When sound waves travel from air to water, which of the following quantities remains unchanged?

When sound waves travel from air to water, the frequency of the wave remains unchanged. This is because frequency is determined by the source of the sound, not by the medium through which the sound travels. For instance, if a tuning fork vibrates at 440 Hz in air, it will continue to vibrate at 440 Hz when the sound passes into water. However, other properties such as speed, wavelength, and intensity may change depending on the medium’s density and elasticity.

The speed of sound depends on the medium’s physical properties — primarily its density and elasticity. Water is denser and more elastic than air, so sound travels faster in water (around 1,480 meters per second) than in air (approximately 343 meters per second). This increase in speed also affects the wavelength of the sound wave. Since the relationship between wave speed (v), frequency (f), and wavelength (λ) is given by the equation:

v = f × λ,

if the speed increases and the frequency remains constant, the wavelength must increase as well. Therefore, when sound enters water from air, its wavelength becomes longer.

The intensity of sound may also decrease when transitioning between media because some energy is reflected at the boundary due to differences in acoustic impedance. This is why a sound heard underwater can seem quieter even if its frequency is the same.

In summary, when sound travels from air to water:

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  Speed changes (increases in water)

  
  


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  Wavelength changes (increases in water)

  
  


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  Intensity may change (due to reflection or absorption)

  
  


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  Frequency remains constant

  
  

Thus, frequency is the only property of a sound wave that does not change when moving from one medium to another, making it the correct answe