A source emits sound of frequency 600 Hz inside water. The frequency heard in air (velocity of sound in water is 1500 m/s and velocity of sound in air is 300 m/s) will be:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 600 Hz
The frequency of a wave is fixed by its source and does not change when the wave passes from one medium into another. Only the speed and the wavelength change, and they change in the same ratio so that \( v = f\lambda \) holds in each medium. The source and the listener are at rest, so there is no Doppler shift.
\( \displaystyle \lambda_{\mathrm{water}} = \frac{v_{\mathrm{water}}}{f} = \frac{1500}{600} = 2.5\,\mathrm{m} \)
\( \displaystyle \lambda_{\mathrm{air}} = \frac{v_{\mathrm{air}}}{f} = \frac{300}{600} = 0.5\,\mathrm{m} \)
The wavelength falls from 2.5 m to 0.5 m, in the ratio of the speeds 5 : 1, while the frequency stays at 600 Hz.