StellarThe Doppler Effect
So far, we have dealt with waves that have a stationary source. However, when this wave source is moving relatively to someone who is observing the wave, this will result in what is known as the Doppler Effect.
When a wave source is moving toward an observer, the wave pulses will reach the observer earlier than if the wave source was stationary. This leads to the frequency being observed to be greater than the actual frequency of the wave source.
When a wave source is moving away from an observer, the wave pulses will reach the observer later than if the wave source was stationary. This leads to the frequency being observed to be lower than the actual frequency of the source.