CIE A Level Physics (9702) 2019-2021

Revision Notes

27.3.2 Line Spectra

Line Spectra

  • Line spectra is a phenomenon which occurs when excited atoms emit light of certain wavelengths which correspond to different colours
  • The emitted light can be observed as a series of coloured lines with dark spaces in between
    • These series of coloured lines are called line or atomic spectra
  • Each element produces a unique set of spectral lines
  • No two elements emit the same set of spectral lines, therefore, elements can be identified by their line spectrum
  • There are two types of line spectra: emission spectra and absorption spectra

Emission Spectra

  • When an electron transitions from a higher energy level to a lower energy level, this results in the emission of a photon
  • Each transition corresponds to a different wavelength of light and this corresponds to a line in the spectrum
  • The resulting emission spectrum contains a set of discrete wavelengths, represented by coloured lines on a black background
  • Each emitted photon has a wavelength which is associated with a discrete change in energy, according to the equation:

Line Spectra equation

  • Where:
    • ΔE = change in energy level (J)
    • h = Planck’s constant (J s)
    • f = frequency of photon (Hz)
    • c = the speed of light (m s-1)
    • λ = wavelength of the photon (m)
  • Therefore, this is evidence to show that electrons in atoms can only transition between discrete energy levels

Absorption Spectra

  • An atom can be raised to an excited state by the absorption of a photon
  • When white light passes through a cool, low pressure gas it is found that light of certain wavelengths are missing
    • This type of spectrum is called an absorption spectrum
  • An absorption spectrum consists of a continuous spectrum containing all the colours with dark lines at certain wavelengths
  • These dark lines correspond exactly to the differences in energy levels in an atom
  • When these electrons return to lower levels, the photons are emitted in all directions, rather than in the original direction of the white light
    • Therefore, some wavelengths appear to be missing
  • The wavelengths missing from an absorption spectrum are the same as their corresponding emission spectra of the same element

 

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