Doppler effect is the compression or extension of a sound wave, which causes a change in its wavelength / frequency (and so its sound). Explanation: It is defined as the effect produced by a moving source of waves in which there is an upward shift in frequency for observers, the source is moving towards and downward shift of frequency from which the source is moving away. used to tell if an object in space is moving toward or away from us.
When a star is moving away the light looks red or redder. And when a star is moving toward you it is blue or bluer. This is important because you can tell which way the star is moving.
