Imagine a police car on the side of the road with lights and sirens on full blast. You’re also parked and you hear the same frequency (say 1,000 Hertz) of the siren. However, if you’re driving at 75 mph toward the police car. you’re going to hear a higher frequency (1096 Hz), and if you’re driving away at 75 mph, you’re going to hear a lower frequency of 904 Hz. Why is that?
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It has to do with a motion-related frequency change called the Doppler Effect.  This effect was initially conceived by Johann Doppler in 1842 and later tested by Buys Ballot in Holland in 1845 with a locomotive drawing an open car with trumpeters. There’s Doppler Effect for not only sound but also light including microwaves, radio waves, and visible light. Police use the Doppler Effect on their radar guns to track your speed. Astronomers use it to find the motions of stars, galaxies, and quasars… it’s really an amazing tool in our scientific toolbox!

Click here to go to next lesson on Standing Waves.

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