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The Crookes radiometer, also known as the light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation has been the cause of much scientific debate.
And wouldn't radiation pressure push harder on the shiny (reflecting) sides where the momentum change is twice the momentum of each photon, than on the black (light absorbing) side on which the momentum transfer is only from converting the photon into excitation energy of some sort in the vane?
I worked out the force (very approx) due to radiation pressure on a 1cm sq vane in direct sunlight.It seems it's about 10e-7Newtons (0.1 microNewtons). Not likely to overcome the friction in even the best needle bearing.But that force can be used, on a huge 'solar sail' to produce quite a few Newtons and it's there 24/7. (at least whilst near the Earth's orbit). It could accelerate a light probe to some pretty high speeds, eventually.