A train whistling at constant frequency 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 (figure). Identify the expected curve.

1.                           2.  

3.                            4. 
(3) Hint: Use the concept of Doppler's effect.
Step 1: Find the frequency when the train is approaching the station.
Let the original frequency of the source is n0
Let the speed of the sound wave in the medium is v.
As observer is stationary. 
Apparent frequency when the train is approaching,
It is a constant value.
Step 2: Find the frequency when the train is receding from the station.
When the train is going away from the observer,
Apparent frequency, na=vv+vsno=na<no
It is also a constant value.
Hence, the expected curve is (3).