Galactic Redshift (Cambridge (CIE) IGCSE Physics): Revision Note

Galactic redshift

• When a stationary object emits waves, they spread out symmetrically

• If the source of the waves moves, the waves can become squashed together or spread out

• This change in wavelength is known as the Doppler effect

• This effect applies to both sound waves and electromagnetic radiation (i.e. light waves)

Doppler effect of light

• If a source of light moves towards an observer, the observed wavelength decreases

  • 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 observed wavelength increases

  • The wavelength of light shifts towards the red end of the spectrum

  • This is called redshift

Redshift and blueshift of light

Observing redshift in distant galaxies

• When astronomers compare light from glowing hydrogen in distant galaxies with light from glowing hydrogen on Earth, the light appears to be redshifted

• This means the observed wavelength has increased as the light travelled from the galaxy to the Earth

• This shows that distant stars and galaxies are moving away (receding) from the Earth (opens in a new tab)

Redshift of light from a distant galaxy

• The greater the observed redshift: 

  • the greater the distance to a galaxy

  • the faster the galaxy is moving away from Earth