The speed of light, 299,792,458 meters per second, is constant throughout the universe and was established hundreds of years ago. It can be figured out easily for the purposes of research and science experiments using high-tech instruments. Or it can be done right in your own home, using a microwave, ruler and bag of marshmallows.
Cover an entire dish with large marshmallows standing side by side.
Place the dish in a microwave, set the microwave on high for a few minutes, and watch.
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Wait until the first four or five marshmallows begin to melt, turn off the microwave and remove the plate.
Measure the distances between the marshmallows that began to melt. The distance should be around 6 centimeters, and represents half of the wavelength of the microwave. Divide by 100 to get the reading in meters.
Find the frequency of your microwave. It is measured in megahertz (MHz) and is usually llisted on the back of the device and in the instruction manual.
Multiply the wavelength (your measurement from step 4, doubled) by the frequency (the MHz from the microwave) to equal the speed of light in meters per second.