Question

A source emits sound of frequency 600 Hz inside water. The frequency heard in air will be equal to:
(velocity of sound in water = 1500 m/s, velocity of sound in air = 300 m/s)
A. 3000 Hz
B. 120 Hz
C. 600 Hz
D. 6000 Hz

Answer 1

Velocity of sound will be different in different media. If the velocity of sound is higher in one medium and lesser in another medium then the first one is called rarer medium and the second one is called denser medium. If a source or observer is moving then due to doppler effect there will be change in observed frequency.
Formula used:
$v = f \times \lambda$

Complete answer:
Frequency is nothing but the number of waves passing per second. It depends upon the source which is producing the sound
Velocity of sound($v$) is given as $v = f \times \lambda$
Where $v$ is velocity of the sound and $f$ is the frequency of the source and $\lambda$ is the wavelength of the sound wave
When a sound wave travels from one medium to the other medium as long as the source is constant its frequency remains the same. But its wavelength and the speed changes during this trip. Velocity of sound in water is greater than velocity of sound in air. From this we can deduce that sound travels faster in liquids than in gases.
Since the source is the same its frequency doesn’t change, the answer would be 600Hz.

Hence option C would be correct.

Additional information:
In case of doppler effect if source is moving towards observer, observer feels more frequency and if source moves away from observer he feels less frequency and same if observer is moving towards observer he feels more frequency and if he moves away from source he feels less frequency.

Note:
One might get confused comparing this question with the doppler effect. The Doppler effect is all about if the source or observer is moving but there is no motion for the source or observer but just the medium is changing and hence frequency remains the same.