Physics · Ch 6 — Optics
Double Slit as Coherent Sources
Double Slit as Coherent Sources
A double slit uses the principle of wavefront division: two slits , both illuminated by a single monochromatic source , act as a set of coherent sources. Waves from and travel in the same medium and superpose in the overlap region beyond the double slit: where the crest of one wave meets the crest of the other (or trough meets trough), the two waves are in-phase, the displacement is maximum, and the region appears bright -- constructive interference; where the crest of one wave meets the trough of the other, the waves are out-of-phase, the displacement is minimum, and the region appears dark -- destructive interference. So on a screen placed in this overlap regi …
What this figure shows. Panel (a) shows two overlapping sets of circular wavefronts (crests as solid lines, troughs as broken lines) spreading out from two coherent slits; wherever two crests (or two troughs) coincide, the waves add constructively and the region is marked bright, while wherever a crest meets a trough the waves cancel and the region is marked dark, tracing out fan-shaped bright and dark zones radiating outward from the two slits. Panel (b) shows the double slit S1, S2 itself, fed by a single upstream source S, generating exactly this coherent overlapp …