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03/01/1996 08:00:00

Question of the month: How long is a radio wave, and how do you measure it?

Question of the Month Answered by: Sterl Phinney, Associate Professor of Theoretical Astrophysics

Radio waves come in a variety of lengths, from as short as an inch or less up to several miles. The waves we usually think of as radio waves, the ones that broadcast music and weather reports, range in length from about 10 feet for FM to about 300 yards for AM stations.

Radio waves are electromagnetic radiation, just like x-rays at the dentist, gamma rays from space, the microwaves that cook your food, visible light that allows us to see, and infrared radiation that we feel as heat. The only difference between these kinds of radiation is their wavelength.

We can't measure radio wavelengths directly, like we can an ordinary object, because they are invisible and move at the speed of light. So we have to measure them in a clever, indirect manner.

The method is comparable to measuring the length of boxcars in a moving train. Because they are moving, we can't use a tape measure on a boxcar. But if we know the speed of the train and how often a boxcar goes by, we can figure it out. If the train is moving at 80 feet per second, and two boxcars go by in each second, then each car must be (80 ÷ 2 =) 40 feet long.

Similarly, radio waves move at the speed of light, about 300 million meters per second. For an FM station at 100.0 on your dial, 100.0 million wavelengths go by in each second. So the wavelength is (300 million ÷ 100 million =) 3 meters, or about 10 feet.