The double slit experiment demonstrates the wave-particle duality of light.
You shoot photons at a barrier that has two slits in it. The pattern on the backstop appears as in the top right panel: an interference pattern, because light is behaving as a wave.
Next, you set up a detector at the slits, so that you can determine which slit each photon passed through, one photon at a time. Now the pattern on the backstop appears as the lower right panel, not an interference pattern, because each photon is acting as a particle.