# Mirror rotates any polarisation by 90°?

• I
• iorveth_
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.You can as well argue with parity, which is imho more to the point.

#### iorveth_

TL;DR Summary
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?

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)

topsquark
scottdave said:
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.

Last edited by a moderator:
vanhees71
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.

sophiecentaur, vanhees71 and iorveth_
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.

iorveth_ said:
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.

## 1. How does a mirror rotate the polarisation of light?

When light reflects off a mirror, the electric field vector of the light is rotated by 180 degrees. This means that if the light is initially polarised in a certain direction, it will be rotated by 180 degrees after reflecting off the mirror, effectively rotating the polarisation by 90 degrees.

## 2. What is polarisation?

Polarisation refers to the orientation of the electric field vector of light waves. Light can be polarised in many different directions, and when light is reflected or transmitted through certain materials, its polarisation can be affected.

## 3. Can any type of mirror rotate polarised light by 90 degrees?

Yes, any type of mirror can rotate polarised light by 90 degrees as long as the light is reflecting off the mirror at a specific angle known as the Brewster angle. This angle depends on the refractive index of the material the mirror is made of.

## 4. What is the significance of rotating polarisation by 90 degrees?

Rotating polarisation by 90 degrees can be useful in many applications, such as optical communications and polarisation-based spectroscopy. It allows for manipulation and control of the polarisation of light, which can be used to extract information or enhance certain properties of light.

## 5. Are there any other ways to rotate polarisation besides using a mirror?

Yes, there are other ways to rotate polarisation, such as using polarising filters or wave plates. These devices use the principle of birefringence to alter the polarisation of light passing through them.