But the car's movement causes the waves to be crowded together, a shorter wavelength . The car is moving away from the previously emitted sound waves. You've all undoubtably experienced this effect. A car or plane or train zipping past you makes sounds that shift from a higher pitch to lower as it passes by. This shifting of sound is called the Doppler Shift .
Light is affected in the same way.
You never see these color shifts in everyday life, but most astronomical objects are moving fast enough for a measurable shift. Not only does the Doppler shift tell us whether the source is moving towards or away from us, but the amount of shift can be used to calculate the speed. Again, we will skip that mathematical part of the text. End Chapter 3.