Emission Spectra (Cambridge (CIE) A Level Physics): Revision Note

Emission spectra

• Astronomers are very limited in how they can investigate objects in space

• All of the techniques used involve analysing the light emitted from the star, or galaxy

• One of these techniques involves analysing the emission and absorption spectra of stars

  • More details on this can be found in the revision note on Line Spectra

• Elements in the star, predominantly hydrogen and helium, absorb some of the emitted wavelengths

• Therefore, characteristic lines are present when the spectrum is analysed

Redshift on an emission spectrum

The top emission spectrum shows spectral lines of hydrogen measured on Earth. The bottom emission spectrum show the shifted spectral lines of hydrogen measured from a distant galaxy

• When astronomers observe light from distant galaxies, they observe differences in the spectral lines to the light from the Sun

• The lines have the same characteristic pattern, meaning the element can still be easily identified, they just appear to be shifted slightly

  • The lines in the spectra from distant galaxies show an increase in wavelength

  • The lines are moved, or shifted, towards the red end of the spectrum