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From: judah@uwvax.UUCP (Judah Greenblatt)
Newsgroups: net.space
Subject: Re: Light Sails
Message-ID: <136@uwvax.UUCP>
Date: Thu, 28-Feb-85 16:24:25 EST
Article-I.D.: uwvax.136
Posted: Thu Feb 28 16:24:25 1985
Date-Received: Mon, 4-Mar-85 04:16:20 EST
References: <722@mordor.UUCP> <133@uwvax.UUCP> <6893@rochester.UUCP>
Organization: U of Wisconsin CS Dept
Lines: 25

> ...  Each paddle has a black side and a
> silver side with radial symmetry.  Placed in light, the paddle(s) on one
> side of the pivot will reflect the light, the one(s) on the other will absorb
> the light.  The absorbing surface gets only the inelastic momentum change,
> or mv, while the reflecting surface gets the 2mv momentum change.  This
> creates a torque on the paddle structure, and the little bugger rotates.
> Available at novelty stores near you!
> Nemo

The principle of light pressure is correct, but unfortunatly, it is not
what drives a radiometer (as these little toys are called).
If you look closely at a radiometer spinning in the sunlight, you will see
that the vanes spin with the SILVER face leading and the BLACK face trailing.
If light pressure drove the spinner, the BLACK face should lead and the SILVER
face (which would be pushed twice as hard by the light pressure) would follow.

The radiometer is actually a simple heat engine: the BLACK side of the vane
heats up slightly more than the SILVER side and the vanes are driven by
the difference in momemtum of gas molicules bouncing off the warmer and
cooler sides.  

Light pressure is actually much weaker than the forces that drive a radiometer.

Judah Greenblatt		ARPA: judah@wisc-rsch.arpa
U. of Wisconsin C.S. Dept.	UUCP: {seismo ihnp4 lbl-csam}!uwvax!judah