SLITS
Darkness made of light — d·sinθ = mλ, and only ⌊d/λ⌋ fringes exist.
Use the simulation above to change the variables and play through the guided stages. The explanation below describes the default starting values; the simulation updates its explanation as you experiment.
Setup
One color of light, two narrow slits, a distant screen. Thomas Young set this up in 1801 expecting to settle what light is — and the screen answered with stripes: places where two open slits deliver less light than one would.
Superpose
Each slit re-radiates the wave, and at the screen the two arrivals simply add. Whole-wavelength path differences stack crest on crest; half-wavelength differences cancel exactly. Darkness manufactured from two lit slits was the scandal — and the proof.
Solve
In the Fraunhofer far field, d·sinθ = mλ selects each bright direction. The small-angle comb has spacing λL/d, with its error printed. A finite screen requires |sinθ| < 1; an order at equality is grazing at ±90° and never lands at finite y.
Build
The pattern assembles: the cos² intensity comb, bright exactly where the path ledger closes on a whole λ. The drawing is honest about its scales — the slit region is zoomed to be visible and says so; the fringe spacing on the screen is true.
Audit
Audited within the Fraunhofer model: d·sinθ equals mλ at every checked bright direction, intensity is I₀ at maxima and zero halfway between, and the uniform-phase average is precisely ½. The same interference arithmetic later counts particles one at a time.