22.3 Wave-Particle Duality

2026 📋 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Understand that the photoelectric effect provides evidence for the particle nature of electromagnetic radiation, while interference and diffraction provide evidence for its wave nature
  2. Describe and interpret the evidence provided by electron diffraction for the wave nature of particles
  3. Understand the de Broglie wavelength as the wavelength associated with a moving particle
  4. Recall and use the equation λ = h / p

📖 Main Content

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1. The Dual Nature of Light — Waves AND Particles

For a long time, scientists argued about whether light is a wave or a particle. The surprising answer is: it's both, depending on how it behaves in a given situation. This idea is called wave-particle duality — the concept that something can show both wave-like and particle-like behaviour.

Light is a type of electromagnetic radiation (EMR) — energy that travels through space as waves. But light can also behave as tiny packets of energy called photons (like tiny "bullets" of light).

2. Evidence for the WAVE Nature of Electromagnetic Radiation

When light behaves as a wave, it shows two key behaviours:

🌊 Diffraction

  • Diffraction is when a wave spreads out after passing through a narrow gap or around an obstacle.
  • For example, when light passes through a very thin slit, it spreads out on the other side instead of travelling in a straight beam.
  • Only waves can do this — particles would simply pass straight through or be blocked.

🌊 Interference

  • Interference is when two waves overlap and either reinforce each other (making a brighter spot) or cancel each other out (making a dark spot).
  • This creates a pattern of alternating bright and dark bands or rings, called an interference pattern.
  • A classic demonstration is Young's Double Slit experiment, where light passing through two slits creates exactly this kind of alternating pattern on a screen.
  • Only waves can interfere — this cannot be explained if light were simply a stream of particles.

Key point: Diffraction and interference are wave behaviours. These experiments prove that light has a wave nature.

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