If you're a long-time reader, you may remember the great leftover Easter Peeps microwave experiment. Well, today we're going to be nuking leftover Valentine's Day chocolate to demonstrate one of the constants of physics, the speed of light. Chocolate makes a very 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 a microwave device being tested for use in radar.

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WARNING: This experiment may take several tries to get right. We are not responsible for any weight gained. To avoid familial strife, be sure to only do this experiment with your own chocolates or with candy which you have been authorized to access. You can probably find some leftover boxes on sale this week.

The demonstration 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 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 that the peaks and valleys line up perfectly, creating "hot spots" (actually, lines of heat).

What you do with the candy is to find the hot spots and measure the distance between them. From that information, you can determine the wavelength. And when you multiply the wavelength by the frequency, you get the speed! Here's what you do:

Make sure the candy is in a microwave-proof box. Better yet, take the chocolate out and put in a microwave safe dish. Remove the turntable in your oven. (You want the candy to stay still while you heat it.) Put an upside-down plate over the turning-thingy, and place your dish of candy on top. Heat on high about 20 seconds. Take the chocolate out and look for hot spots. Depending on the candy you use, you may have to feel the candy to see where it has softened. With the cherry cordials we used, we saw several shiny spots and one place where the chocolate shell melted through, releasing the sweet syrup inside. Measure the distance between two adjacent spots. This 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 or 2,450,000,000 (2.45 X 109 for my son who is just learning scientific notation).

In our trial, we measured a distance of roughly 6 centimeters. 6 x 2 x 2,450,000,000 = 29,400,000,000 centimeters per second, or 294,000,000 meters per second. This is awfully close to 299,792,458 meters per second, which is the speed of light. Not bad for some leftover chocolate and a kitchen appliance!

I discovered this experiment at Null Hypothesis, although it can be found all over the Internet, including many versions with fancy charts and animations. By the way, melted chocolate bars are perfect as ice cream topping. Just saying.

Kathy Ceceri also blogs at Home Physics.