what exactly is the blue and red shift? what happens? why does this happens?
In astronomy, blue shift is when the frequency of light received is higher than the frequency at which it was emitted (in the reference frame of the emitter). Redshift is when the frequency is lower.
This can be due to Doppler shift when the emitter and receiver are moving relative to each other, similarly to how an approaching object sounds higher in pitch and a receding object sounds lower. This is because the emitter is either catching up with or lagging behind each wavefront that it emits, making the waves closer together or farther apart.
In general relativity, shifting of light can also occur because of gravitational time dilation (gravity influencing the rate at which wavefronts are emitted), or from the expansion of space itself stretching light and redshifting it.
Welcome to Physics Forums, alchemist!
Well said, Ambitwistor.
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