Mirror rotates any polarisation by 90°?

[iorveth_]

TL;DR

When looking in a mirror using passive 3D glasses and closing one eye, you cannot see the open eye, but the closed eye. Tilting the head doesn't change this.

Over each eye is a linear polariser and they are orthogonal to each other. So I conclude from the experiment that the polarisation must have been rotated by 90° by the mirror.
That reminded my of phase plates but they only work because their refractive indix depends on the angle. Along two directions they don't do anything to the polarisation. But I can tilt my head.
I also remembered that for some reflective surfaces linearly polarised light cannot reflect in certain directions. But this is not what I am seeing here. I can see everything in the reflection. Except my open eye.
Any ideas?
Can you reproduce this?

 

[scottdave]

I speculate that it is circular polarization. The mirror would reverse that. This stackexchange post suggests the same.

https://physics.stackexchange.com/q...f-in-the-mirror-through-polarizing-3d-glasses

 

[vanhees71]

What's happening concerning reflection and refraction is described by Fresnel's equations, which you find in any textbook on classical electrodynamics/optics. A nice treatment is in

G. Joos, Theoretical Physics, Dover (1989)

 

[iorveth_]

I speculate that it is circular polarization. The mirror would reverse that. This stackexchange post suggests the same.

https://physics.stackexchange.com/q...f-in-the-mirror-through-polarizing-3d-glasses

You're right! I didn't know this excited but it turns out that RealD glasses use circularly polarised light. This observation would have been impossible with linearly polerised light as symmetry prohibits 90° rotations (+90 and -90 cancel). If we assume that the mirror indeed turns right into left polerised light, this explains the observation. Starting from the eye, the light passes a poleriser in the (1,1) direction. Then the (0,1) direction is retarded by π/2 so that we have left polerised light. After reflection the light is right polerised but still turns left as we are looking antiparallel to the propagation. It first hits the retarder so we have a π retardation now, the light is polarised along (1,-1) and gets blocked.

You can also use time reversal symmetry to see that left polerised light can pass towards the eye.

 

[Claude Bile]

This is a really cool experiment!

The "handedness" of circular polarisation is conventionally defined from the perspective of the receiver. Simply put, the mirror reverses the direction of propagation, thus the "sender" becomes the "receiver" and vice versa, hence reversing the handedness of the polarisation.

This configuration is often referred to as a "poor man's optical isolator", because it can be used in conjunction with a polarising optic to separate forward and backward propagating beams, albeit with less fidelity than a proper Faraday isolator.

 

[vanhees71]

You can as well argue with parity, which is imho more to the point. A spatial reflection ($\vec{x} \rightarrow -\vec{x}$) flips momentum $\vec{p} \rightarrow -\vec{p}$ but doesn't change the angular momentum of the em. wave, and thus helicity flips.

BTW: An electromagnetic wave does not have an additional property you could call "chirality", but since it's a massless particle you can simply define "chirality" as being the same as helicity although it's a bit confusing terminology.

 

[A.T.]

You're right! I didn't know this excited but it turns out that RealD glasses use circularly polarised light.

Older 3D glasses were linearly polarized, but people got tired of having to hold the heads perfectly straight to avoid double images. Circular polarization fixes this.