How do radiometers work? Well, the bulb in which the blades spin is a partial vacuum. Partial being the tricky part - another clever scientist by the name of Lebedev noticed that the effect disappeared in a hard vacuum. Curious, yes? The light side of the blades are slightly warmer than the internal air temperature but cooler than the black side. Basically, the principal is the air that hangs out by the cool side of the blade flows slowly to the warm side of the blade. That process is called thermal transpiration. Science is cool.
This Radiant Light Mill looks awesome sitting on your desk or windowsill. It works best in direct sunlight, but moves pretty well even hit with a flashlight. Of course, in our own highly scientific testing here at ThinkGeek World Domination HQ we discovered that a laser, while intense, was too focused to significantly move the vanes very much at all. Now what if we told you that sunlight, artificial light, and even infrared radiation can cause the blades to spin? What other sources can you come up with to create thermal transpiration? And what happens when you cool it?