A train whistling at constant frequency n is moving towards a station at a constant speed v . The train goes past a stationary observer on the station. The frequency n of the sound as heard by the observer is plotted as a function of time t . Identify the correct curve.

Step-by-step solution

Since, the observer is stationary, so when train is approaching towards the observer or moving towards the observer, apparent frequency is given as, n^ =n ( v v-v_ s ) where, v_ s is the speed of sound. It is clear from the above expression that n^ >n . However, when the train is going away from the observer, then apparent frequency, array ll & n^ =n ( v v+v_ s ) & n^ < n array Hence, graph represented in option (d) is correct.