Double-slit interference

Wavelength λ Slit separation d Screen distance L

Follow the light

Whole experiment

S1 open S2 open Troughs Paths

Move point P

What arrives at P?

Magnified

● S1 ● S2 Solid: crest · Dashed: trough

Add the fields at P

Time in the cycle

S1 → P

S2 → P

Path difference

Why does this happen?

A bright fringe stays bright as crests, zero crossings, and troughs pass through it. Brightness measures the average intensity over many cycles, not the field at one instant.

At a fully dark fringe, equal and opposite contributions cancel throughout the cycle.

Teal and orange identify the two sources; the light has the same wavelength. All distances use model units on a consistent scale. The model assumes equal amplitudes, with no spreading or single-slit diffraction envelope.

Arrival buttons pause time and align S1’s crest or trough with P. If S1 is blocked, they use S2.