Suppose ocean waves are hitting a shore at a frequency of 20 waves per minute. Two swimmers are in the water. One swimmer says the frequency is 25 waves per minute and the other says the frequency is 15 waves per minute. How can the Doppler effect explain this apparent difference?

Answer and explanation; One swimmer is swimming out to sea and is moving against the direction the waves are traveling, which causes the swimmer to encounter a wave 25 times per minute. The other swimmer is swimming toward shore in the same direction the waves are traveling, so that swimmer encounters waves less frequently. Therefore; swimmer who sees 25 waves a minute is swimming toward the waves, while the other swimmer is swimming with the waves. As you move toward a wave, the Doppler effect makes the frequency seen faster, and visa versa.