Measure the Speed of Light with a Microwave
Overview: The speed of light is 300,000,000 m/s. Can we figure that out using a microwave oven and a chocolate bar? Of course we can.
Supplies: Microwave oven, ruler, calculator, paper plate, paper towels, and food to melt (chocolate bar, fruit roll ups, marshmallows, etc.)
Objectives: Use a basic household appliance (the microwave) and food to measure the speed of light
Setup:
- Plug in the microwave
- Remove the turntable from the microwave
How to run the demo:
- Place the food item in the microwave on a plate or paper towel. It works best if the item fills a large portion of the interior of the microwave and goes wall to wall left to right
- Nuke the item for 5-6 seconds and look for melted marks. CAREFUL NOT TO BURN THE FOOD ITEM!!
- Pull the food item out of the microwave
- Measure the distance (in cm) between the melted parts of the food item (should be around 6cm)
- Each spot is half a wavelength and we want the distance of a full wavelength, so multiply by 2 to get 12cm
- 12cm is .12m
- The speed of light is frequency x wavelength
- The frequency of most microwaves is 2450MHz (check the back of your microwave if you are unsure)
- 2,450,000,000 x .12 = 294,000,000 m/s
What’s Happening? Light can come in many forms, aka wavelengths. We always think of light as what we can see, but there are may frequencies of light that we cannot see. Those frequencies include infrared light, ultraviolet light, and yes, even microwaves, to name a few. The speed of light is always a constant, so no matter what frequency you are dealing with the speed will never change. As the wave propagates across the microwave it travels with crests and troughs. Every time the wave hits a trough it cooks the item of food in that particular spot. Since the wavelength is always the same you can accurately and reliably use those troughs as half wavelength measurements.
Learn more: (external links)