The Doppler Effect refers to the phenomenon of electromagnetic waves changing in frequency based on the relative motion of the source and the observer. Essentially, if there is a greater relative velocity between the source and the observer, there is a greater difference in the frequencies measured by the source and the observer.
The three main cases are:
- When a wave source is moving just as fast as the observer, then the observed frequency is equal to the rest frequency
- When a wave source is moving toward an observer, the observed frequency is greater than the rest frequency
- When a wave source is moving away from an observer, the observed frequency is less than the rest frequency
When dealing with visible light on the electromagnetic spectrum, we know that colour seen by an observer is based on the frequency of the wave. Thus, the Doppler Effect leads to the same colours being seen differently by different observersβdependent on the relative velocity of the observers.