The Doppler Effect: Definition, Examples & Applications

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• published: 17 Mar 2016
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What is the Doppler effect? How does it affect our perceptions of sound and light? In this lesson, learn how traveling wave sources can change the way we observe sound and light waves. Explore the applications of the Doppler effect on shock waves, sonic booms, and the movements of distant galaxies. What Is the Doppler Effect? Have you ever noticed that sounds change their pitch when they're traveling past you? Let's say you're sitting at a stoplight, minding your own business, when all of a sudden you hear an ambulance. The siren wails a high pitch from half a mile down the road. You see the approaching vehicle and wait patiently while the ambulance speeds through the intersection. Once it passes, you notice the siren sounds different. It has a lower pitch than it did before. Why did the sound of the siren change? This common occurrence happens because of something we call the Doppler effect. You may have heard about this before. The Doppler effect explains how we perceive changes in sound when the source of the sound is moving. While the ambulance siren doesn't change pitch at all, we perceive that it changes as the vehicle moves past us. The Doppler effect isn't just about ambulance sirens, though. It's an important phenomenon that occurs in all types of waves: sound waves, light waves and even water waves. Scientists have used the Doppler effect to make some amazing discoveries. Let's learn a little more about this phenomenon, so we can see why it's so important. Actual vs. Apparent Frequency To an observer, the apparent frequency will be higher than the actual frequency. Apparent Frequency Diagram We already know that a wave is caused by the disturbance of a medium. Let's say you're standing at the edge of a lake, when Nessie the Loch Ness Monster pops her head up right in the center. Nessie's head causes a disturbance that travels outward as a circular wave. It takes a bit of time to reach you standing at the bank. But if the lake is perfectly circular, then the wave reaches the opposite edge at exactly the same time. If Nessie keeps bobbing her head up and down, she creates more circular waves moving outward from her head. These waves all reach the banks at a certain frequency: the same frequency of Nessie's head bobbing. If she bobs her head two times every second, then that's the frequency of the waves hitting the banks. Now, suppose Nessie begins swimming toward you, still bobbing her head two times every second. The circular waves begin to look much different. Because Nessie is traveling some distance between each head-bob, each successive wave originates from a slightly different spot on the water. The waves bunch together right in front of Nessie's head and spread out behind it.