# Homework Help: Reflection of a plane mirror and phase shift

1. Jan 8, 2012

### Niles

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

I think I have confused myself: If I have a linearly polarized beam incident on a plane mirror with higher index, will the 180-degree phase shift change the direction of the polarization? Say, from lying in the 1st-3rd quadrant to lying in the 2nd-4th quadrant? Personally I would say no, since the mirror has no way to "tell" what the 1st-3rd quadrant actually is. But I am not 100% sure my argument works.

Best,
Niles.

2. Jan 8, 2012

### ehild

The pi phase change during reflection does not change the direction of polarization.
But: the coefficient of reflection is different for waves polarized in the plane of incidence and for those, perpendicular to the plane(TM and TE mode waves).
If the incident wave has both parallel and perpendicular components, the ratio of the components will change upon reflection, and so does the direction of polarization.
When the mirror is from metal, the situation is even more complicated at non-normal incidence, as not only the amplitudes of the wave components change but their phases, too. Therefore the reflected light becomes elliptically polarized.

ehild

3. Jan 9, 2012

### Niles

Hi

Thanks, do you have a reference where these things are explained?

Best,
Niles.

4. Jan 9, 2012

### ehild

It is any handbook of Optics, O.S. Heavens, Optical Properties of Thin Films, Academic Press, 1955, for example. Google "Fresnel coefficients", "polarization of light by reflection".

ehild

5. Feb 7, 2012

### Niles

Hi again

Ok, so from Fresnel's equations I see that the component perpendicular to the plane of incidence gets an additional phase of 180 degrees upon reflection. That means that there is a net relative change between the components of 180 degrees upon reflection. But this is practically just what a half-wave plate does?

Where am I wrong here?

Last edited: Feb 7, 2012