The effect named after the austrian physicist Christian Doppler relates to the propagation of waves in the case that the source of the wave and the observer move relative to each other.

If the source and the observer move towards each other, the observer experiences a higher frequency compared to the frequency at the source. If they separate in time the observer measures a lower frequency. This effect can easily be experienced for sound waves.

In the case of relativity theory the Doppler effect relates to light waves.