A whistle produces a continuous note of frequency 800Hz at the surface of a swimming pool. Consider the sound as it travels first through air and then into the water of the pool.
(a) State whether the frequency of the sound changes when the wave enters water. Give reason.
(b) Compare qualitatively the wavelengths of this sound in air and in water, explaining your reasoning.
(c) Taking the speed of sound in air as 330m/s and in water as 1500m/s, calculate the ratio of the wavelength in water to that in air.
(d) Using the result obtained in part (c), explain why the pitch of the sound heard by a swimmer under water appears unchanged even though the wavelength is different.