ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related institutions.
*****************************************************************************

I took the liberty of posting part of this in a physics group, and got the 
following response.

It is the speed of light in a waveguide, or resonate chamber, which is about
20 to 30% slower than c, free space.

Another respondent likened the idea to that of determining the length of 
either a open or closed pipe with a tunning fork of known frequency. The 
tuning fork is held at the open and until resonance is found. Some math 
based on whether the one end is open or closed determines the length of the 
pipe.

The same respondent goes on with a much longer comment as follows:
- The speed of light in vacuum is not the same as the speed of light
   in chocolate chip cookies. It's not even the same for microwaves,
   though the difference may be smaller for microwaves than visible
light.
   I have no idea what the index of refraction of melted cheese is for
   microwave frequencies. Hmmm... You said you are getting results of
   c being too large? Probably not the index of refraction then.
   If you were getting c too small, I might guess the index was 1.25.
- The cold/warm spots may not be due to "standing waves" or "antinodes"
   or the like. They may be due to reflection off the inside of the
oven.
   And that may be due to a combination of things that are not instantly
   obvious. For example, the oven may have odd shapes, and may be a
   non-perfect reflector. It has a window and a fan grill, for example,
   and these may reflect differently from the solid parts of the wall.
   So, your warm spots may be either directly in front of the microwave
   source, or directly in front of a reflection of the source.
- The source may have complicated power distribution patterns. Things
   like diffraction from the portal where the micrwaves enter the oven.
   These could depend on the wavelength in a complicated way. You might
   be seeing the pattern from "single pinhole diffraction." For example,
   see this URL.

<http://www.phy.davidson.edu/StuHome/cabell_f/fraunhofer/pages/experiments.htm>

- The frequency may not be constant over operation. If it is changing
   up and down by any significant amount, then the standing waves may
   move around. A warm device may operate differently to a cold device.
   So, any nodes/antinodes may wander a bit during operation. Indeed,
   you may not see a standing wave much of the time. What is the spectral
   characteristic of the typical microwave oven anyway?
- If I'm doing my math correctly (doubtful of course) then you are looking
   for 12 cm waves. So assuming you really are seeing half wavelengths,
   you should be seeing about 6cm hot-to-hot.  How accurately can you
   measure this by noticing hot spots in food? So, 25% high  means you
   are seeing 4.5 cm hot-to-hot? Could it be simply that your food is
   not that good a measuring device?
- Could warm spots be interfering with each other? Two hot spots in
   your cheese could be slightly warming eachother, so seem to move
   towards eachother. That is, they could be melting on the side
   towards the other hot spot.

For even more fun with microwaves, there is a web site on the net
some place where various things are put in microwave ovens. A grape,
sliced in half and placed cut-face-down on a plate produces quite
interesting effects. Supposedly a lit candle will produce interesting
effects also, though my old now-replaced low power unit did not show
them. I've been reluctant to expose my new oven to this.

A third respondent offers:

What is being measured is the distance between bright spots in an 
interference pattern (ie the antinodes). If the standing wave was produced 
by two waves travelling parallel to the layer of food, in opposite 
directions, then the distance between antinodes is wavelength/2. However, if 
the waves are not going exactly parallel, then the distance will be larger. 
In the extreme case of both waves travelling perpendicular to the food 
layer, then the antinodes will be an infinite distance apart.

What interference pattern results from a single wave reflecting from the 
bottom of the microwave oven and interfering with itself? Sit down and draw 
a diagram showing the wavefronts for an angle of incidence of 45 degrees. 
What is the distance between anitnodes along the layer? The next step is to 
use the 25% high figure to find what this angle of incidence actually is!

It looks like this can get pretty involved. Hope this helps.

Sandy Eix wrote:
> ISEN-ASTC-L is a service of the Association of Science-Technology Centers
> Incorporated, a worldwide network of science museums and related institutions.
> *****************************************************************************
> 
> Hello clever and creative folks.
> 
> Have any of you ever tried to "measure" the speed of light with a microwave?  The idea goes like this: you put a layer of melty food (chocolate chips or processed cheese slices) in the microwave on a plate or tray, which you cleverly position so that the turntable doesn't turn it around. You zap it for about 20 seconds, or until isolated spots start to melt. The melted bits represent antinodes of the standing waves in the oven. So, the distance between melted bits should be half the wavelength of the microwaves. The frequency of the microwaves is printed on the oven itself, usually 2450 MHz. wavelength x frequency = c.
> 
> Now, I've tried this with many different foodstuffs, at the bottom and the middle of the several ovens, with and without a cup of water to absorb microwaves, on low and high powers, and the closest I get to the real value of c is still 25% off. In fact, I'm quite consistently 25% high. And yes, I have double and triple checked the frequency.
> 
> I'd love to hear from anyone else who's tried this... or who would like to try it and tell me what happens! If nothing else, you'll end up with a lot of half-melted chocolate chips to share with your colleagues.
> 
> thanks,
> sandy
> 
> ***********************************************************************
> More information about the Informal Science Education Network and the
> Association of Science-Technology Centers may be found at http://www.astc.org.
> To remove your e-mail address from the ISEN-ASTC-L list, send the
> message  SIGNOFF ISEN-ASTC-L in the BODY of a message to
> [log in to unmask]
> 
> 

-- 
          Wayne T. Watson (Watson Adventures, Prop., Nevada City, CA)
              (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
               Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet

             "Here's a tip on how to save energy in your home. Use a fan.
              It mixes the warm air from the ceiling with the colder air
              below." -- Bill Wattenburg, scienctist, KGO radio

                     Web Page: <home.earthlink.net/~mtnviews>

***********************************************************************
More information about the Informal Science Education Network and the
Association of Science-Technology Centers may be found at http://www.astc.org.
To remove your e-mail address from the ISEN-ASTC-L list, send the
message  SIGNOFF ISEN-ASTC-L in the BODY of a message to
[log in to unmask]