This week I received a package from the USA, containing my own Crookes radiometer (only a few bucks at ThinkGeek.com (Yes, I am a geek, but the good well developed kind ;) )). This device is a small mill with black and white vanes in a vacuum pumped light bulb which turns around when you put it in sunlight (any light works!). It looks like this (pictures speak a thousand words, well in my case a few dozen)
My physics teacher used this device to explain that light particles have momentum (he should have known better), and due to the different reaction with the white and black colored vanes, the mill would turn. During my whole life, I was really sure about that this explanation was the effect causing the light-mill in vacuum to turn. Until this week.
My old master supervisor (not that he is old, but I am no longer a master student, he is my colleague now) entered my office room with a cup of coffee (this is of course essential for the story) and saw the Crookes radiometer. After making a comment about me doing Disneyland physics, he pointed at the radiometer and asked: “Do you know why the vanes of the radiometer turn?”. During my master we always had good discussions about physics, which I always lost, but now I could tell him that I knew. “Wrong”, was his response, “Just calculated the acceleration of the vanes due to the light particles” and he stepped out of my office, leaving me behind in total confusion. He always did this during my master research, instead of explaining the principle, he encouraged myself to come up with the answer (I know, this is more pedagogic correct).
As a good scientist does, I consulted the encyclopedia, or in this era of technology, Wikipedia. The Crookes radiometer page has an excellent documentation about this device. It is NOT radiation pressure due to electromagnetic radiation (fancy words for light, heat and other things you can not see), which is far to small to cause the huge (well, I think they are tiny) vanes to rotate. They give four historical explanations of which two are to small to explain the movement. The other two have to be combined to fully explain the motion. This is real science!!! Initial theories are disputed by better ones, and even in the end there is not one explanation but multiple.
Ok, but what are the effects that cause the radiometer to turn. Even Einstein thought this was a great exercise and did some calculations (well if Einstein worked on it, the problem should be very complex). It sort of is. Air molecules in the near-vacuum (vacuum conditions can not be obtained, this will even stop the radiometer from turning) environment exchanging different amounts of momentum with the vanes due to different temperatures. Just read it here. In the end light has something to do with the motion, it is however not the pressure of the photons.