Measure the Speed of Light using your Microwave
What You Need:
•    Microwave
•    Microwave safe dish at least 5-6 inches across (bigger is better)
•    Mini-marshmallows
•    Ruler
•    Calculator

Try This:
1.    Place the mini-marshmallows in the dish one layer thick.
2.    If your microwave has a rotating platform, remove it. We don’t want the dish to rotate.
3.    Put the dish in the microwave for 10 seconds.
4.    When you remove the dish, you’ll notice only certain parts are melted. (Time may depend on the microwave – if all or none of your marshmallows melted, adjust the time.)
5.    Measure the distance between melted marshmallows using your ruler. Measure in centimeters. This is half the wavelength of a microwave.
6.    Look for a sticker on your microwave that tells you its frequency in Hertz (Hz). Most microwaves are around 2450 MHz. Note: MHz = 10^6 Hz
7.    Use the following equation to find the speed of light:

Speed of light = 2 x (distance between melted spots) x (frequency of microwave)

8.    The actual speed of light is 3.00 x 10^10 cm/s. How close were you?

What’s going on?
Microwaves are a form of electromagnetic radiation, just like light, but they are beyond the visible spectrum so we cannot see them. A microwave oven works by creating a standing wave in the microwave.

Microwave - Standing Waves

The peaks of the wave heat faster and melt the marshmallows first. What you measure is half the wavelength of the microwave light. The arrow shows one full wavelength of microwave light.

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This page updated on September 18, 2007