The Photon (OCR A Level Physics)

The Photon Model

• Light waves can behave like particles (i.e. photons) and waves

  • This phenomenon is called the wave-particle nature of light or wave-particle duality

• Light interacts with matter, such as electrons, as a particle

  • The evidence for this is provided by the photoelectric effect

• Light propagates through space as a wave

  • The evidence for this comes from the diffraction and interference of light in Young’s Double Slit experiment

Light as a Particle

• The photon model of light explains that:

  • Electromagnetic waves carry energy in discrete packets called photons

  • The energy of the photons are quantised according to the equation E = hf

  • In the photoelectric effect, each electron can absorb only a single photon - this means only the frequencies of light above the threshold frequency  will emit a photoelectron

• Although the wave theory provided good explanations for phenomena such as interference and diffraction, it failed to explain the photoelectric effect

Defining the Photon

• Photons are fundamental particles which make up all forms of electromagnetic radiation

• A photon is defined as:

  A massless “packet” or a “quantum” of electromagnetic energy

• This means 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

Energy of a Photon

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

E = hf

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

• 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 shows:

  • 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