The Doppler Effect is the change in frequency of a wave due to the relative motion between the source and the observer.

• When the source approaches the observer, it 'catches up' to its own outgoing wave crests, effectively shortening the wavelength ahead of it.
• A shorter wavelength leads to a higher observed frequency ($f^{\prime }>f$).
• Conversely, receding motion increases the wavelength, decreasing the frequency (Redshift).