A bat emitting an ultrasonic wave of frequency $4.5 \times 10^4 \mathrm{~Hz}$ at speed of $6 \mathrm{~m} / \mathrm{s}$ between two parallel walls. The two frequencies heared by the bat will be

• A

  $4.67 \times 10^4 \mathrm{~Hz}, 4.34 \times 10^4 \mathrm{~Hz}$
• Correct answer
  B

  $4.34 \times 10^4 \mathrm{~Hz}, 4.67 \times 10^4 \mathrm{~Hz}$
• C

  $4.5 \times 10^4 \mathrm{~Hz}, 5.4 \times 10^4 \mathrm{~Hz}$
• D

  $4.67 \times 10^3 \mathrm{~Hz}, 4.34 \times 10^4 \mathrm{~Hz}$

Solution

Frequency received by bat after reflection from wall (1). $\begin{aligned} t_2 & =f\left[\frac{v-u}{v+u}\right] \\ & =4.5 \times 10^4\left[\frac{330-6}{300+6}\right] \\ & =4.34 \times 10^4 \mathrm{~Hz}\end{aligned}$

Hence, the correct answer is option: 'B'.