Microwave ovens cook unevenly because a pattern of standing waves that
form inside the oven chamber.The pattern creates an array of hotspots throughout
the oven's volume. An operating frequency of around 2000 MHz will produce
a wavelength of around 10cm, and the hotspots should be at halfwave
points, or every 5cm, but in a complex 3D pattern.
Procedure
First open the marshmallows and place them in the casserole dish, completely
covering it with a layer one marshmallow thick. Make sure that the flat
side of the marshmallow is facing upwards.
Take the turntable out of the microwave.
Next put the dish of marshmallows in the microwave and cook on low heat.
The microwave will not cook evenly after the removal of the turntable and
the marshmallows will begin to melt at the hottest spots in the microwave.
Heat the marshmallows until they begin to melt in 4 or 5 different spots.
Remove the dish from the microwave oven and observe the spots.
Take the ruler and measure the distance between the melted spots. You will
soon find that one distance repeats over and over. This distance will correspond
to the wavelength of the microwave. That should be a little over 12 centimeters.
Now, turn the microwave around and look for a small sign that tells you
the frequency of the microwave. Most commercial microwaves operate at 2450
MHz.
NOTE: Only do one trial. Obtain the wavelength data from other lab groups.
If no frequency is visible on the microwave, use 2450 MHz.