Doppler Effect

The Doppler effect is what happens to waves which are coming from an object or source that is moving toward or away from you. A common example is an ambulance siren.

As you listen to the sound of this siren, you will probably be reminded of an ambulance racing toward you, passing you, and finally moving away. But how do you know this just by listening? When the ambulance is approaching you the siren has a higher pitch than when it is moving away. The pitch of a sound wave tells us how often the waves encounter the listener. As the source of the sound waves comes closer, the waves bunch up close together. When it is moving away, the waves are stretched out. The amount that the waves are bunched up or stretched is what determines the pitch, or what we call the frequency of the waves.
This change in pitch with the motion of the source is called the Doppler effect and it works for all kinds of waves, including both sound waves and light. In fact, it is the Doppler effect of light coming out of the surface of the Sun that tells us how the surface of the Sun is moving.
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