Explain what is the Doppler effect in sound and state its any four applications.

Hint: We know that whenever there is a relative motion between the source of the sound, medium and the observer, the frequency of the sound as received by the observer is different from the frequency of sound emitted by the source.

Complete Step by Step Solution:
Let us first see the doppler effect:- the apparent change in frequency of sound when the source, observer, and medium are in relative motion with each other is called the Doppler effect.
Applications of doppler effect:

1. Sirens: The siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer.
2. Astronomy: The Doppler effect for electromagnetic waves such as light is of great use in astronomy. It has been used to measure the speed at which stars and galaxies are approaching us.
3. Radar and satellite communication also uses the concept of doppler's effect.
4. Medical imaging and blood flow measurement is also based on the doppler's effect.

Additional information:
When a source moves and the observer is stationary, the following equation can be used.
$f=\left( \dfrac{c}{c+{{v}_{s}}} \right){{f}_{0}}$
Where
f is the observed frequency, ${f}_{0}$ is the actual frequency, C is the velocity of waves in the medium, Vs is the velocity of the source
It should be noted that Vs is positive when the source is moving away from the observer, and it is negative when the source is moving toward the observer.
Now in the same way, when a source is stationary and the observer is moving relative to it, the equation is as follows :

$f=\left( \dfrac{c+{{v}_{0}}}{c} \right){{f}_{0}}$$f=\left( \dfrac{c}{c+{{v}_{s}}} \right){{f}_{0}}$
The symbols stand for the same quantities as mentioned above and Vo is the velocity of the observer. Unlike before, Vo is positive when the observer is moving toward the source and negative when it is away from the source.

When both the source and observer are moving relative to one another, these equations can be combined to form the equation below.
$f=\left( \dfrac{c+{{v}_{0}}}{c+{{v}_{s}}} \right){{f}_{0}}$

Note: We know that the Doppler effect is a wave phenomenon, it holds not only for sound waves but for also electromagnetic waves such as microwaves, radio waves, and visible light .however we can notice the Doppler effect only when relative velocity between the source and observer is an appreciable fraction of wave velocity.