# How the Doppler Effect Works

## Wave Frequency

Let's begin our dissection of the Doppler effect by considering a source that creates waves in water at a certain frequency. This source produces a series of wave fronts, with each moving outward in a sphere centered on the source. The distance between wave crests -- the wavelength -- will remain the same all the way around the sphere. An observer in front of the wave source will see the waves equally spaced as they approach. So will an observer located behind the wave source.

Now let's consider a situation where the source is not stationary, but is moving to the right as it produces waves. Because the source is moving, it begins to catch up to the wave crests on one side while it moves away from the crests on the opposite side. An observer located in front of the source will see the crests all bunched up. An observer located behind the source will see the waves all stretched out. Remember, the frequency equals the number of waves that pass a specific point per second, so the observer in front actually sees a higher frequency than the observer in back of the source.