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Message: <blockquote data-quote="freyar" data-source="post: 5030592" data-attributes="member: 40227">Actually, that's not right.  (<a href="http://en.wikipedia.org/wiki/Crookes_radiometer" target="_blank">Here's</a> the description on wikipedia, btw.)  If the motion were due to the impacts of the photons, it would spin in the opposite direction (from white to black) due to conservation of momentum:  when light is absorbed by the dark side, it imparts its original momentum; when the light bounces off the white side, it gains some negative momentum and therefore must impart more than its original momentum to the spinner.  The momentum gained by the spinner is greater on the white side, making white move away from the light.The real reason for the motion has to do with how the different colored sides heat the minute amount of gas remaining in the bulb.  Wikipedia gives two explanations about that, but <a href="http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html" target="_blank">this site</a> favors one of those specifically (the author of it is usually pretty good with getting things right).</blockquote>