How does a radiometer work?

Answer 1

Whenever a radiometer is advertised in a catalog, it is said to be contained in a partial vacuum. This deceives the shopper into thinking the radiometer is in a vacuum, and there is no air.

Because of this, the movement of a radiometer is often wrongly attributed to the momentum of light particles (photons), but not the movement of air. The shiny (or white) surface reflects most incoming photons, and gains twice the photon momentum of the black surface, which absorbs most photons, and gains their momentum only once. In a vacuum, this would result in a rotor movement in the direction of the black surface. In other words, the radiometer rotates so that the leading surface is the black side.

A look at your radiometer reveals that the opposite is true. In fact, this movement is caused by the black surface absorbing photon energy and heating the air near it. Hot air molecules move faster than cold ones and, by bumping onto the surface, some transmit their momentum to the black sides of the rotor. The force of the photon momentum is very small compared to that of the hot air molecules, thus the rotor moves towards the cooler, shiny (or white) surface.