# Optically active materials + Faraday Rotation

1. Feb 5, 2005

### freemind

Hello folks,

I have been told that for a linearly polarised wave, the direction of rotation due to the Faraday effect (caused by a constant magnetic field) is independent of the direction of wave propagation, dependent only upon the direction of the magnetic field. Fair enough. Does this property hold for normal optical activity? Is the direction of rotation the same in an optically active material regardless of direction of propagation?

Thanks.

2. Feb 8, 2005

### Laser Jock

I think that the answer to your question is no (although someone may correct me). The magnetic field (in the Faraday effect) breaks the symmetry of the situation. Suppose you send linearly polarized light (say s-polarized) through a Faraday rotator that rotates the light 45 degrees. If you place a mirror after the rotator that sends it back the way it came, the light will be p-polarized after it goes through the rotator a second time. However, if you replace the rotator with a normal optically active material, the output polarization will be what it was originally (s-polarized). That is why a Faraday rotator (coupled with a polarizer) works as an optical isolator (light can only go one direction). Hope this helps.

3. Feb 8, 2005

### Gonzolo

If by "direction of propagation", you mean one way or the other along the optic axis, then no, the Farady effect and optically active materials behave differently, as was said above. An optically active material could be said to be either clockwise or anticlockwise, while for the Faraday effect, "clockwise-ness" depends on direction.

If you include directions that are not along the optic axis, the question is a bit more complicated.

4. Feb 8, 2005

### freemind

I see how the direction of rotation is dependent upon the direction of propagation. Thanks a lot for the replies.