Measuring the Speed of Light With a Microwave

Materials: ★★☆ Available in most school laboratories or specialist stores
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Electromagnetic Spectrum and Waves, Light

Alternative titles: Microwave Marshmallow Speed of Light Experiment, Finding Wavelength of a Microwave

Summary

By heating a layer of marshmallows in a microwave without the rotating tray, you can observe melted spots that mark the peaks of a standing wave. Measuring the distance between these spots and combining it with the microwave’s frequency allows you to calculate the speed of light.

Procedure

Place mini-marshmallows in a microwave-safe dish in a single, even layer.
Remove the rotating tray from your microwave so the dish does not move.
Microwave the marshmallows for about 10 seconds (adjust if all or none melt).
Remove the dish and observe where marshmallows melted compared to where they did not.
Use a ruler to measure the distance between two adjacent melted spots (this represents half a wavelength).
Find the frequency of your microwave (often listed on a sticker, usually around 2450 MHz).
Use the equation: Speed of light = 2 × (distance between melted spots in cm) × (frequency in Hz).
Compare your result to the accepted value of 3.00 × 10^8 m/s.

Links

Measuring the Speed of Light with Marshmallows - Wise Wonders:

📄 Measure the Speed of Light Using Your Microwave - The Wonders of Physics: https://wonders.physics.wisc.edu/measure-the-speed-of-light/

Variations

Try using chocolate chips instead of marshmallows.
Test different microwaves to see if results vary.
Use larger dishes to get more melted spots and more accurate measurements.

Safety Precautions

Always use oven mitts when handling hot dishes.
Do not overheat marshmallows to avoid burns or fire.
Ask an adult for help when removing the rotating tray.

Questions to Consider

Why do the marshmallows only melt in certain spots? (Because those spots are at the peaks of the standing wave, where the microwave energy is strongest.)
Why is the measured distance only half a wavelength? (Because the distance between adjacent peaks is half the wave’s full cycle.)
How close did your calculation come to the actual speed of light? What might cause differences? (Errors may come from uneven heating, inaccurate measurement, or microwave timing.)
What does this experiment show about the nature of light and microwaves? (That light, like microwaves, behaves as a wave with measurable wavelength and frequency.)