Young's Experiment

A one-color light source passing through a pair of very closely spaced slits becomes two coherent beams of light. Instead of two lines, a pattern of bright and dark bands is observed on a screen far from the slits. Young demonstrated the wave nature of light.

The bright bands happened when the path difference is n l, ( n = 0, 1, 2, or 3... )

The dark bands happened when the path difference is (n+? l, ( n = 0, 1, 2, or 3... )

The band is bright due to constructive interference, and dark due to destructive interference.