In this activity, students observe solar energy being transferred into thermal and mechanical energy using a radiometer.
When light rays hit the vanes of a radiometer, the black sides of the vanes absorb the rays better than the white sides. This causes the black side to become hotter than the white side (thermal energy). When molecules in the air hit the vanes heat energy is transferred to them. The molecules that hit the black side gain more energy and hence recoil with greater force than those that hit the white side, causing the vanes to spin (kinetic energy).
The solar energy is changed first to thermal energy, and then to kinetic energy.
A partial vacuum inside the radiometer glass allows the vanes to spin easily. The brighter the light, the more heat energy is transferred, and faster the vanes will rotate.