why does the frequency change in the doppler effect?

Answer 1

Because as the source moves toward you it is catching up to the sound waves that have already left. This makes the next wave closer to the last so the pitch is higher. As it moves away the waves get farther apart so not as many reach you per second sounding as a lower pitch.

Answer 2

It is important to realize that the frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in a man's direction. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The converse is true if the thrower is moving away from the man. So it is actually the wavelength which is affected; as a consequence, the perceived frequency is also affected. It may also be said that the velocity of the wave remains constant whereas wavelength changes; hence frequency also changes.