Question

An underwater sonar source operating at a frequency of $60 \,kHz$ directs its beam towards the surface. If velocity of sound in air is $330 \,ms^{-1}$, wavelength and frequency of the waves in air are

• A. 5.5 mm, 60 kHz
• B. 330 m, 60 kHz
• C. 5.5 mm, 30 kHz
• D. 5.5 mm, 80 kHz

Answer 1

Answer: (A)

When wave travels from one medium to another, its velocity changes while frequency remains same.
Also, there is change in intensity of wave.
Hence, frequency of sonar will be $60 \,kHz$.
Also, $n=\frac{v}{\lambda}$
where $n$ is frequency, $v$ is speed of sound in air,
and $\lambda$ is wavelength.
$\therefore \lambda=\frac{v}{n}=\frac{330}{60 \times 10^{3}}$
$=5.5 \times 10^{-3} m $
$=5.5 \,mm$