Solution to 1992 Problem 67

\begin{align*}A + B \cos 2 \theta = A + 2B  \cos^2 \theta - B = A - B +  2 B \cos^2 \theta\end{align*}
This is the intensity one would get if the incident light consisted of 1) light with intensity 2 B polarized along \theta = 0 and 2) unpolarized light with intensity 2(A-B). In order to be able to add the intensities as we have done, we need to assume that parts 1) and 2) are incoherent. %(see Giancoli page 873-874 and 877).
Assuming this, then answer (C) is correct.

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