The Crookes radiometer: -
A lifelong misapprehension enlightened
19/06/2015
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Today at work in a quiet moment as I checked out today's Astronomy Picture of The Day, a young colleague asked me what the picture was.
I explained it was a light sail, something used to move a spacecraft by the pressure of light. He was reluctant to accept that this was possible, saying the sail could not work because light can't push anything. I replied "Yes it can! Hang on a sec, I will show you something." What had sprung to mind was a device I remembered purely from an illustration from my education in the Sixties, a glass bulb with a vacuum inside, containing some small and very light vanes on a thin shaft which turned when light was shone on it. Certain this would be a convincing demonstration, I quickly opened a YouTube search for "light mill". Sure enough, up came a number of postings showing the remembered device in action. It turned out to be called the Crookes Radiometer. (Because it was invented in1873 by the chemist Sir William Crookes, and it meters light radiation). The first post I clicked on is shown at Left. I didn't have the sound on, being as it is a workplace, but before my colleague had to move on, I just managed to remark something about "See, it is very light and the pressure of the light makes it go round." |
A bit later, when I could discreetly use my earplugs, I thought I should get the facts on how it works, because I was wondering why one side of the vanes was black, and the other reflective. It occurred to me that either the photons would be reflected by the shiny side, which must exert a force bouncing off the vane, or be absorbed by the dark side, which would also amount to absorbing energy from the photons. So where was the turning force coming from?
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I watched the clip at left, and to my horror found I was completely mistaken in my conception that this was a device powered by photon pressure, and in my memory that the glass contained a vacuum. |
While it is true that light makes the vanes turn, it is not by photon pressure, but by heat-driven gas displacement. To work, the device cannot be completely evacuated, but must contain some gas molecules which can be heated by conduction from the dark, photon-absorbing side. In a manner which is not clear to me, this temperature difference between the sides causes a flow of gas from the cooler side to the warmer side. This gas movement causes the vane to move in the opposite direction, cooler side leading. See if you can understand the page at Wikipedia: -Crookes radiometer.
I felt compelled to rush off and explain this discovery of my error to my colleague, while still asserting that this did not invalidate the reality of photon pressure. However, as I find is often the case, not everyone is as concerned to to be correct as I am, and the point was not really appreciated. In the same vein, I also note from Wikipedia that Crookes radiometers "are often used in science museums to illustrate "radiation pressure" – a scientific principle that they do not in fact demonstrate."
I must remain content to have once again learned the lesson that human memory is far from perfect, and must be checked, especially when applied to subjects 50 years or so in the past!
I felt compelled to rush off and explain this discovery of my error to my colleague, while still asserting that this did not invalidate the reality of photon pressure. However, as I find is often the case, not everyone is as concerned to to be correct as I am, and the point was not really appreciated. In the same vein, I also note from Wikipedia that Crookes radiometers "are often used in science museums to illustrate "radiation pressure" – a scientific principle that they do not in fact demonstrate."
I must remain content to have once again learned the lesson that human memory is far from perfect, and must be checked, especially when applied to subjects 50 years or so in the past!
p.s. Below is a more 'techo' but drier YouTube video more to my taste, which still glosses over the exact nature of the temperature-difference driven gas movement.
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Update 20/01/16
"Physics Girl", my favourite Science Vlogger, posts the below item on Solar Sails, which mentions the Planetary Society's Lightsail-1, whose prototype was Lightsail-A above. |
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