The Doppler effect is the change in frequency or wavelength of a wave for an observer moving relative to its source. It occurs with all types of waves, including sound waves and light waves. When a source of waves moves towards an observer, the observed frequency increases (wavelength decreases), and when it moves away, the observed frequency decreases (wavelength increases).

Doppler effect is the shift in frequency of sound sources with respect to the observer. Doppler effect is observed in sound as well as a light wave. It was given by Johann Doppler in 1842.

• Write  the apparent velocity definition.

The apparent velocity is the true velocity when the surface is parallel to the refractor.