When wave energy like sound or radio waves travels from two objects, the wavelength can seem to be changed if one or both of them are moving. This is called the Doppler effect.
The Doppler Effect causes the received frequency of a source (how it is perceived when it gets to its destination) to differ from the sent frequency if there is motion that is increasing or decreasing the distance between the source and the receiver. This effect is readily observable as variation in the pitch of sound between a moving source and a stationary observer. Imagine the sound a race car makes as it rushes by, whining high pitched and then suddenly lower. Vrrrm-VROOM. The high pitched whine is caused by the sound waves being compacted as the car approaches you, the lower pitched VROOM comes after it passes you and is speeding away. The waves are spread out. 
When the distance between the source and receiver of electromagnetic waves remains constant, the frequency waves is the same in both places. When the distance between the source and receiver of electromagnetic waves is increasing, the frequency of the received wave forms is lower than the frequency of the source wave form. When the distance is decreasing, the frequency of the received wave form will be higher than the source wave form.
Besides sound and radio waves, the Doppler Effect also affects the light emitted by other bodies in space. If a body in space is "blue shifted," its light waves are compacted and it is coming towards us. If it is "red shifted" the light waves are spread apart, and it is traveling away from us. All other stars we have detected are "red shifted," which is one piece of evidence for the theory that the universe is constantly expanding, perhaps from a "big bang."
The Doppler Effect is something which occurs when something which emits sound or light moves relative to an observer. The object, observer, or both can move, causing an apparent change in the frequency of the wavelengths being emitted by the object. The Doppler Effect explains why a rude driver's car horn appears to change in frequency as he or she zooms by while leaning on it, and an understanding of the Doppler Effect can help scientists make a variety of observations about the world around them.
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