For an observer, the redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum. This phenomenon is due to

Doppler effect in physics is defined as the increase (or decrease) in the frequency of sound, light, or other waves as the source and the observer move towards (or away from) each other.

Waves emitted by a source traveling towards an observer gets compressed. In contrast, waves emitted by a source traveling away from an observer get stretched out.

Doppler effect formula:

​\(λ_{o}=λ_{s}\left ( 1+\frac{v}{c} \right )\sqrt{\frac{1}{1-\frac{v^{2}}{c^{2}}}}\)    

λ' = wavelength observed by the observer

λ = actual wavelength

v = relative velocity of the source and observer

c = velocity of the light(m/s)

The velocity v is positive for motion away from an observer and negative for motion toward an observer.