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The Doppler effect is a certain phenomenon that concerns waves. The most familiar example of it that you know has to do with sound. If you have ever heard the wail of a siren on a fire truck or ambulance you may have noticed something interesting about the sound.
If you are in a position to see the fire truck or ambulance approaching you might have realized that the pitch of the siren changes, becoming higher as it gets closer to you. When the vehicle passes you by you may notice that the pitch of the siren starts to get lower as it leaves you. This phenomena is due to the Doppler effect. The Doppler effect doesn’t just work for sound waves. It works for any wave even light.
The Doppler effect is essentially about the compression of waves. As the source of the wave gets closer to the observer, the wavelength of the waves becomes shorter. The opposite occurs if the source of the wave is retreating from the observer. The wavelength of the waves lengthen as it takes longer for them to reach the observer.
This effect has some interesting implications. For example the principle results in light actually changing color when applied. Scientists have used this property to discern certain stars that are closer to the earth and other that are retreating. This helped astronomers and astrophysicists to conclude that the universe is actually expanding. When the Doppler effect is applied to light it is called red shift and blue shift.
Red shift occurs when a light source such as a star is moving away from the observer. Blue shift happens when the light source is approaching.
The Doppler effect also has other practical applications. For example radar and sonar work using this principle. By measuring changes in wavelength and frequency sonar and radar can be used to detect unseen objects and their distance from the observer. Law enforcement uses the Doppler Effect every time a radar gun is used to track the speed of motorists on the highway.
The concept of the Doppler Effect has helped to expand our knowledge of waves. This has helped scientist to hypothesize on the formation of the universe and important theories in physics such as the theory of relativity.
You can also listen to Astronomy Cast. There is a great episode about the Doppler Effect.