Re: radiometer paradox

[Bob Sciamanda <Sciamanda@worldnet.att.net>]

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

Joseph,
	You are speaking of the Crookes Radiometer.  Conventional Wisdom treats
this as a thermal phenomenon. The container is NOT a high vacuum.  The
blackened side of the vane absorbs better than the reflective side and
so attains a slightly higher temperature.  The result is a higher air
(not radiation) pressure on the blakened side.  I'm sure that Jearl
Walker's "Circus" book gives more details and references for this - I
don't have it handy.  (In a high vaccuum, the radiation pressure would
dominate, and your expectation would be realized.)
-- 
Bob Sciamanda		sciamanda@edinboro.edu
Dept of Physics		sciamanda@worldnet.att.net
Edinboro Univ of PA	http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA		(814)838-7185