What the difference between red shift and blue shift?

Answer 1

The frequency of light from a star is reduced (its wavelength increased) when the star is moving away from the observer. Red light has longer wavelengths than blue light. So the light from a star which is moving away from the earth appears to be redder than it actually was. This phenomenon is similar to the pitch of an ambulance siren dropping as it passes you and is moving away. The true spectrum for light from a star depends on the temperature of the star, but there are characteristic absorption lines (dark lines) in the spectrum which are specific to chemical elements which are present in the outer regions of the stars - such as hydrogen and helium. Light from within the star is absorbed by the atoms of these elements when they move from a low-energy state to one with a higher energy level. A comparison of where in the spectrum these lines appear to be against where they should be (for a stationary) star are a measure of the red-shift (or blue shift).
A red shift is observed if an object is moving away from the earth while a blue shift is observed if it is approaching the earth.