The Crookes radiometer is a vintage instrument that reacts to light, more specifically to infrared or heat radiation. The inside of the bulb contains a low pressure gas. Each vane has a black and a white side. Radiation is easily absorbed by the darker surface, it becomes warmer so the gas molecules that hit it are reflected with more momentum than the molecules that hit the white side. This generates a net force on the dark side and makes the propeller turn.