IB Physics SL

Revision Notes

7.1.1 Discrete Energy Levels

Discrete Energy Levels

The Photon Model

  • Photons are fundamental particles that make up all forms of electromagnetic radiation
  • A photon is a massless “packet” or a “quantum” of electromagnetic energy
  • What this means is that the energy is not transferred continuously but as discrete packets of energy
  • In other words, each photon carries a specific amount of energy, and transfers this energy all in one go, rather than supplying a consistent amount of energy

Calculating Photon Energy

  • The energy of a photon can be calculated using the formula:

E = hf

  • Using the wave equation, energy can also be equal to:

2.5.1 Photon Energy Equation

  • Where:
    • E = energy of the photon (J)
    • h = Planck's constant (J s)
    • c = the speed of light (m s-1)
    • f = frequency (Hz)
    • λ = wavelength (m)
  • This equation tells us:
    • The higher the frequency of EM radiation, the higher the energy of the photon
    • The energy of a photon is inversely proportional to the wavelength
    • A long-wavelength photon of light has a lower energy than a shorter-wavelength photon

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


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