Line Spectra (Cambridge (CIE) A Level Physics): Revision Note

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:

$∆E=hf=\frac{hc}{\lambda }$

• 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

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

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