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.

The Crookes Light Mill (radiometer) consists of a glass bulb filled with low pressure gas. Kinetic theory of gases offers the following explanation of the radiometric effect that can be observed in the mill: the average kinetic energy of a molecule increases with rising temperature. When molecules of gas hit the black sides of the vanes they bounce off with greater velocity then when they hit the white sides. Due to the difference between the momentum deposited by molecules on black and white side of each vane respectively, pushing force is created.

Radiometric effect does not appear at very low pressure (sufficient quantity of molecules is needed) or at higher pressure (e.g. atmospheric pressure). It is because at higher pressure molecules hitting against the 'hot' side of a vane lose some of their energy (and momentum) when colliding with 'cooler' molecules and consequently pushing force is weaker, in spite of the fact that more molecules participate in the process of depositing energy and momentum.

The pushing force of photons is far too weak to set the mill in spinning motion. The pushing force of solar photons (1000 W/m² of power) is only 9 N per square km.