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Re: radiometer paradox



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!

You are already on your way to resolving this. How many people have
observed radiometers without *investigatin* as you apparently have, by
shining a small beam on one surface! Excellent scientific method!

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).


Someone will surely give a complete answer with references. But off the
top of my head, there are two processes involved if there's any residual
gas in the "vacuum" tube (and there usually is). The black side gets
hotter, from photon absorption, and any molecule or atom hitting it
rebounds with greater kinetic energy. That, and conservation of momentum,
must be taken into account. The shiny side remains cooler. But then
there's the question of rapidity of thermal conduction through the fin
from black to shiny side. It's really quite involved. Too much for my
foggy mind tonight.

-- Donald

......................................................................
Dr. Donald E. Simanek Office: 717-893-2079
Prof. of Physics Internet: dsimanek@eagle.lhup.edu
Lock Haven University, Lock Haven, PA. 17745 CIS: 73147,2166
Home page: http://www.lhup.edu/~dsimanek FAX: 717-893-2047
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