IB Physics SL

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7.1.3 Emission & Absorption Spectrum

Emission & Absorption Spectrum

Line Spectra

  • Line spectra is a phenomenon that occurs when excited atoms emit light of certain wavelengths which correspond to different colours
  • This comes from differences in discrete energy levels when electrons move between energy levels within an atom
  • 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:

2.5.2 Difference in Energy Levels 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

Hydrogen Emission Spectra, downloadable AS & A Level Physics revision notes

Emission spectrum of Hydrogen gas

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

Hydrogen Absorption Spectra, downloadable AS & A Level Physics revision notes

Absorption spectrum of Hydrogen gas

The Hydrogen Spectrum

  • A larger version of the hydrogen spectrum from the infrared to the ultraviolet region looks like this

The full hydrogen spectrum

  • In the spectrum, we can see sets or families of lines
  • Each element will have several series with electrons able to jump between specific energy levels producing specific energy photons
  • The Lyman series converges on the ground state for electrons
    • The Balmer series converges on the second energy level n = 2
    • The Ritz – Paschen converges on the third energy level n = 3 and so on
  • The Lyman series photons will have the least energy since they have the longest wavelength

Electron jumps in the hydrogen spectrum

  • The finding of these electron jumps helped scientists to understand how electrons work and produce photons of specific wavelength and energy
  • The jumps can be summarised as follows:

Electron Jumps & Energy Table


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