The coherence length of light is the path difference over which interference can be observed. For sodium light (Δλ = 0.6 nm, λ = 589 nm), the coherence length is approximately:

Two coherent slits S₁ and S₂ act as secondary sources; at each point on the screen, the path difference between light from S₁ and S₂ determines whether the waves arrive in phase (bright fringe) or out of phase (dark fringe), producing the characteristic alternating fringe pattern.