Crookes radiometer

A Crookes radiometer, also called a light mill, is a special type of toy or instrument that uses light to spin a wheel. The radiometer is made up of four thin, black vanes connected to a metal axis. The vanes are fitted into a glass bulb that has been pumped free of air, so it is a vacuum. When light falls on the vanes, they get hot on the side facing the light, and cool on the side facing away from the light. This causes air molecules to push the vanes away from the light, making them spin like a windmill. The inside of the radiometer is too empty to feel the air molecules, so the movement you see looks like magic!