So when we did the experiment with
marshmallows in a microwave oven I could see that the "hot" and
"cold" spots made a regular pattern, but I don't understand why
it's kind of like a checkerboard. If it was just the crests and
troughs of the waves causing the hot spots, I would have thought
the pattern would look like this:
Well, that is what it would look like if the waves were only coming
from one direction, but that's not how microwaves work. If the wave you
imagined was moving from side to side in the microwave, there is also a
wave going front to back, like this:
Oh, and now we have to combine the two waves, like we had to in the Two Slit Experiment, except
here we have to add two whole surfaces, not just two lines...
Exactly. It's a little harder to visualize, but its the same kind of
thing. The result looks like this:
Cool! The high and low areas in that picture match the pattern we
saw in the marshmallow experiment. But wait a second, I have two
problems with this. First, if the waves are moving at the speed of light,
shouldn't the hot spots be moving, too? Also, does this mean that all
microwave ovens have TWO "microwave guns"? I mean, one for the back and
forth waves
and one for the front to back waves?
Not only are those both good questions, but they have the same answer.
The "microwave guns" are called "magnetrons" and a microwave oven only
has one of them. We get two different wave patterns because the physics
of a microwave oven leads to something called
Standing Waves...
So when we did the experiment with 
"cold" spots made a regular pattern, but I don't understand why
it's kind of like a checkerboard. If it was just the crests and
troughs of the waves causing the hot spots, I would have thought
the pattern would look like this:
Well, that is what it would look like if the waves were only coming
from one direction, but that's not how microwaves work. If the wave you
imagined was moving from side to side in the microwave, there is also a
wave going front to back, like this:
Oh, and now we have to combine the two waves, like we had to in the 
Exactly. It's a little harder to visualize, but its the same kind of
thing. The result looks like this:
Cool! The high and low areas in that picture match the pattern we
saw in the marshmallow experiment. But wait a second, I have two
problems with this. First, if the waves are moving at the speed of light,
shouldn't the hot spots be moving, too? Also, does this mean that all
microwave ovens have TWO "microwave guns"? I mean, one for the back and
forth waves
and one for the front to back waves?
Not only are those both good questions, but they have the same answer.
The "microwave guns" are called "magnetrons" and a microwave oven only
has one of them. We get two different wave patterns because the physics
of a microwave oven leads to something called
Standing Waves...
