The Photon (OCR A Level Physics)

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Katie M

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Katie M

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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

Examiner Tip

Make sure you learn the definition for a photon: discrete quantity / packet / quantum of electromagnetic energy are all acceptable definitions.

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:

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 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

Worked example

Light of wavelength 490 nm is incident normally on a surface, as shown in the diagram.The power of the light is 3.6 mW. The light is completely absorbed by the surface.Calculate the number of photons incident on the surface in 2.0 s.

2.5.1 The Photon Model Worked Example

Examiner Tip

The values of Planck’s constant and the speed of light will always be available on the datasheet, however, it helps to memorise them to speed up calculation questions!Since Planck's constant is in J s, you may need to convert from eV to J and vice versa for the energy

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.