Question

A bat is emitting the ultrasonic sound of frequency $100\text{kHz}$ in air. What will be the wavelength of sound heard by a person inside a lake over which bat is flying?
[speed of sound in air $=340{\text{ms}}^{-1}$ and speed of sound in water $=1500{\text{ms}}^{-1}$]

• A. $15\text{mm}$
• B. $20\text{mm}$
• C. $25\text{mm}$
• D. $30\text{mm}$

Answer 1

The correct option is A $15\text{mm}$

When the sound wave travelling in air enters water, its frequency does not change.
We know that velocity of sound wave is given by,
$v=\lambda f$, where $\lambda$ is wavelength and $f$ is frequency.
$\Rightarrow \lambda =\frac{v}{f}$
$\Rightarrow \lambda =\frac{1500}{100\times {10}^3}=0.015\text{m}=15\text{mm}$