Cosmological Redshift
- Usually, when an object emits waves, they spread out symmetrically
- If the source of the waves moves, the waves can become squashed together or spread out
- If a source of light moves towards an observer, the waves become squashed, causing the wavelength to decrease
- The wavelength of light shifts towards the blue end of the spectrum
- This is called blueshift
- If a source of light moves away from an observer, the waves spread out, causing the wavelength to increase
- The wavelength of light shifts towards the red end of the spectrum
- This is called redshift
Redshift and Blueshift of Light
When light moves away from an observer, they observe redshift and when light moves towards an observer, they observe blueshift
- When astronomers compare the absorption spectra of light from distant galaxies with spectra from nearby objects, such as the Sun, they observe the dark lines are shifted towards the red end of the spectrum
- This means the wavelength has increased as the light travelled from the galaxy to the Earth
- This shows that distant stars and galaxies are moving away from the Earth
Redshift of Light from a Distant Galaxy
Comparing the absorption spectra produced by the Sun and a distant galaxy shows the light from the distant galaxy has been shifted towards red wavelengths (it has stretched) due to it moving away
Examiner Tip
You need to know that in the visible light spectrum, red light has the longest wavelength and the smallest frequency.
Absorption spectra usually display wavelength as increasing from left to right, so the red end (longer wavelength) is usually on the right and the violet end (shorter wavelength) is usually on the left
