What happens to a sound wave as it travels from air into water? (a) Its intensity increases. (b) Its wavelength decreases. (c) Its frequency increases. (d) Its frequency remains the same. (e) Its velocity decreases.
Answer (d). When a sound wave travels from air into water, several properties will change. The wave speed will increase as the wave crosses the boundary into the water causing the spacing between crests (the wavelength) to increase, because crests move away from the boundary faster than they move up to the boundary. The sound intensity in the water will be less than it was in air because some sound is reflected by the water surface. However, the frequency (number of crests passing each second) will be unchanged, since a crest moves away from the boundary every time a crest arrives at the boundary.