The Radiometer consists of 4 vanes on a spindle that are black on one side and white on the other. When light rays strike the vanes, the black sides absorb the rays far better than the white sides, which reflect the rays.
This causes the black sides of the vane to heat up and as the air around them warms, the excited molecules of air exert pressure on the dark side. Since there is a partial vacuum of air inside the glass bulb, the vanes can spin very easily so they start to rotate as they are exposed to light. The brighter the light, the faster the vanes will rotate to a point where they will spin up to 3,000 RPM.
