Red Shift — Light from a Moving Source

🚀 Red Shift — Light from a Moving Source

Figure 1: The Doppler effect for light — a source moving away produces red shift (longer wavelengths), a source moving towards you produces blue shift (shorter wavelengths)

Red shift is the light equivalent of the Doppler effect — the change in observed frequency or wavelength of a wave when the source and observer are moving relative to each other.

How Red Shift Works

When a galaxy moves away from us, the light waves it emits are stretched as they travel through space — their wavelength increases. Longer wavelengths in the visible spectrum appear red, so this stretching is called red shift.

When a galaxy moves towards us, light waves are compressed — their wavelength decreases. Shorter wavelengths appear blue, so this is called blue shift (only observed in galaxies very close to us, like the Andromeda Galaxy, which is heading towards the Milky Way).

How We Detect Red Shift

Scientists detect red shift by analysing the absorption spectrum of starlight. Every element absorbs specific wavelengths of light, creating dark lines at known positions in the spectrum. When a galaxy is red-shifted, all these lines are shifted towards the red (longer wavelength) end of the spectrum by the same proportion. By measuring how far the lines have shifted, scientists can calculate the galaxy's velocity.