Overview
This lecture covers the concept of wave-particle duality, focusing on light's behavior as both a wave and a particle, and explains key discoveries such as the photoelectric effect.
Wave Nature of Light
- Light was historically known to behave as an electromagnetic wave, also called electromagnetic radiation.
- Light waves are characterized by wavelength (distance between peaks) and frequency (waves passing per second).
- Wavelength and frequency are inversely proportional; shorter wavelengths have higher frequencies.
- The speed of light (c) equals wavelength multiplied by frequency, c ≈ 3.00 × 10⁸ m/s.
- The electromagnetic spectrum ranges from gamma rays (short wavelength) to radio waves (long wavelength); visible light is a small section.
The Photoelectric Effect
- The wave theory could not explain why light could eject electrons from metal only at certain frequencies.
- The photoelectric effect: light above a threshold frequency causes electron ejection, regardless of intensity.
- Below the threshold frequency, no electrons are ejected, no matter how intense the light.
Quantum Theory and Photons
- Max Planck proposed that energy is quantized, existing in discrete units called quanta.
- Albert Einstein extended this idea, proposing that light is made of quanta called photons (particles of light).
- A photon can eject an electron if it has sufficient energy, explaining the photoelectric effect.
- Photon energy depends on frequency and is given by the equation: Energy = frequency × Planck's constant.
Wave-Particle Duality
- Light exhibits both wave-like and particle-like properties, a concept known as wave-particle duality.
- This discovery marked the beginning of quantum theory, changing physics fundamentally.
Key Terms & Definitions
- Wavelength — Distance between successive peaks of a wave.
- Frequency — Number of wave cycles passing a point per second.
- Photoelectric Effect — Ejection of electrons from a metal when exposed to light above a specific frequency.
- Photon — A quantum (particle) of light carrying energy dependent on its frequency.
- Quanta — Discrete packets of energy.
- Planck's Constant (h) — The proportionality constant relating photon energy to frequency.
Action Items / Next Steps
- Review the photoelectric effect and solve practice problems related to photon energy.
- Read about the electromagnetic spectrum and Planck's quantum theory.