Redshift

From Slackerpedia Galactica

When electromagnetic radiation is shifted down in frequency due to the Doppler Effect. The amount of redshift is dependent upon the velocity away from the observer of the object in question. If the object is approaching, it is shifted up in frequency and is called blueshift. It doesn't have to be just visible light that is shifted in frequency; radio, ultraviolet, infrared, X-rays, gamma rays, and manta rays (ok, not manta rays) can be shifted too.

There is a whole lot of math involved to determine just how much the light is shifted. It gives me a headache just thinking about it. If you can't live without knowing it, check out the Wikipedia listing. They even have pretty pictures.