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Pushing stuff with light: how Crookes radiometers work

April 26, 2011 - Posted in Science and Society , Technology Posted by:

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Yesterday, I bought a Crookes radiometer.  And proceeded to play with it.

crookes radiometer

This post has come out of people asking me ‘how does that work?’

A Crookes radiometer is a simple little device, generally now produced as a novelty item.  It allows the quantitative (i.e. wif all numbers in) measurement of the intensity of electromagnetic radiation: heat, light, etc.

It’s made of a glass bulb containing a partial vacuum, a low-friction spindle and some lightweight metal vanes, spaced evenly and with one side reflective/white, and the other side dark.  The partial vacuum is because some small amount of air is necessary to allow the currents which push the vanes, but lots means there’s too much for the vanes to push against.

And, when you put it in sunlight, bounce frikkin’ laser beams off it, point IR at it or even put it near a heat source, those vanes spin (black side trailing/moving away from light).  How fast they spin is due to the intensity of the radiation.  Also, they spin backwards if you cool the radiometer, which is kinda neat :)  And makes me want to put things in freezers.

Now, to answer the question of how it works:

There are two forces involved – one shown by a German, and one an Irishman

First, the German.  Einstein, in fact. It had been proposed that the warmer dark side of the vanes warmed up gas molecules coming into contact with them slightly more than the light side.  This then increased the speed at which the molecules bounced off the vanes, in turn exerting a tiny net ‘push’ on the dark side.

However, these faster gas molecules would also impede other such beasties from coming into contact with the darker side of the vanes; in theory, therefore, the two forces would cancel each other out.  It took Einstein to show that this was not, in fact, the case at the edges of the vanes because of the temperature difference there.

BUT.  This didn’t quite explain the speeds seen – more was needed.

And it was provided by one Osborne Reynolds*, who was a big man in fluid dynamics**.  He found that if a porous (i.e. wif all holes in) plate was kept hotter on one side than the other, gas molecules would flow through the holes, from the cooler side to the hotter side***.  Kinda like people flowing from a couch onto a dancefloor.  Maybe.

ANYWAY, while the vanes in a Crookes radiometer aren’t porous, the space around their edges behaves like pores, with gas flowing from the cooler (light) to the warmer (dark) side.  This causes a pressure difference****, pushing the vanes forward.

Anything else that you may read in booklets/museums/anywhere else is incorrect.  It’s NOT light pressure pushing on the vanes; nor is it the dark sides outgassing.

So, both of these forces are in play, although apparently, we’re not sure which (if either) has the greater effect.

And that, dear friends, is how it works.  Hooray for science :)

End note: a friend of mine also has one, but on his,  each vane has a single, large black dot on the light side. Apparently there is still debate as to which design is better.


* However, the research was actually published by one of my favourite Scots of all time: James Clerk Maxwell.  Not only is he a pivotal figure in electromagnetism as well as a bunch of other stuff, he also liked to write rather clever (but still silly) poetry.

From a Tennysonian parody, the first stanza:

The lamp-light falls on blackened walls, And streams through narrow perforations,
The long beam trails o’er pasteboard scales,
With slow-decaying oscillations.
Flow, current, flow, set the quick light-spot flying, Flow current, answer light-spot, flashing, quivering, dying,

The poem’s called Lectures to Women on Physical Science (not quite sure I agree with some of the sentiment expressed here, but yes :P)

More of his poetry here – some of it’s rather beautiful

** Posts of mine involving fluids: oscillating animals and teapots.  And that’s _not_ counting the ones about alcohol…

*** Remember, kids – temperature is simply a measurement of how fast particles are moving.

**** Ahem.  To be more accurate – this happens when the pressure ratio is less than the square root of the absolute temperature ratio.

  • aimee whitcroft

    If anyone is interested in the development of laser-bearing sharks (or people), please contact me at benevolentworldruler (at) gmail.com

  • http://sethop.com Seth Wagoner

    Hah! Somehow not surprised to see PC magazine has by far the best article of the three ;-) Now, as they point out, they’ve so far only managed a laser somewhat smaller than a human hair, probably so pissweak it wouldn’t require a Class 1 cert to sell on Trademe, but well, lets remember the Apollo Rockets had less computing power than my wristwatch, apply Moore’s law, accelerating returns in nanotech and biophysics, plus the brainpower of a million nerds who’ve watched Austin Powers, some of whom have access to biotech labs…I’d say we’ve got less than a decade before we have to start worrying about these things in our shipping lanes ;-)

  • http://sethop.com Seth Wagoner

    Or me, at evilgeniusesinternational (at) gmail.com if you’re looking for some bizdev assistance with that global shipping extortion / cyber-shark-pirate start-up you’ve just started brainstorming. Pretty sure I know where to look for some VC…

  • aimee whitcroft

    Yes, I’m interested more in a leadership-level role. My maniacal laughing skills are superb. And I have documentation assuring people I would be a nice world leader (seriously, I do). Seth – perhaps a collaboration?

  • http://sethop.com Seth Wagoner

    Most excellent. We’ve always got room for people like you in our organization. We actually rather appreciate the “benevolent” in your email address now we know it’s a suitably Orwellian kind of “benevolence” ;-)

  • aimee whitcroft
  • http://sciblogs.co.nz/code-for-life/ Grant Jacobs

    Lets see, we already have glowing pet fish. The fish tank may yet become an ugly scene…

    Ya get ya frikken scaly bod outa my feed. Pzzzzip.

  • http://sci.waikato.ac.nz/bioblog alison

    Follows link…..
    …. oh, OK, good testimonials, I can see that you will definitely be a most benevolent overlord (should that be overmistress???).

    /replaces hat so zombies don’t get the rest of der braiiinnnzzzzz.

  • http://sethop.com Seth Wagoner

    Oh my. You *do* come highly recommended. We are well familiar with Dr Maccio’s work and his fine team at Fake Science Laboratories. Perhaps as your assistant director for Appeasment he could be set to some useful exploratory tasks in this nacent joint venture of ours?

  • Ross

    You mean Shiva might be…….reincarnated……ahem….


  • http://sethop.com Seth Wagoner

    I think the first proof of concept my investors would like to see would involve scaling up the existing experiment. If we can get to say, 5 watts in vitro, they might begin to see the possibilities. Even cybersharks that merely act as rangefinders for shipboard artillery systems should be sufficient as a minimum viable product. The more significant challenge is probably the control mechanism. I am thinking a modified cybug – http://www.msnbc.msn.com/id/31906641/ns/technology_and_science-science/t/military-developing-robot-insect-cyborgs/ – architecture would probably give us shorter time-to-market than waiting for neurophotonic mind-control APIs ( http://www.ted.com/talks/ed_boyden.html ).

  • http://gasstationwithoutpumps.wordpress.com gasstationwithoutpumps

    The cells did not lase, they simply contained the dye. External pumping and mirrors were needed. This article was a pure puff piece, typical of Nature journals, who go for flash before substance frequently.

  • http://sethop.com Seth Wagoner

    They also *produced* the dye, which is a bit more interesting than just acting as a vessel. However, it’s obvious there’s some way to go before Dr Evil’s great vision is finally implemented. In fact, I’m pretty sure it’s decades away, maybe centuries. You’d have to be completely mad to start thinking about how to engineer Cyclopean sharks before we get the initial patents filed at this very early stage.

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