Path difference. The light from the two slits travels different distances, r₁ and r₂, to reach P. The difference is Δr = |r₂ − r₁|. For small angles, Δr ≈ d sinθ ≈ dy/D.

Constructive and destructive. If Δr is a whole number of wavelengths (mλ), the waves arrive in step, crest with crest, and add up: a bright fringe. If it is a half-integer number (m + ½)λ, a crest meets a trough and they cancel: a dark fringe. The graph at the bottom shows the two waves arriving at P and their sum.

Δr = mλ (bright), Δr = (m + ½)λ (dark), I = I₀ cos²(πΔr/λ)

Fringe spacing. Neighbouring bright fringes are Δy = λD/d apart.

The picture is not to scale: the slits are really only 100–300 μm apart and the screen is 1 m away, and the wavelength drawn along the rays is a stand-in so that you can see it. The numbers (path difference, intensity) are exact.