Double-slit experiment

The double-slit experiment is a physics experiment which demonstrates wave-particle duality.

The experiment is easy to do, but difficult to understand. It consists of setting up a light source on one side, facing a barrier (such as a piece of cardboard) that has two slits in it to permit some light to pass through to a distant wall on the other side. If both slits are open, then one pattern is observed on the distant wall. But if one slit is covered, then a completely different pattern is then seen on the distant wall.

This simple experiment demonstrates that the wave-particle duality of light. Light behaves as a wave when both slits are open, such that the wave simultaneously passes through both slits and creates an interference pattern on the distant wall, but as a particle when only one slit is open, such that there is no interference pattern.

The experiment can also be done with particles, such that a series of particles are directed at a grid, or other similar device, while observing the pattern on the other side.