SACE Stage 2 Physics — Light & Atoms 3.1

Double Slit

A coherent plane wave from the left lights both slits in step. Each slit then radiates secondary cylindrical waves (Huygens) that overlap on the way to the screen. Where their path lengths differ by p = d sin θ = mλ, you get a bright fringe; by (m + ½)λ, a dark fringe. Spacing on the screen is Δy ≈ λL/d.