It is a great example of the energy carried by light, and how that energy can be converted into heat
The radiometer is a light-bulb shaped glass device with a little weather vane in the middle of it. When you put it in the light, sunlight or lamp, it starts to spin, turning the light energy into mechanical energy.
Originally developed in the mid-nineteenth century by Sir William Crookes, the radiometer was developed to measure the intensity of radiant solar energy.(1875)
Inside the glass bulb is a near perfect vacuum with nearly 99% of the air removed. This makes the air molecules move about more easily. The different colors on either side of the "vane blades" create the convection currents and momentum that causes the blades to spin really fast.
Each vane has one side polished and the other blackened; the vanes are arranged so that the polished side of one faces the blackened side of the next. When radiant energy strikes the polished surfaces, most of it is reflected away, but when it strikes the blackened surfaces, most of it is absorbed, raising the temperature of the surfaces. The air near a blackened surface thus becomes hotter, exerts a greater pressure on the blackened surface, and causes the rotor to turn. The rate of rotation provides an indication of the intensity of the radiation.
This radiometer makes a really cool educational science gift for ages 8 and up
Light energy becomes mechanical energy plus heat energy. Just put your radiometer in the sun, by a lamp or by many other light sources and watch it spin faster and faster - how fast it goes depends on the intensity of the light.
Below original radiometer from the Science Museum in London,U.K.
An original double radiometer (on the right) and an original Maltese Cross tube, a well-known experimental cathode-ray tube devised by Crookes.