Q. Unpolarized light of intensity Io is incident on a stack of 2 polarizers, and intensity I emerges from the last polarizer. If the angle between the polarizers is 45 degrees, what is the ratio I/Io?

A 0.707
B 0.500
C 0.866
D 0.250
E 0.354

Thanks in advance

Malus' law is

I=I_o\,cos^2(\theta)

where \theta is the angle of rotation of the plane of polarization. One can rearrange the equation by dividing both sides by zero to get

\frac{I}{I_o}=cos^2(\theta)

If the angle, \theta is 45 degrees, the problem becomes trivial:

\frac{I}{I_o}=cos^2(45^o) and

cos(45)=\frac{\sqrt2}{2} so

\frac{I}{I_o}=\frac 12

So, your answer is B