Experiment: Shining a laser pointer through two narrow slits and observing the interference pattern on a distant screen.
Observation: When the laser light is on, there is an interference pattern on a distant screen. These
patterns could only be produced if light were acting like a wave.
Explanation: When light goes through a slit, diffraction causes it to bend and spread across the screen, making a predictable banded pattern. When light goes through two slits, new dark regions appear. The dark and light regions are produced by interference of the light passing through the slits. As light coming through one slit reaches the screen, it overlaps with light coming through the other slit. When the crest of one wave of light overlaps with the crest of another wave, the two waves combine to make a bigger wave and you see a bright blob of light. When the trough of one wave overlaps with the crest of another wave, the waves cancel each other out and you see a dark band. The appearance of dark bands when two light sources strike a screen shows that light is a wave phenomenon.