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?