Overview
This lecture explores the mysterious nature of light, its dual wave-particle behavior, key physics experiments, and what this reveals about the weird, probabilistic foundation of our universe.
The Nature of Light
- Light delivers energy and enables vision on Earth.
- Isaac Newton theorized in the 1700s that light consists of particles called corpuscles.
- Thomas Young’s 1801 double-slit experiment showed light acts like a wave, creating an interference pattern.
Wave Behavior and Interference
- Waves oscillate and can constructively (add up) or destructively (cancel out) interfere.
- Light passing through two slits creates interference patterns, proving it behaves like a wave.
The Photoelectric Effect and Particle Behavior
- In 1905, scientists noticed that shining light on metal ejects electrons (the photoelectric effect).
- Increasing light’s intensity only increased the number of electrons, not their speed.
- Increasing light’s frequency (not intensity) increased the ejected electrons’ speed.
- Albert Einstein explained this by proposing light is made of energy packets called photons.
Wave-Particle Duality and Observation
- Modern experiments can send single photons through slits; each behaves like a particle at detection.
- Multiple single photons still form an interference pattern, suggesting each photon interferes with itself as a wave.
- If detectors check which slit the photon passes through, the interference pattern disappears.
- Light’s behavior changes when it is observed or measured—a phenomenon called wavefunction collapse.
Quantum Probability and the Polariser Paradox
- Polarizing lenses only let photons with the correct orientation pass; photons "snap" to an allowed orientation when measured.
- Adding a third polarizer at an angle between two perpendicular ones allows some light through, revealing photon behavior is probabilistic, not fixed.
Discreteness and Quantum Nature
- Light exists in discrete amounts (quanta); you can't have half a photon.
- Light and all matter at small scales follow probabilistic quantum rules, not predictable classical laws.
Matter and Energy: Universal Quantum Behavior
- Not just light, but all matter (like electrons and atoms) exhibit wave-like, probabilistic behavior at the quantum scale.
- Reality itself is fundamentally uncertain and governed by probability, not absolutes.
Key Terms & Definitions
- Photon — A discrete packet (quantum) of light energy.
- Wave-Particle Duality — The concept that light and matter exhibit both wave-like and particle-like properties.
- Interference Pattern — A striped pattern created when waves overlap, indicating constructive and destructive interference.
- Photoelectric Effect — Ejection of electrons from a material when it absorbs light of sufficient frequency.
- Polariser — A filter that only allows light waves of a certain orientation to pass through.
- Wavefunction Collapse — The process by which a quantum system becomes a definite state upon observation.
Action Items / Next Steps
- Review key quantum experiments: double-slit and photoelectric effect.
- Practice explaining wave-particle duality and the significance of observation in quantum mechanics.