Young’s double-slit experiment is a powerful demonstration of the wave-particle duality of light — the idea that light behaves both as a wave and as a particle, depending on how it is observed.
Wave Nature:
- When a beam of light (or even single photons) passes through two slits and falls on a screen, it produces an interference pattern of bright and dark fringes.
- This pattern occurs due to constructive and destructive interference, which is a characteristic of waves.
- Even when light is sent one photon at a time, over time the pattern still forms — suggesting that each photon interferes with itself, as if it traveled through both slits simultaneously like a wave.
Particle Nature:
- When light intensity is extremely low and photons are detected individually, they strike the screen as discrete particles (small dots).
- These dots are recorded one by one, but collectively they build up the same interference pattern, revealing the underlying wave behavior.
Duality Revealed:
- The individual detection events show light is made of particles (photons).
- The interference pattern shows that those photons exhibit wave behavior.
This coexistence of wave and particle characteristics is what defines the wave-particle duality of light, and Young’s experiment beautifully demonstrates both aspects in a single setup.