Intensity of light through two polarizers

1. Nov 13, 2015

Mddrill

1. The problem statement, all variables and given/known data

Unpolarized light of intensitry 3 W/m^2 goes through two polarizing films. Their axes are 60 degrees apart. What is the intensity of light transimitted by the second film
2. Relevant equations
$I_2 = I_1*cos^2 phi$
Where I_1 is the intensity of light after going through the first film and I_2 is the intensity of light after going through the second film.

I also read online that the intensity of light going through one polarized lens is one half the original intensity, I don't know if I should use that since we never learned it in class and its not in the lab manual
3. The attempt at a solution
I don't know whether to just use the first equation and plug the 3W/m^2 in for I_1, or to use both equations(use half of 3 W/m^2 as I_1), which I don't think their expect since I was only given the first equation.

2. Nov 13, 2015

Staff: Mentor

The problem clearly states that you start with unpolarized light. Why could you then assume that it all goes through the first polarizer?
Can't you figure that factor out with the equation you are given?

3. Nov 13, 2015

Mddrill

The equation I was given was I_2=I_1cos^phi. How would I figure ou the second equation from that?

I just want to make sure I'm not missing something because I feel like I should be able to solve the problem with just the equation I'm given.

4. Nov 13, 2015

Staff: Mentor

For unpolarized light, you can consider that all polarizations are equiprobable. So you can calculate, on average, how much of that light will pass through the first polarizer using that equation.