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Re: Speed of Light With a Microwave



At 09:25 AM 5/16/97, Van E. Neie wrote:

In the most recent _Physics Teacher_, there appeared a one-page article
which described how one could use a pan of marshmallows in a microwave oven
to determine the speed of light. The method involves measuring the
distances between hotspots or more-cooked places on the top of the layer of
marshmallows. It is asserted that this distance is the wavelength of the
microwaves. I am having trouble understanding why this is so. It seems
reasonable that a standing wave may be set up inside the microwave. It also
seems reasonable that the hot spots would correspond to the anti-nodal
points. What I don't get is why the distance between the anti-nodes should
be a wavelength. One-half wavelength makes more sense to me as the distance
between consecutive antinodes. Help me, someone, please.


I have a more fundamental question: Is the Klystron in a MW oven a
single-frequency device? If so, is it reasonable that the cavity size of
the oven is such that standing waves will be set up for this frequency?

------------------------------------------------------------------------------
Van E. Neie ven@physics.purdue.edu
Dept of Physics PH: (765) 494-5511
Purdue University FAX: (765) 494-0706
1396 PHYS Bldg
W. Lafayette, IN 47907-1396

It is usual to find a magnetron fitted to a MW oven.
The power supply is a rough and ready rectification at line frequency.
With regulated voltage a magnetron will mode hop. Indeed this is the
purpose in life of 'mode-straps' which circle the cavities in hopes of
stabilizing the oscillation mode of a magnetron.
With an unregulated power supply such as an oven uses, the frequency
variability can be expected to be greater.

But I always enjoyed watching the gold glaze on a tea-cup sparkling
in the radiation....

Regards
brian whatcott <inet@intellisys.net>
Altus OK