Lamp and spinning fan
Q. In DR we saw a little fan in a glass bulb. When we turned on a light shining on the fan it began to spin. Why?
A. Because the light from the lamp is not in direct contact with the fan (there is no conduction) and there is no air that moves from the light to the fan since the fan is enclosed (there is no convection) the only method of heat transfer from the lamp to the fan is radiation. The radiation from the lamp creates some radiation pressure on the blades of the fan, but this effect is very small and negligible. More notably, the radiation heats up the blades of the fan. The black side of the blades is a better absorber than the white side. Because the black side is absorbing more heat it is also radiating more heat. Thus, the air near the black side of the fan is heated up more than the air near the white side of the fan. Hot air means that the molecules have more energy and bounce around more. This random motion means that the hot air has higher pressure; it wants to expand. The high pressure region near the black side of the fan blade pushes on the blade. This causes the fan to spin.