The engine of a train sounds a whistle at frequency $\nu$, The frequency heard by a passenger is:
A) $ > \nu$
B) $ < \nu$
C) $ = \dfrac{1}{\nu}$​
D) $= \nu$

Hint: Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer.

Complete step by step solution:
When wave energy like sound or radio waves travels from two objects, the wavelength can seem to be changed if one or both of them are moving. This is called the Doppler effect.

As given in the Question, since the source and the observer travel both at the same velocity there is no relative motion. Therefore there is no Doppler effect and visible frequency shift.

Additional information:
The Doppler effect works on both light and sound objects. For example, when a sound object moves towards you, the frequency of sound waves increases and leads to a higher pitch. Conversely, if it moves away from you, the frequency of the sound wave decreases and the pitch goes down. Ambulance sirens fall on the pitch when passed, and red light shifts are a common example of the Doppler effect.
Edwin Hubble made the discovery that the universe expands as a consequence of the Doppler Effect. It has important applications in the fields of astronomy and space technology.

Note: The Doppler effect refers to the change in wave frequency during a relative motion between a wave source and its observer. It was discovered by Christian Johann Doppler, who described it as the process of increasing or decreasing the brightness of a star based on the relative motion of the star.
There are various applications of Doppler Effect. It is used in:
1. Sirens
2. Astronomy
3. Radars
4. Medical imaging and blood flow management
5. Flow management
6. Velocity profile management
7. Satellite communication
8. Audio
9. Vibration measurement