Light behaves like a particle (a stream of tiny packets called photons) and also like a wave (ripples that can interfere and diffract). This dual nature is key to modern physics and is tested by two classic experiments: the photoelectric effect and interference/diffraction patterns.
⚡️ When light shines on a metal surface, electrons are ejected. The energy of each electron depends on the light’s frequency, not its intensity. This shows that light comes in discrete packets (photons) with energy \$E = h\nu\$.
This behaviour cannot be explained by waves alone; it requires photons, each carrying a fixed amount of energy.
🌊 When light passes through two close slits, it creates a pattern of bright and dark fringes on a screen. This pattern is produced by the constructive and destructive interference of waves.
The spacing of the fringes depends on the wavelength: \$d \sin \theta = m\lambda\$, where \$d\$ is slit separation, \$\theta\$ is the angle, \$m\$ is an integer, and \$\lambda\$ is the wavelength. This is a classic wave equation.
When light passes a narrow slit or around an obstacle, it spreads out and forms a diffraction pattern. The pattern’s shape is governed by the wave nature of light, not by particles.
| Aspect | Particle Evidence (Photoelectric Effect) | Wave Evidence (Interference/Diffraction) |
|---|---|---|
| Energy Transfer | \$E = h\nu\$ – depends on frequency | Depends on wavelength \$\lambda\$ – interference pattern |
| Intensity Effect | More photons → more electrons, but same energy per electron | Higher intensity → brighter fringes, same fringe spacing |
| Threshold | Requires minimum frequency | No threshold – any wavelength shows diffraction |