A car sounding its horn at $480Hz$ moves towards a high wall at a speed of $20m{s}^{-1}$. If the speed of sound is $340m{s}^{-1}$, the frequency of the reflected sound heard by the girl sitting in the car will be closest to?

The correct option is A

$540Hz$

Step 1: Given data

Frequency of horn, $f_s=480Hz$

Speed of sound, $v=340Hz$

Speed of Observer, $v_o=20m{s}^{-1}$

Speed of source, $v_s=20m{s}^{-1}$

Step 2: Find the frequency heard by observer

The sound heard by the listener changes if the source of that sound and the listener are moving relative to each other. This is called the doppler effect.

According to Doppler effect, Frequency heard by observer is,

$$\begin{gathered} f_o=f_s\frac{v+v_o}{v-v_s} \\ \Rightarrow f_o=480\frac{340+20}{340-20} \\ \Rightarrow f_o=540Hz \end{gathered}$$

Hence, option (A) is correct.