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.)