The pitch of a whistle from a fast moving train sounds higher as the train approaches an observer than it does when it has passed by. What is this due to ?

The phenomenon where the pitch of a whistle from a moving train sounds higher as it approaches and lower as it recedes is known as the Doppler effect. This effect is defined as the apparent change in the frequency of a wave due to the relative motion between the source and the observer [t3][t8]. When a train (the source) approaches a stationary observer, the sound waves are compressed, leading to a reduction in wavelength and a consequent increase in the observed frequency or pitch [t1][t3]. Conversely, as the train moves away, the waves are stretched, increasing the wavelength and decreasing the pitch [t4][t6]. Other options like the Compton effect and Raman effect involve inelastic scattering of light/photons [t2], while the Kerr effect relates to changes in the refractive index of a material in response to an electric field, none of which explain the acoustic frequency shift described.

Sources

1. [1] https://www.sc.iitb.ac.in/~nkhaneja/books.pdf