# POlarisation and intensity

1. Jul 22, 2011

### Lindsayyyy

Hi,

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

I have the following question:

I found two formulas given for the intensity after polarised light went through a polarisation filter.
2. Relevant equations

$$I_{1}=I_{0}*\frac 1 2$$

and

$$I_{1}= I_{0}*cos^{2}(\phi)$$

3. The attempt at a solution
Well I don't know when to use which equation. Does it depend on the filter used or does it depend on the angle?

edit:

I think when the axis of the filter is parallel to the incoming rays its just 1/2 and when theres an angle between the incoming rays and the filter I have to use the one with cosine.

Last edited: Jul 22, 2011
2. Jul 22, 2011

### IDOGAWACONAN

The first one is used when the light is natural light;
And use the second when it is linearly polarized light.

3. Jul 22, 2011

### Staff: Mentor

As IDOG says, the first equation is for unpolarized light incident on the polarizer, and the 2nd is for polarized light with its axis of polarization at an angle phi to the polarizer's axis.

You can use the 2nd equation to derive the first equation, BTW. What is the average value of cos^2(phi) ?