LIGHT & OPTICS · LAB 21

Double-slit experiment

Release particles one at a time and watch a wave-like interference pattern emerge from individual impacts.

EXPERIMENTCOHERENT

Particles arrive as points; their distribution reveals the probability wave

WavefrontsParticle pathProbability

EMITTED0

0 detected

FRINGE SPACING—

Δy ≈ λL/d

CENTRAL ENVELOPE—

w ≈ 2λL/a

VISIBILITY100%

fringe contrast

UNCERTAINTY—

counting estimate

WHAT TO NOTICE

One particle, two paths

Each detection is a localised point, yet many points build the wave interference distribution. When path information is available, coherence is lost and the fringes fade into a single-slit envelope.

Δy ≈ λL/dw ≈ 2λL/a

Educational Fraunhofer approximation; displayed apparatus dimensions are schematic.