Department of Physics and Materials Science

Radiometer

The Physics Behind It

This bulb is called a Crookes radiometer. It’s in partial vacuum, meaning the air inside the bulb is at a lower pressure than the air outside. As you noticed, shining a light on it makes the vanes rotate. As simple as this is, physicists couldn’t figure out what caused the rotation for years! Initial explanations suggested that the vanes rotate because light carries momentum, some of which is transferred to the vanes when the light reaches them. This does happen, but if it were the root cause, the vanes would rotate the other way, and much more slowly.

Instead, note the white and black sides of each vane (see the figure on the left). When light shines on them, the black sides absorb more light than the white sides and heat up faster. This temperature difference causes air currents to flow along the edges of each vane from the white side to the black side. As the air flows over the vanes, it pushes back on them, causing rotation.