Emission & Absorption Spectrum (DP IB Physics) : Revision Note

Spectra & Atomic Energy Levels

• Atomic spectra are observed when atoms emit or absorb light of certain wavelengths

  • These are known as emission spectra and absorption spectra

• Atomic spectra provide evidence that electrons in atoms can only transition between discrete atomic energy levels

Emission Spectra

• Emission spectra can be produced by heating a low-pressure gas

  • Heating provides energy to excite electrons to higher energy levels

  • When an electron transitions back to a lower energy level, it emits 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

Emission spectrum of hydrogen gas

A typical hydrogen emission spectrum contains several spectral lines in the visible region of the electromagnetic spectrum

Absorption Spectra

• Absorption spectra can be produced by passing white light through a cool, low-pressure gas

  • Only photons with the exact energy required to excite electrons will be absorbed

• Each absorbed photon corresponds to a specific wavelength of light which correlates to an observable dark line in a continuous spectrum of wavelengths

• The resulting absorption spectrum contains a set of discrete wavelengths, represented by dark lines on a coloured background

  • These lines correspond to the same lines observed on an emission spectrum for the same element

Absorption spectrum of hydrogen gas

A typical hydrogen absorption spectrum is the inverse of its emission spectrum

Spectra & Chemical Composition

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

• Each element produces a unique pattern of spectral lines

• No two elements produce the same set of spectral lines, therefore, elements can be identified by their atomic spectrum

Emission spectra of different elements

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