Happy October, friends! I apologize for my lack of posts recently…graduate school has started for me, and it’s been very tough for me to find any free time. I’m going to attempt to quickly write up this post about an awesome apparatus that puzzled many great minds (including my own)—the Crookes Radiometer.
A Crookes radiometer, known to some as a light mill, is a pretty low-tech apparatus consisting of a light bulb with a partial vacuum inside (very few air particles inside the bulb compared to normal air we breathe) and a bunch of vanes attached to a spindle. It was designed by Sir William Crookes in 1873—he was doing chemistry experiments in a partial vacuum and noticed the effect he later built this apparatus to measure. That guy had an awesome beard. Despite the age of this contraption, you can still easily find one for your own (typically these are just items used for novelty and to stimulate the minds of young graduate students like me).
To understand what the radiometer is supposed to measure, we must examine some properties of light. Light has always been, and still is, a confusing phenomenon. Scientists don’t really understand what light is…sometimes it acts like a wave, sometimes it acts like a particle. This has led to what we call the “wave-particle” duality of light, and it’s really just a representation of the fact that we have more than one model of how light works, and they both explain certain phenomena and can’t explain others. Indeed, this concept is so important that it helped pave the way for the formulation of quantum mechanics. The specific property of light I want to tell you about is light radiation pressure.
Imagine holding a piece of black construction paper in mid-air (you’re so awesome that you can hold it completely still), and shining light on it. Remember that if something is black, it is absorbing light that hits it, and so you see an absence of color. So here you have light shining on the construction paper, and the light is completely absorbed. Because light has momentum associated with it (like when you are running down a hill and have a hard time slowing down), the absorption creates a force over the area of the paper being illuminated (i.e. a pressure!). This is called radiation pressure.
Now imagine you’re holding white construction paper instead of black. Here, the paper is reflecting the light off of it, rather than absorbing. In this case, light also creates a radiation pressure, but due to how we’ve observed (and modelled) how light works, we know it creates twice as much pressure when reflecting than it does when being absorbed!
So now, armed with knowledge, let’s go back to our radiometer. The vanes on the spindle have two sides, one black, one white. Crookes observed this effect and wanted to measure it—he wanted to shine light (in his day, with the sun, in ours, perhaps with a flashlight or laser) on the vanes and see how they moved. The white side should reflect the light, and the black side absorb it…so the spindle should spin, with the white side trailing the black. But he instantly noticed a problem…the black side was trailing the white…it moves backwards!
Don’t believe me? Try it out for yourself. The great physicists of the time pondered this puzzle, and it took some time for them to figure out the answer. I will explain the answer to this conundrum in my next entry…for now, I challenge you all to think about this puzzle and try to come up with an answer!