Re: radiometer paradox

[Pete Lohstreter <petel@tenet.edu>]

At 06:38 PM 6/13/97 CDT, you wrote:
> I am puzzled by how a radiometer works.  It makes sense to me that 
> when you shine light on the black fin that radiation pressure would 
> cause the radiometer to spin.  However, why doesn't it spin even 
> faster when you shine light on the white fin?  In fact, it doesn't 
> spin at all!   If  I think of 
> reflection as a photon of light bouncing off of the white 
> fin, then the change in momentum of the photon would be twice as 
> great as if the photon were absorbed.  
Ahhh....But it's not the photons that are doing the work here.  This is
another mis-conception that I have tried to squash for many years now.

The inside of the glass container is only at a partial vacuum.  The radiant
energy on the black surface heats up the gas inside more than near the white
surface.  The increased kinetic energy of the molecules near the black
surface impart a larger 'kick' and push the black one away.

I have not tried it, but, i'm told that if a millitorr vacuum is drawn on
the glass, it won't work at all.

Pete