Re: radiometer paradox

["A. R. Marlow" <marlow@beta.loyno.edu>]

On Fri, 13 Jun 1997, Joseph Gress 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.  
>
> I looked up radiation pressure in several text books.  The closest 
> example indicated that radiation pressure = the light irradiance 
> (watts per m^2) /  speed of light; i.e.  P=I/c if the light is 
> completely absorbed whereas P=2I/c if the light is reflected.  This 
> matches my simple picture of what I thought was happening when light 
> hits the radiometer fins.
>
> So, why doesn't light shining on the white fin (which is reflected) cause it to spin 
> faster than light shining on the black fin (which is absorbed).
>
> Joseph Gress
> **************************************
>
> Sincerely,
> Joseph Gress
> Department of Physics and Astronomy
> Moorhead State University
> Moorhead, MN 56563
> e-mail: gress@mhdha.moorhead.msus.edu
As I recall the explanation, radiation pressure is not the driving factor
in the motion of the radiometer  --  instead, it is swamped by the effects
of the unequal heating of the two sides of the blades.  The temperature of
the black sides of the blades is raised more by absorption of radiation
than is that of the shiny reflecting sides.  The black sides then
contribute more kinetic energy and momentum to adjacent gas molecules than
do the shiny sides, and it is the reaction to this interaction with gas 
molecules that drives the motion of the radiometer.

A. R. Marlow                          E-MAIL:  marlow@beta.loyno.edu
Department of Physics, Box 124        PHONE:   (504) 865 3647  (Office)  
Loyola University                                    865 2245  (Home)
New Orleans, LA  70118                FAX:     (504) 865 2453