The phenomenon responsible for the Doppler effect is the change in frequency or wavelength of a wave in relation to an observer moving relative to the source of the wave. This effect is commonly experienced with sound waves, where a moving sound source, such as a siren, appears to have a higher pitch as it approaches and a lower pitch as it moves away. The Doppler effect is significant in various fields, including astronomy, radar technologies, and medical imaging techniques like Doppler ultrasound.