What is Doppler effect ?Derive expressin for apparent frequency when
source is mving towards a stationary listener

Explanation

Let u be the frequency of sound wave emitted by source and v be the velocity of sound waves.
When the source is at rest, the wavelength of sound wave is `lambda = v/u`. If the source is moving with velocity `v_s` away from stationary observer, the new wavelength is given by `lambda. = (v+ v_(s))/u`
`therefore` Apparent frequency
`u. = v/lambda^(.) = v/(v + v_(s))u rArr v. lt v`
Hence apparent frequency is less than the actual frequency of source.
Example: The frequency of whistle blown by a fast running train appear to decrease as the trai| goes away from platform.