Re: How does a radiometer work?

Dear Laura,

The movement of a radiometer is often wrongly attributed to the momentum of light particles (photons). The shiny surface gains, by reflecting incoming photons, twice the photon momentum, whereas the black surface, by absorbing, gains it only once. This would result in a rotor movement in the direction of the black surface.

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 gas near it. Hot gas molecules move faster than cold ones and, by bumping onto the surface, some transmit their momentum to the rotor. The force of the photon momentum is very small compared to that of the hot gas molecules, thus the rotor moves towards the cooler shiny surface.

Both particles and waves can and do have momentum. There is no distinction between particles and waves in quantum physics.

Greetings from Indonesia,

Frank Berauer