Measure the Speed of Light With Chocolate

(From The Geek Dad’s Guide to Weekend Fun, adapted from an idea by Kathy Ceceri)

It sounds crazy, but you can indeed verify the speed of light by briefly heating some chocolate in a microwave. Now, there are many other materials that would work just as well, but chocolate makes an appropriate medium because the heating property of microwaves was first discovered by a scientist whose candy bar melted in his pocket when he got too close to the microwave device he was testing for use in radar. Plus, you can eat the results.

The experiment works because microwave ovens produce standing waves—waves that move “up” and “down” in place instead of rolling forward like waves in the ocean. Microwave radiation falls into the radio section of the electromagnetic spectrum. Most microwave ovens produce waves with a frequency of 2,450 megahertz (millions of cycles per second). The oven is designed to be just the right size to cause the microwaves to reflect off the walls so the peaks and valleys line up perfectly, creating “hot spots” (actual lines of heat).

When you put the chocolate in a microwave for a short time, the peaks and valleys of the microwave will form hot spots on the chocolate, which will then cause localized melting. First, find the hot spots and measure the distance between them. From that information, you can determine the wavelength of the electromagnetic waves. We already know the frequency (the 2,450 MHz mentioned earlier—though you may want to check the sticker inside your microwave to make sure), and when you multiply the wavelength by the frequency, you get the speed!

Age range:8+

Materials

• Chocolate (truffles, liquid-filled bonbons, or bars of solid chocolate—darker is better; you need enough to cover an area at least 8 inches square)
• Microwave-safe pan
• Microwave oven
• Ruler

Instructions

Step 1: Arrange the Chocolates

Place the chocolate in a microwave-safe dish. If you’re using bonbons or truffles, arrange them in a tight grid so there’s no space between them. Or lay two chocolate bars side by side to create a square.

Step 2: Bombard Them With Electromagnetic Radiation

Remove the turntable from the microwave. (The turntable is there to keep your food moving so it cooks evenly, but we want the chocolate to remain stationary.) You may need to put an upside-down plate over the center pillar that rotates the turntable, and then you can place your dish of chocolate on top. Close the door and heat the chocolate on high for 20 seconds.

Step 3: Inspect the Irradiated Candy

Open the door, and using a flashlight to increase glare, look for hot spots. Depending on the candy you use, you may have to feel the candy to see where it has softened. With liquid-filled cordials you may see several shiny spots and even spots where the chocolate shell melted through, releasing the sweet syrup inside. Chocolate bars may just show a series of small shiny dots. If you see none of these, close the door and run for another 10 seconds. Check and repeat if needed until you see the spots. Take the pan out of the microwave.

Step 4: Measure and Calculate

Using your ruler, measure the distance between two adjacent spots (geeks like to use metric, so that’s what we’ll do here). The space between two spots should be the distance between the peak and the valley (crest and trough) of the wave. Since the wavelength is the distance between two crests, multiply by 2. Finally, multiply that result by the frequency expressed in hertz—i.e., 2,450,000,000 (2.45 x 10⁹for those learning scientific notation). Remember: If your microwave uses a different frequency, use that instead.

• If you want to use some proper scientific protocol, you can perform a series of tests and do some simple statistics to refine the answer by establishing your standard deviation and determining your results within a specific degree of certainty. This will require you to eat quite a lot of chocolate so it doesn’t go to waste after each test. The things we do in the name of science!