2.6 physics

• Line spectra


  Produced to help identify elements present in stars
• Emission of light


  1. When substances are very hot, they emit light

  2. Light can be viewed through a diffraction grating as a continuous spectrum

  3. Hot gases don't emit a whole spectrum, only specific frequencies or wavelengths

  4. Using knowledge of which frequencies correspond to which elements, we can identify the composition of the gas in a star
• Emission spectra


  • A line emission spectrum, produced from light emitted by an element in a hot gas, is specific to that element
  • A hot gas cloud will release an emission spectrum comprising a series of light frequencies collating the emission spectra of all the individual elements contained within the cloud
  • Line emission spectra act like a chemical fingerprint
• Absorption spectra


  • When a gas cools, it absorbs the same frequencies of light that it would emit if hot
  • If a continuous spectrum of light is passed through a cool gas, absorbed frequencies appear as dark lines as they are 'taken in' by the gas cloud between the observer and the light source
• Using absorption spectra


  Passing a continuous spectrum of light through a cool gas and observing the absorption spectrum produced allows us to figure out what elements are present in the gas
• In the early 19th century, Joseph von Fraunhofer invented the spectroscope allowing absorption spectra to be observed for the first time
• Doppler effect


  If a sound source is moving relative to an observer, the wavelength of the emission (pitch) heard by the listener will be compressed or extended depending on whether the source is moving towards or away from the listener
• Red shift


  As the planet moves away from the observer, the light wavelength is stretched so the light changes to be closer to the red end of the spectrum
• Blue shift


  As the planet moves towards the observer, the light wavelength is compressed so the light changes to be closer to the blue end of the spectrum
• Cosmological red shift was first hypothesised and observed by Edward Hubble in the 20th century
• Cosmological red shift


  • Hubble observed the absorption spectra of many different galaxies and saw that they displayed the same relative pattern as that for our sun, but the absorbed lines appeared to be red shifted
  • This means these galaxies must be moving away from the Milky Way and provides evidence for an expanding universe
• Hubble's law states that the further away the galaxy is, the more red shifted the emitted light is
• Hubble's constant


  The gradient of the line on the graph showing the correlation between the velocity of recession and distance from Earth
• The reciprocal of Hubble's constant can be used to estimate the age of the universe, which Hubble estimated to be ~14 billion years old
• Cosmic Microwave Background Radiation (CMBR)


  • Radiation from the Big Bang explosion that is still detectable today as microwave radiation
  • The CMBR provides good evidence for a continually expanding universe, as the high energy, short wavelength gamma radiation released in the initial explosion has red-shifted into the microwaves detectable today