What's a Doppler Effect?

Doppler effect is stretching or compression of a wave as it moves. A good illustration would be to listen to a car zoom by you. As the car passes you, its sound changes pitch, because sound waves are either compressed or stretched out, depending on whether the car is approaching you, or moving away. Similarly, the wavelength of light changes when its source is moving.

http://science.intellichristian.com/what-s-a-doppler-effect