Emission & Absorption Spectrum (SL IB Physics)

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 specific wavelength of light which correlates to an observable spectral line

• 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

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 one side of a cool, low-pressure gas it is found that certain wavelengths of light are missing when detected out the other side
  • This type of spectrum is called an absorption spectrum

• An absorption spectrum consists of a continuous spectrum of all wavelengths (i.e. white light) with dark lines where specific wavelengths have been absorbed by the gas
  • These dark lines correspond exactly to the differences in energy levels in an atom

• When electrons return to lower energy 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 correspond to the coloured lines on an emission spectrum for the same element

Absorption spectrum of hydrogen gas

• The chemical composition of a substance can be investigated using emission and absorption 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

Emission line spectra are unique to each element, like a fingerprint

• For example:
  • Hydrogen is known to produce strong spectral lines in the red portion of the visible spectrum, at 656 nm
  • When sodium is burned, a characteristic yellow flame is observed due to it producing strong spectral lines in the yellow portion of the spectrum, at 589 nm
  • When mercury is burned, most of the emission lines are below 450 nm, which produces a characteristic blue light

• Elements such as sodium and mercury are known for their use in street lights, as well as neon for its use in colourful signs
• This can be achieved when
  • An electrical discharge is applied to the vapourised substance
  • The energy supplied excites orbital electrons within individual atoms to a higher energy state
  • When the electrons move back down to the ground state, a specific wavelength of light is emitted