Pushing stuff with light: how Crookes radiometers work

Yesterday, I bought a Crookes radiometer.  And proceeded to play with it. This post has come out of people asking me ‘how does that work?’ A Crookes radiometer is a simple little device, generally now produced as a novelty item.  It allows the quantitative (i.e. wif all numbers in) measurement of the intensity of electromagnetic radiation: heat, light, etc. It’s made of a glass bulb containing a partial vacuum, a low-friction spindle and some lightweight [...]