Edexcel A Level (IAL) Physics-2.30 The Particle Nature of EM Radiation- Study Notes- New Syllabus

The Photoelectric Effect as Evidence for the Particle Nature of Electromagnetic Radiation

The photoelectric effect is the process where electrons are emitted from a metal surface when electromagnetic radiation shines on it. This effect cannot be explained by classical wave theory, but is fully explained by the photon (particle) model of light. Thus, the photoelectric effect provides strong evidence that electromagnetic radiation behaves as particles.

 Key Observations of the Photoelectric Effect

• Electrons are emitted immediately when light strikes the metal.
• There exists a threshold frequency \( f_0 \): no electrons are emitted below this frequency, regardless of intensity.
• The maximum kinetic energy of emitted electrons depends on frequency, not intensity.
• The number of electrons emitted depends on intensity (when above threshold).

These results directly contradict the predictions of wave theory.

Why the Wave Theory Fails

• If light were a wave:
  • Increasing intensity should always cause emission (more energy delivered).
  • Emission should be delayed at low intensities (time needed to build up energy).
  • Frequency should not affect emission.

None of these predictions match experimental observations.

Photon Model Explanation

• Light consists of discrete particles called photons.
• Each photon has energy:

\( E = hf \)

• A single photon interacts with a single electron (1:1 interaction).
• If the photon has energy less than the work function \( \phi \), the electron cannot escape.
• If \( hf \ge \phi \), the electron is emitted with kinetic energy:

\( \dfrac{1}{2}mv_{\text{max}}^{2} = hf – \phi \)

This perfectly matches all experimental data.

 How the Photoelectric Effect Proves Particle Nature

• Immediate emission → energy must be delivered in discrete packets, not spread out like a wave.
• Electron emission depends on frequency → photon energy depends on frequency \( (E = hf) \).
• Intensity affects number of electrons → more photons per second, not more energy per photon.
• Threshold frequency exists → a minimum photon energy is required.

All of these observations support the photon model and contradict wave theory.

 Conclusions

• Light behaves as particles (photons) when interacting with electrons.
• The photoelectric effect was one of the first pieces of evidence for quantum theory.
• Einstein’s explanation earned him the Nobel Prize in Physics (1921).