The engine of a train moving with speed $10 \mathrm{~ms}^{-1}$ towards a platform sounds a whistle at frequency $400 \mathrm{~Hz}$. The frequency heard by a passenger inside the train is: (neglect air speed. Speed of sound in air $=330 \mathrm{~ms}^{-1}$ )

<p>The phenomenon of frequency change due to relative motion between a source and an observer is called Doppler effect. However, if the observer is at rest relative to the source (as in this case where the passenger is inside the train), then the frequency heard by the observer is the same as the frequency produced by the source.</p> <p>This is because the relative velocity between the source (whistle) and the observer (passenger in the train) is zero. The Doppler effect only applies when there is a relative velocity between the source and the observer.</p> <p>Therefore, the frequency heard by the passenger inside the train is the same as the frequency of the whistle, i.e., 400 Hz.</p> <p>So, the correct answer is:</p> <p>400 Hz.</p>