moving toward the Earth appears to have shorter wavelengths (and therefore seems more blue), while light from objects moving away from the Earth appears to have longer wavelengths (and therefore seems more red).
Example:
A car traveling at 136km/h is being chased by a police car sounding its siren, which emits sound with a frequency of 1300Hz. The police car is traveling at 161km/h. Take the speed of sound to be 331m/s. Suppose the driver jumps out of the car (miraculously remaining uninjured) and watches as the police car speeds by. What does the frequency of the siren now appear to be to the driver?
