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There are three disks, with transmission axis of the 1st being 10 deg. from the vertical, the 2nd at an angle theta from the vertical, and the 3rd at an angle 120 deg from the vertical. Unpolarized light, I(0) is incident on the first disk,. We need to find the angle theta for 2nd disk so that transmitied light from 3rd disk is of max intensity.

So I know that the unpolarized light automatically loses 1/2 of its intensity; I(1)=I(0)/2 and that the intensity of I(3) is given by [I(0)/2]cos^2(theta-10)cos^2(120-theta). I think the formula is correct. So how should I go about finding the max? Should I take the derivative of this function and find the zeros, or is there a better way?