Energy of a Photon (Edexcel International A Level Physics)

Revision Note

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Lindsay Gilmour



Energy of a Photon

  • Photons are fundamental particles which 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:


  • 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 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

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

Exam Tip

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

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!

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Lindsay Gilmour

Author: Lindsay Gilmour

Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.

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