Astro Bob: How to calculate the speed of light using your kitchen microwave

Use your microwave oven to perform a simple experiment to measure a fundamental quantity.

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Cooking chocolate bars in the microwave? Why not? It's a simple, homegrown way to find the speed of light. (Bob King)

Did you know you've been cooking your food with light? Most of us have a microwave oven in our kitchens, and that's exactly what they do. When you press start, a device called a magnetron emits a stream of electrons that interacts with a magnet to produce microwaves.

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Visible light makes up just a small sliver of the entire electromagnetic spectrum of radiation. Waves that are shorter and have higher frequencies like ultraviolet (UV) and x-rays (left) are more energetic and can damage human tissue. Longer-wavelength, lower frequency light like infrared and microwaves lie beyond the red end of the visible spectrum. (NASA)

Microwaves are invisible the same way we can't see ultraviolet (UV) light or infrared light (heat) radiating from a sun-warmed rock. Together with x-rays, gamma rays and radio waves they form the electromagnetic spectrum. Our eyes detect only a sliver of the spectrum called visible light which includes the familiar colors of the rainbow. All the other forms of radiation are also light but only detectable with instruments that extend the range of the human eye.


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Light waves like water waves oscillate up and down in a series of crests and troughs. The distance between two crests or troughs equals the wavelength of the light. In a microwave oven food absorbs billions of these waves and heats up. (Wikipedia / Pluke CC0 with additions by the author)

Light is made of waves. Picture a cork riding a wave. As the wave passes, the cork rides up the side of the wave, crests at its top and then drops down the other side into a trough before riding the next wave. The distance between two crests is the wavelength, and the speed at which the wave travels — how many crests pass a fixed point every second— is called the wave's frequency and described in cycles per second or hertz (Hz). One million cycles (hertz) per second equals 1 megahertz or 1MHz. All forms of light travel at the same speed of 186,000 miles per second (300,000 km/ sec).

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The wall of a soap bubble is only as thick as the waves of visible light. (Jeff Kubina / CC BY-SA 2.0)

Visible light waves are really, really small, with wavelengths between 380 and 700 nanometers. One nanometer equals one-billionth of a meter. Light waves are only about as wide as the thickness of a soap bubble membrane. For comparison, a sheet of paper is 100,000 nanometers thick. Visible light frequencies vary between four and eight hundred trillion cycles per second. Microwaves lie between infrared and radio with wavelengths that are much easier to visualize, ranging from one meter (3.3 feet) to one millimeter, about the thickness of a credit card.

All forms of light carry energy. Food absorbs microwaves, and the waves transfer energy to water molecules in your tea or pizza and make them vibrate. The vibrations produce the heat that cooks the food. Crazy but true, a forkful of baked potato feels hot in your mouth because of violently jiggling molecules.

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Skip the zeroes! Stars are so distant that the light-year is a much more convenient unit of measurement. The closest star system after the sun is Alpha Centauri at 4.4 light years. It's located to the east of the Southern Cross and visible on spring and summer evenings from southern latitudes. (Stellarium)


Now that we know a little bit about how a microwave oven works let's use it to determine one of the most fundamental quantities in the universe: the speed of light. The speed of light is a useful shorthand for measuring astronomical distances. Traveling at 186,000 miles a second (300,000 km/sec) a light beam covers 6 trillion miles (9.7 trillion km) over the course of a year, equal to one light-year. Instead of saying that Alpha Centauri is 26,000,000,000,000 miles away it's much more economical to say 4.4 light-years.

The speed of light, called c, is given by this formula: c = frequency of the light x wavelength. You'll find your oven's frequency on a label affixed to the backside or inside the door. If not there, check the manual. Consumer ovens like the one in our kitchen are typically rated at 2,450 MHz or 2,450,000,000 (2.45 billion) hertz.

To determine the wavelength, place a large sacrificial slice of cheese or chocolate bar on a plate. Open up the oven door and remove or turn off the rotating tray — the food must be stationary. Next, set the oven's power level to low (~30 percent) and the time to 20 seconds. You want the slice slightly melted not nuked. Press start. Keep your eye on the process and adjust the time as needed until you see melted pockets in the bar or slice.

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My Hershey's bar developed a couple blobby holes after heating in the microwave at 30 percent power for a total time of about 1 minute. The separation between them came to about 6 centimeters. (Bob King)

When finished, you'll see that the bar has two or more melted spots. These "hot spots" are where the crests and troughs of the myriad, oscillating microwaves transferred their energy. Carefully measure the distance between the centers of two divots using a metric ruler marked off in centimeters. You can also use a standard ruler and then convert inches to centimeters with this handy converter . My melted spots were about 6 centimeters apart.

The number you record will be the microwaves' half wavelength or the distance from crest and trough. Double it to get the full wavelength, measured from crest to crest. Then multiply your result by the frequency in hertz. For my oven that's 2,450,000,000 x 12 (6x2) centimeters = 29,400,000,000 centimeters per second or 294,000 kilometers a second. That converts to 182,683 miles per second. Not bad! And I only ruined four slices of cheese and one chocolate bar. By the way, you can also use mini-marshmallows and measure the distance between the melted marshmallows.

Give it a try. While you're sure to get quizzical looks from your family tell them it's all in the name (and fun) of doing science.


"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at .

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