# I S vs. P polarization (classical EM)

1. Apr 18, 2017

### pierce15

Hello,

I am having trouble wrapping my head around S vs P polarization of light. First, if linearly polarized light strikes an interface of two linear media, what determines whether it is S or P polarized? Also, why are these the only two options, i.e. why can't the polarization of the incident light be oblique relative to the plane of incidence?

2. Apr 19, 2017

### vanhees71

Do you have a source, where this terminology is used, i.e., where it is defined what S and P polarization means? I know it from the German literature as synonyma for TE and TM waves (transverse magnetic or transverse electric) for a mirror, where the electric or magnetic field are in the plane spanned by the direction of incidence and the mirror-normal vector respectively. S and P stand for "senkrecht"=perpendicular and "parallel" (denoting whether the magnetic field is perpendicular or parallel to the mirror surface). For some historical reason the names thus refer to the magnetic field of the em. wave. I'm always confused by this, and I'd prefer TE and TM, where it's clearly said that the electric or magnetic field are transverse (i.e., perpendicular to the mirror plane).

3. Apr 19, 2017

### nasu

Who said it cannot be oblique? If it is oblique you can analyse it as a combination of S and P components. Same as you resolve a vector along two perpendicular axes in mechanics problems. The reflection and transmission coefficients for the two components are given by Fresnel's equations. At Brewster angle the reflection coefficient for one of the components is zero. For any other angle both components are reflected (and transmitted) but as they have different coefficients, the polarization of the reflected wave will be different than that of the incident light.
See figure 57 on this webpage for more details.
http://farside.ph.utexas.edu/teaching/em/lectures/node104.html

Last edited: Apr 19, 2017
4. Apr 19, 2017

### pierce15

Here: https://en.wikipedia.org/wiki/Fresnel_equations

That makes much more sense. I had seen the derivation of both Fresnel equations but didn't realize that the most general case of polarization could be decomposed and the Fresnel equations subsequently applied. Thanks.

5. Apr 20, 2017

### vanhees71

Ok, so it's one more notion taken from German (despite Bremsstrahlung and Zitterbewegung) :-)).