Question

A bat emits an ultrasonic sound of frequency $1000\text{kHz}$ in air. If this sound wave is incident on water surface, what is the wavelength of the reflected sound?
Speed of sound in air is $340\text{m/s}$ and in water $1486\text{m/s}$. $\left[2\text{marks}\right]$

Answer 1

Frequency of the ultrasonic sound, $f=1000\text{kHz}={10}^6\text{Hz}$
The frequency does not change with the medium but in this case the reflected wave will lie in the same plane and medium as the incident ray, so the medium of reflected light will be air.
Speed of sound in air, $v_a=340\text{m/s}$ $[$given$]$
The wavelength $\left({\lambda }_r\right)$ of the reflected sound is given by the relation:
${\lambda }_r=\frac{v_a}{f}$
$=\frac{340}{{10}^6}=3.4\times {10}^{-4}\text{m}$
$\left[2\text{mark}\right]$