radiometer

[einsteinsdog]

How does a radiometer work?

[Space]

I assume you're talking about the ones that look like light bulbs with little silver and black vanes inside, spinning on a needle...

Sunlight warms the dark side of each vane more than the light side. Gas molecules bounce off the hotter, black side with greater velocity than off the colder, white side. This gives the vanes a little thrust on the black side, that is why they spin.

[mibn2cweus]

In a vacuum the vanes spin in the opposite direction. This is due to absorption of photons on the flat-black side and photons bouncing off the shiny side.

[einsteinsdog]

So presumably if it was possible to control the pressure in it you could make the vane change direction at will