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 waves emitted by source and v be the velocity of sound waves.
When the source is at rest, the wavelength of sound waves is : `lambda = v/u`.
If the source is moving with velocity `v_(s)` towards 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. gt v`
Hence, apparent frequency is greater than actual frequency of source. Example: The frequency of whistle blown by a fast running train appear to increase as the.trait approaches railway platform.