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Because of the Doppler effect, light from a source that is fast moving away from the observer (e.g. another galaxy) appears lower in frequency than it does at the source. Since red is at the low end of the spectrum, this is called "red shift"; white light would appear red.

Conversely, light from sources moving towards the observer undergoes "blue shift".


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Doppler Effect:

This is easily demonstrated when thinking about when you hear an ambulance coming towards you with the siren going. As it's approaching you, the pitch of the siren is high, and as it passes you it lowers in frequency/tone. This is due to Doppler shifting, and it works the same way with light. So for something that's approaching, the waves are coming at you at a higher frequency than if it's going away from you. Another way to illustrate this is by imagining a drop falling into water, and then the ripples which will then flow in all directions. If the drop hits the water each second, then the ripples will be 1 second apart, yet if you were to...lets say move towards the area where the drop hits the water, the ripples will hit you more frequently due to your speed at which you are moving...and the same goes for sound and light.

So stars which are approaching us, will have it's light waves hit us at a higher frequency, and shift it to the blue spectrum, and those moving away from us will have it's wave lengths increased an thus towards the red spectrum.

So redshifting is the term used to say that a planet is having it's light waves lengthened due to the Doppler Effect, towards the red spectrum due to moving away from us.

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