Emission & Absorption Spectrum (DP IB Physics: SL)

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:

• 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

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

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 most energy since they have the shortest wavelength
• The Pfund series photons will have the least energy since they have the longest wavelength

Electron jumps in the hydrogen spectrum

• The discovery of these electron jumps helped scientists to understand how the movement of electrons is able to produce photons of specific wavelength and energy