# Polarization formula

1. Apr 21, 2005

### sghaussi

Hello! I'm having difficulty solving this problem, I was wondering if you can help me solve it.

Three polarizing filters are stacked, with the polarizing axis of the second and third filters at angles of 28.4 and 57.0, respectively, to that of the first. If unpolarized light is incident on the stack, the light has an intensity of 76.5 W/cm^2 after it passes through the stack.

If the incident intensity is kept constant, what is the intensity of the light after it has passed through the stack if the second polarizer is removed?

I know I need to use the formula: I = I_max cos^2 phi

Do I solve for I_max first given the intensity 76.5 W/cm^2 then change the angle to fit the second situation? And what do they mean by "light is incident" on the stack? Does that mean I am looking for I or I_max?

I've read the section on Polarization 5 times so far and I still don't understand this problem. I hope you can help me. Thank you. =)

Last edited by a moderator: Jan 7, 2014
2. Apr 21, 2005

### Staff: Mentor

Here's one way to approach it. Call the intensity of the light after passing through the first filter $I_1$. Now use your formula (which is Malus' Law) to calculate the intensity of the light for two cases:
(a) after passing through filter 2, then through filter 3
(b) after passing directly through filter 3​
Compare (a) with (b), realizing that the intensity you calculate for (a) is given as 76.5 W/cm^2.

3. Apr 21, 2005

### sghaussi

Does this mean that filter one is positioned at zero degrees? Also, am I supposed to be using 76.5 W/cm^2 as my I_max or is that just the intensity given after the light has passed through all three filters? Do I just use the given intensity for find I_max? Thanks for your quick response - it's greatly appreciated!

4. Apr 21, 2005

### Staff: Mentor

Yes.
As the problem stated: "...the light has an intensity of 76.5 W/cm^2 after it passes through the stack"
Do what I suggested and see what happens.