Why It Goes

Day 143: I hadn't really thought to make a series of this, but since yesterday was "Who," today I bring you "Why," as in "Why do the vanes in a radiometer spin when it is exposed to light?"

The widespread notion that the rotor is impelled as photons strike the white sides and are absorbed by the black is not scientifically accurate. Were it so, the spin would always be in one direction: away from the black faces. Stick a radiometer in a sunny window until the rotor is spinning rapidly and then move it to a cooler bright location; you will soon prove that this is not the case, although some pressure is definitely supplied by radiation. In actuality, the amount of vacuum within the radiometer is critical, and it is the transfer of heat in this partial vacuum which supplies the action. As the vanes heat, the radiometer spins in one direction, and as it cools, the spin reverses. Try it! These cool science toys are readily available on the internet and at the very least, will make your guests ask why you have a non-lighting lightbulb in your window.