Kostyas question for angular momentum and half wave plates Half wave plate and a

Question

Kostyas question for angular momentum and half wave plates Half wave plate and angular momentum made me think a little bit. It took me some seconds to "swallow" the answer. :=)

Then I started to think further, whether such a plate, rotating in the right direction, could lead to a slightly shorter wavelength?

And further, what happens in an optical active substance, which "rotates" the plane of polarisation of a light beam? Of course no momentum is changed, there is only a extremely small difference of interaction of the two circular polarized waves (representing the plane polarized beam) with the substance. Is there any "mechanic" reaction of the "sample"? Might a small torque be excerted along the axis parallel to the direction of the beam?

Explanation / Answer

About wavelength change: In general, if you have light at frequency f1 and it's modulated (in any way for any reason) at frequency f2, you'll get sidebands of light at frequencies f1+f2 and f1?f2.

So yes, if you spin a half-wave plate at a constant speed, the light traveling through it will acquire sidebands at a shifted frequency and wavelength.

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