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?

  • $\begingroup$ I believe this goes in the field of "optical tweezers". People are being very active in using light to directly act in mechanical ways on small objects (proteins, nanoparticles, etc.) $\endgroup$ – genneth Feb 2 '11 at 21:00
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    $\begingroup$ Could you link to the question of Kostya's that you mean? $\endgroup$ – ptomato Feb 3 '11 at 0:15

About wavelength change: In general, if you have light at frequency $f_1$ and it's modulated (in any way for any reason) at frequency $f_2$, you'll get sidebands of light at frequencies $f_1+f_2$ and $f_1-f_2$.

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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