Crookes' light mill

When you switch on the source of light and direct it on the mill it starts to turn. Unfortunately, even scientists frequently cannot guess in which direction.

White surfaces of the plates reflect light, while the black ones absorb light. So, the temperature of the black sides of the plates is higher than the white ones. The rise of temperature means the higher velocity of gas molecules. When the gas molecules bounce from the black surface they do it with higher velocity than when they bounce from the white one. When they bounce from a warmer surface they pass on greater momentum so they exert more pressure on black surface than on the white one.

Even more experienced scientists are prone to think that the plates turn as a result of light's pressure (which exists). In that case, however, a deep study of this experiment should show that the plates turn in opposite direction to which it does in our experiment.