Young's double-slit experiment works by allowing waves from two narrow slits to overlap and undergo superposition. The resulting interference pattern contains alternating bright and dark fringes, providing evidence for the wave behaviour of light.
The Mechanism
This is part of C.3 Wave phenomena and applies to both SL and HL. Monochromatic light first illuminates two closely spaced slits. The slits act as coherent sources, meaning they produce waves with the same frequency and a constant phase difference.
The waves spread out by diffraction and overlap on a distant screen. Each slit must be narrow enough for diffraction to spread the light, while the slit separation must be small enough for the diffracted waves to overlap. At each point, their displacement amplitudes combine according to the principle of superposition. Whether a bright or dark fringe forms depends on the path difference between the two waves.
| Fringe | Path-difference condition | Result |
|---|---|---|
| Bright | , where |