Observation on a Radiometer


by Simon Bridge
Tags: observation, radiometer
Simon Bridge
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Jan11-14, 09:33 PM
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Off a g+ post:
https://plus.google.com/+PeterTerren/posts/Weo9gg8F5d9

Quote Quote by Peter Terrin
The Crookes tube has vanes that turn rapidly in sunlight. Lesser lights such a 5mW green laser her also works. Also a 3 and a 40mW violet laser work well. What don't work are a 1mW red diode and 10mW red HeNe laser.
So why doesn't the stronger red laser work?
(I haven't measured the power on calibrated equipment). This doesn't make sense from a heat driven action point of view. Any ideas from the physics guys.
The device pictured (see link) is turning away from the dark side so thermal transpiration dominates.

Data is a little hazy, ill-defined terms, and there is poor control of variables.
But I'd put it down to the the laser spectra and the absorption spectra of the surfaces.

Also reference:
How does a light mill work by Philip Gibbs July 1996 (Usernet Physics FAQ)

Note: Terrin is one of those happy individuals who loves to "play" with science equipment for entertainment but does not automatically assume some groundbreaking discovery whenever something happens against his expectations.
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Jan12-14, 08:34 AM
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The laser hits a single side, so radiation pressure and thermal expansion work in the same direction.

The absorption spectrum of the surface is certainly a possible explanation, the absorption spectrum of the glass could be interesting, too.
Alternatively: can we be sure the 10mW-laser is really emitting 10mW?

I can block the green from the green laser with an IR transmission filter and it still works to turn the vanes. No visible green passes at all.
That is another interesting observation.
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Jan12-14, 07:35 PM
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The laser hits a single side, so radiation pressure and thermal expansion work in the same direction.
... nice observation: can suggest pointing the laser at the other side, see if radiation presure dominates - however:

I can block the green from the green laser with an IR transmission filter and it still works to turn the vanes. No visible green passes at all.
The green laser must be emitting an IR side-band.
Looks like radiation pressure may be irrelevant.

Alternatively: can we be sure the 10mW-laser is really emitting 10mW?
No.
Even if that is what the manufacturer wrote on the tube, and it is still accurate, it may be the power consumption rather than the output... or it may be the output in the specified bands rather than the total output or anything.

the absorption spectrum of the glass could be interesting, too.
Good point - had not considered the glass. I bet lots of people have that blind-spot.
I've heard that a lot of glass absorbs infra-red well ... on the face of it, that would seem a bad kind of glass to make a radiometer out of. OTOH: the glass would heat up and emit it's own infrared (etc) spectrum...

... that sounds like it could just work as an explanation: the radiation pressure makes little difference and the thermal pressure needs IR, which it gets from the glass not the laser ... the different sources heat the glass differently?

... what else am I missing?

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Jan13-14, 01:03 PM
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Observation on a Radiometer


Direct heating is much more efficient and should work with visible light (as the surface is black), and apparently it absorbs at least some IR as well.
Heating via the glass tube is possible, but the glass will emit in all directions, only a small fraction hits the radiometer.


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