What causes a radiometer to spin in bright light?

[Tranceform]

Homework Statement

The photograph shows a device called a radiometer. The four regular panels are black on one side and shiny like a mirror on the other side. In bright light, the panel arrangement spins around in a direction from the black side of a panel toward the shiny side. Do photon collisions with both sides of the panels cause the observed spinning?

Homework Equations

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The Attempt at a Solution

I thought a radiometer could only spin by shining light (photons) on it. And since the shiny side reflects more light, light shining on this side would make it spin in a direction from shiny towards the black side. But here it says it spins from the black panel towards the shiny side? How is that possible? What is it then that causes this spin?

 

[NascentOxygen]

The photograph shows a device called a radiometer. The four regular panels are black on one side and shiny like a mirror on the other side. In bright light, the panel arrangement spins around in a direction from the black side of a panel toward the shiny side. Do photon collisions with both sides of the panels cause the observed spinning?

Fixed. Somehow you didn't have the appropriate image URL.

Yes, equal numbers of photons are hitting both colours. So you think the photons must be pushing harder on one side than the other?

 

[HallsofIvy]

The radiometer does NOT spin because of photons hitting it. You are right that if that were true, the bright side, where the photons bounce off, so transmit twice the momentum as photons "sticking to" to dark side, the rotation should be the reverse. What causes the rotation is that the black side heats the air around it more than the bright side, creating convection currents.

 

[NascentOxygen]

The radiometer does NOT spin because of photons hitting it. You are right that if that were true, the bright side, where the photons bounce off, so transmit twice the momentum as photons "sticking to" to dark side, the rotation should be the reverse. What causes the rotation is that the black side heats the air around it more than the bright side, creating convection currents.

This is a homework question. Forum rules say providing poster with the answer is not allowed.

 

[Tranceform]

Yes, equal numbers of photons are hitting both colours. So you think the photons must be pushing harder on one side than the other?

Yes. Well, I have never seen a radiometer IRL, so from what direction is the light shining on it? If it's put in such a direction that equally many photons hit both shiny and black side, I would assume it would rotate from shiny to black.

The radiometer does NOT spin because of photons hitting it. You are right that if that were true, the bright side, where the photons bounce off, so transmit twice the momentum as photons "sticking to" to dark side, the rotation should be the reverse. What causes the rotation is that the black side heats the air around it more than the bright side, creating convection currents.

Ok, that makes sense. But strangely in my book that is not explained anywhere, actually the word "radiometer" is not even mentioned in the book before it suddenly appeared in this question. The text in the book just before this question came up was about solar sails which seem to work by radiation pressure (i.e. photons bouncing on a shiny surface thereby causing momentum in the other direction) so one would assume a radiometer would be somehow related to that.

So if I understood correctly, a radiometer has nothing to do with radiation pressure? It's only about the black side getting warmer and the resulting transfer of heat?

This is a homework question. Forum rules say providing poster with the answer is not allowed.

Actually you don't need to worry. The point of me asking this here on the forum is not to "simply getting the answer", since that is already written in the end of my book! The point of me asking here is learning and understanding, in order for me to - not only - answer the question at hand, but also understand the ideas and concepts around it.

 

[NascentOxygen]

So if I understood correctly, a radiometer has nothing to do with radiation pressure? It's only about the black side getting warmer and the resulting transfer of heat?

No, that's not the whole story at all. IIRC, in order to explain the outcome you need to know whether the bulb is evacuated (to a reasonable vacuum) or not.

BTW, your description of rotation as being "from black to shiny" is ambiguous and therefore conveys nothing. Try CW and CCW when viewed from above.

 

[vela]

You should read http://math.ucr.edu/home/baez/physics/General/LightMill/light-mill.html. It's pretty interesting. I never knew that's how a radiometer worked.