🚀 go-pugleaf

kevinl@earthlink.net (Kevin Luttrell) wrote:
>Hi,
>
>I teach 9th grade science and I would like a definitive word on how a
>radiometer works. They are the "toys" that consist of a clear glass bulb
>with freely rotating vanes inside. The vanes are painted white on one
>side, black on the other. The vanes will spin in the presence of bright
>light.
>
>I believe that momentum conservation requires the photons to be imparting
>their momentum to the vanes. Since photons have no rest mass, the mass
>they contribute to momentum is relativistic, m = E/c^2. Since they are
>traveling at speed c, the momentum becomes E/c. Is this view correct?
>
>Another teacher I work with thinks it has something to do with convection
>currents set up inside the bulb. Yet I always assumed the buld was
>evacuated  (a vacuum).
>
>Thanks for helping out!!
>
>                                    "Is this a train robbery"?    
>Thanks! Sincerely,                  "No...it's a science experiment".
>Kevin Luttrell                               - Back 2 the Future, III

	In most classroom radiometers, the bulb is NOT fully evacuated 
and the blades spin by convection currents.  Convection currents make the
white parts act like they are pulled by the light.  In a truly evacuated
radiometer, the blades spin by momentum transfer.  Then, the white 
parts act like they are pushed by the light.  The two cases spin in 
opposite directions.  Which direction do your radiometer blades spin?
drosen@inxs.chem.duke.edu