Doppler Effect - Full explanation The Doppler Effect is the change in the frequency or wavelength of a wave as observed by someone moving relative to the source of the wave. In Simple Terms: When a source of sound (or light) moves towards you, the waves are compressed, you hear a higher pitch (higher frequency). When the source moves away, the waves are stretched, you hear a lower pitch (lower frequency). Common Example: Imagine an ambulance driving past you: • As it approaches, the siren sounds