely unpolarized
B. The light is completely plane polarized
C. The light is partly plane polarized and partly unpolarized
D. The light is partly circularly polarized and partly unpolarized
E. The light is completely circularly polarized

(GR9277 #67)

Solution:

Malus' Law: for plane-polarized light, I = I₀ cos²θ
Trig identity: cos²θ =  ½ (1 + cos 2θ) 

If A = B, the light is completely plane polarized, but A > B > 0

When cos²θ = 0, I = A = constant → unpolarized.

Thus, the light is partly plane polarized and partly unpolarized

Answer: C