AQA A Level Physics复习笔记2.5.2 Energy Levels & Photon Emission

Line Spectra & Energy Levels


Atomic Energy Levels

  • Electrons in an atom can have only certain specific energies
    • These energies are called electron energy levels


  • They can be represented as a series of stacked horizontal lines increasing in energy
  • Normally, electrons occupy the lowest energy level available, this is known as the ground state
  • Electrons can gain energy and move up the energy levels if it absorbs energy either by:
    • Collisions with other atoms or electrons
    • Absorbing a photon
    • A physical source, such as heat


  • This is known as excitation, and when electrons move up an energy level, they are said to be in an excited state
  • If the electron gains enough energy to be removed from the atom entirely, this is known as ionisation
  • When an electron returns to a lower energy state from a higher excited state, it releases energy in the form of a photon


Electron energy levels in atomic hydrogen. Photons are emitted when an electron moves from a higher energy state to a lower energy state


Line Spectra

  • Line spectra is 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:



  • 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


Difference in Discrete Energy Levels

  • The difference between two energy levels is equal to a specific photon energy
  • The energy (hf) of the photon is given by:

ΔE = hf = E2 - E1

  • Where:
    • E1 = Energy of the higher level (J)
    • E2 = Energy of the lower level (J)
    • h = Planck’s constant (J s)
    • f = Frequency of photon (Hz)


  • Using the wave equation, the wavelength of the emitted, or absorbed, radiation can be related to the energy difference by the equation:


  • This equation shows that the larger the difference in energy of two levels ΔE (E2 - E1) the shorter the wavelength λ and vice versa

Worked Example

Some electron energy levels in atomic hydrogen are shown below.Worked-Example-Calculating-Discrete-Energies-e1615281726920The longest wavelength produced as a result of electron transitions between two of the energy levels is 4.0 × 10–6 m.a) Draw and mark:

  • The transition giving rise to the wavelength of 4.0 × 10–6 m with letter L.
  • The transition giving rise to the shortest wavelength with letter S.

b) Calculate the wavelength for the transition giving rise to the shortest wavelength.

Part (a)22.3.3-Calculating-Discrete-Energies-Worked-Example-Calculating-Discrete-Energies-Answer



    • Photon energy and wavelength are inversely proportional
    • Therefore, the largest energy change corresponds to the shortest wavelength (line S)
    • The smallest energy change corresponds to the longest wavelength (line L)

Part (b)2.5.2-Energy-Levels-Worked-Example