You have two linear polarizers. You place one on top of the other, and can see t

Question

You have two linear polarizers. You place one on top of the other, and can see through them clearly. Turning one by 90 degrees completely blocks out all light going through the polarizer. Leaving the two polarizers in this position, how would you allow light through?
a.Add a third polarizer on top of the first two, but at an angle different than either of the first two.
b.There is no way to allow light through two crossed polarizers.
c.Flip both polarizers over together, as light can go through the opposite way
d.Add a third polarizer in between the first two, but at an angle different than either of the first two.
e.Flip the top polarizer over (without changing the angle), as this will reverse the polarization and allow light through

Explanation / Answer

d. add a third polarizer in between at a different angle.

For polarizers: I_transmitted = I_0*cos^2(theta), where theta is the angle from one polarizer to the next. For our initial set up, theta = 90, and since cos(90) = 0, I_transmitted = 0.

By placing a third between the first two, we have two thetas, both less than 90, so for both, cos(theta)>0, meaning I_transmitted>0

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