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Line Spectra (CIE A Level Physics)

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Line spectra

  • Line spectra are 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:

increment E space equals space h f space equals space fraction numerator h c space over denominator lambda end fraction

  • 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 spectrum

hydrogen-emission-spectra

The colours on the emission spectrum refer to photons emitted from de-exciting from one level to another

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 spectrum

hydrogen-absorption-spectra

The dark lines on the absorption spectrum refer to photons absorbed from exciting from one level to another

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Ashika

Author: Ashika

Expertise: Physics Project Lead

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.