How does a Solar Radiometer work?

22 June 2008

Part of the show How do Atoms make Colours?

Crookes_radiometer.jpg

Credit:

Question

My father who’s now 90 has a very peculiar little object. He’s an ex-electrician and likes gadgets. He’s got this little object on his window facing the sun. It consists of a glass tower, almost the shape of the Eiffel Tower, in the centre of which is a glass bulb about 5cm diameter with a tip on the top of it. Suspended in this bulb is a set of vanes, four, in diamond shape in pattern. They attach to a glass rod which seems to be sitting without touching two glass bearings. When the sun hits them one side of each of these four blades seems to be coated with a material and it spins rapidly.

Answer

It's called a solar radiometer, it turns in a circle and it's amazing to think this thing can turn just by sunlight shining on it. There's no motor in there. It's just literally balanced on this tiny needle point and it spins round in the sun. How does it do it? If you look closely at those vanes, at those panels you'll see that they have a light side and a dark side. One side is soaking up the light, the other is reflecting it. This is the best evidence there really is that light can be a particle and a wave. Light can impact a punch or a kick when it hits something and it can push it along. This is literally the light pushing this thing along. People are talking about building solar sails so you can make a craft, send it up to space, light will bounce off it and you'll get a very tiny push by each photon of light bouncing and pushing it back

An alternative explaination is that the Crooke's radiometer which you've got works in a slightly different way. They've got two sides - one side shiny and one side's black. If the sunlight hits the black side it's going to heat up more than when it hits the shiny side. There's a very low pressure gas inside the radiometer. If the low pressure gas is near the hot side then it's going to get hot and expand and get pushed away and therefore push the radiometer round a bit. If it hits the shiny side it's not going to be nearly as hot. It doesn't get nearly as much kick so the black side gets pushed back and the shiny side gets pushed forward and it spins round.