Young's double-slit · two-beam interference simulation

Pass light through two slits — why a striped comb instead of two bright lines, what sets the stripe spacing, and why is one stripe missing?

▶ Run the simulationSee the measured result

Units: metres (helium–neon laser line, 632.8 nm)

How the lab tests it

Model the far-field intensity I=cos²(πdy/λL)·[sinβ/β]² (β=πay/λL) on a screen distance L from two slits of separation d and width a; scan a noisy intensity profile, locate the bright-fringe peaks with a hysteresis finder, fit y_m=(λL/d)·m, and read λ off the fringe spacing Δy=λL/d.

What it checks

two-beam interference — the bright fringes form an EVEN comb at y_m=m·λL/d, so the fringe spacing Δy=λL/d is a ruler for the wavelength (recover λ≈633 nm given L and d). The comb is not a property of either slit: block one and it collapses to a single broad blob. The cos² fringes ride under the single-slit (width a) diffraction envelope [sinβ/β]², whose zero at y=λL/a KILLS every (d/a)-th fringe — here d/a=5, so the 5th order is MISSING: a gap that fingerprints the slit WIDTH a as distinct from the slit SPACING d. The wavefront-division sibling of Newton's amplitude-division rings.