Galactic Redshift (CIE IGCSE Physics)

• 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

When a light source moves away from an observer, redshift is observed, and when a light source moves towards an observer, blueshift is observed

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

Redshift of light from a distant galaxy

Light emitted from glowing hydrogen in distant objects appears to be shifted towards the red end of the spectrum, showing they are moving away from Earth

• The greater the observed redshift: 
  • the greater the distance to a galaxy
  • the faster the galaxy is moving away from Earth