The Crookes Radiometer, Part 1

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!

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7 Responses to The Crookes Radiometer, Part 1

  1. Athena!!! says:

    Could it be that with the double amount of pressure from light on the white side caused the white side to in a way move faster creating the black side to appear moving backwards? Since the black side has only half the amount of pressure from light than the white side, from absorbing the light, it doesn’t have much of this “momentum” created by the light when being absorbed into the black as opposed to the white where the force just continues by bouncing off the white? —I havnt drawn out the diagram or googled what it looks like so I may be totally off! But this was still a fun way to start out my morning brain juice flow! Thanks Dan :)

    • Athena, you’ve correctly thought out and stated what is supposed to happen! The white side should have twice the pressure, making it look like the black side is going backwards and the white side forwards (what I mean by saying the white side trailing the black side). But in fact, it goes the other way, with the white side seemingly going backwards! The first link in the article is a movie on wikipedia showing you the motion, and may help you visualize the problem. Keep thinking! ;)

  2. Alex says:

    What about the small amount of air molecules bouncing around in the near vacuum? Are they heated up on the black side, and so bouncing off of that surface more frequently? Just an initial guess here.

    Here’s another question – what happens when this radiometer is not inside a vacuum?

    • You are very close Alex, but not quite there! It is true that the black side heats up, and so gas particles bounce off with more energy on the black side, imparting a force. This was one theory pushed forth by some scientists at the time. However, the particles also can stop other particles from reaching the vanes (collisionally) and so this net force gets balanced out (this is why this explanation was rejected at the time).

      I’m not sure what happens in normal air, that’s a good question. However, in a much better vacuum, the vanes will not move at all.

      • Alex says:

        hmmm…. I was thinking the temperature difference would cause a difference in pressure on either side of the vane driving the contraption to move in a particular direction.

        I’ll have to keep thinking about this one.

  3. Katie says:

    Why is it that the flags are white and black and not with other colors. Such as yellow and green

    • Good question! It’s all about how the experiment was designed–when it was originally made, Crookes wanted to test the fact that light reflection was stronger than absorption. When you have a white surface, it’s totally (or almost totally) reflective, while black surfaces are total absorbers. This provides a much better contrast for an experiment than yellow or green surfaces, which are partial reflective and partial absorptive (the light reflected will be interpreted by your eyes as the color of the surface, the rest is absorbed).

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