The Double-Slit Experiment

The Concept: Interference

This experiment shows that when a single light wave passes through two closely spaced slits, it splits into two new waves that interfere with each other. Where the wave crests align (constructive interference), they create bright bands. Where a crest meets a trough (destructive interference), they cancel out, leaving dark bands. Thomas Young first did this with light in 1801. Amazingly, the same pattern builds up even when photons (or electrons!) are sent through one at a time. This wave–particle duality is a cornerstone of quantum mechanics.

What to look for

• The wave passes through the barrier, creating two new circular waves from the two visible slits.
• These waves interfere, creating a labeled intensity pattern on the screen.