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| A Crookes' radiometer is a mostly evacuated glass bulb, inside which a propeller sits on a low friction stand. Each vane of the propeller is polished on one side and black on the other, and when placed in light, the propeller rotates. A popular theory explains the rotation as due to photon momentum; however, this theory predicts the propeller rotates the opposite direction. The rotation is actually due to forces described by Einstein and Reynolds, involving pressure and temperature gradients between the two sides of each vane of the propeller. |
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