The Photon (Cambridge International A-Level Physics)

The Particle Nature of Light

• In classical wave theory, electromagnetic (EM) radiation is assumed to behave as a wave
• This is demonstrated by the fact EM radiation exhibits phenomena such as diffraction and interference
• However, experiments from the last century, such as the photoelectric effect and atomic line spectra, can only be explained if EM radiation is assumed to behave as particles
• These experiments have formed the basis of quantum theory, which will be explored in detail in this section

The 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

  • They are sometimes referred to as 'particles of light'

• 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

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