Diffraction and Wave–Particle Duality
#Presented by Arjun, Faizan, and Anirudh
Introduction
#Diffraction is the tendency of waves to spread out and bend around obstacles or through narrow openings. It is one of the clearest signatures of wave behaviour, and observing it in light was historically one of the strongest pieces of evidence that light is a wave rather than a stream of particles.
In this demonstration, light from a laser or LED source is directed through a narrow single or double slit mounted on a board. The light that emerges does not simply cast a sharp shadow of the slit — instead, it spreads out and produces a pattern of bright and dark bands on a screen. The pattern, and its dependence on the slit width and wavelength, is explained entirely by wave optics. The accompanying chalkboard diagrams show the transition from single-slit to double-slit diffraction patterns and connect the experiment to the broader concept of wave–particle duality: the same quantum objects that show a particle-like nature in other experiments here reveal unmistakably wave-like behaviour.
Precautions
#Never look directly into the laser beam or its reflection from a surface. Keep the beam directed away from bystanders and ensure all observers are positioned to the side of the apparatus, not in the beam path. Darken the room as much as possible to make the diffraction pattern clearly visible.
Materials Required
#- A laser pointer or narrow LED light source
- A diffraction slit board (single slit and/or double slit of known width)
- A flat white screen or wall to project the pattern onto
- A ruler or measuring tape (to measure fringe spacing)
Science Behind It
#When a wave passes through a slit whose width is comparable to its wavelength, the wave fans out from every point along the slit opening — each point acts as a new source of circular waves, according to Huygens' principle. These secondary wavelets travel outward and overlap, reinforcing each other in some directions (constructive interference, giving bright fringes) and cancelling in others (destructive interference, giving dark fringes). The result is a characteristic intensity pattern: a bright central maximum flanked by progressively dimmer secondary maxima, separated by dark minima.
For a single slit of width a, the dark fringes appear at angles where a·sin θ = mλ (m = ±1, ±2, …), where λ is the wavelength of light. For a double slit, bright fringes from the two-slit interference are modulated by the single-slit envelope. The spacing of the fringes depends on the wavelength of the light and the geometry of the setup, making the pattern a precise measurement tool.
The deeper significance is the wave–particle duality it reveals. Light behaves as a particle (photon) when it is emitted or absorbed, but as a wave when it propagates — and the diffraction pattern is a direct consequence of this wave nature. The same duality was later discovered for electrons, atoms, and even molecules, forming one of the foundations of quantum mechanics.