This hand made in Germany, Solor Radiometer, also known as a light windmill is an instructive physical object for demonstrating a theory which was invented by the English physicist Sir William Crookes in 1873. When warm light hits the wings, they start to turn. Inside the glass bowl there is a partial vacuum produced by the air being tinned in order to keep power of the solor energy. The black surfaces of the wings absorb more light than the silver surfaces and that creates more pressure on the black surfaces. The more the molecules in the bowl heat up, the more steadily the wings turn.