The double-slit experiment was one of the foundational experiments of quantum mechanics. Before the inception of quantum mechanics, the scientific society was under the impression that waves and particles were two completely separate phenomena. Waves, as understood by sound waves, were disturbances in media, like air. Light, however, did not follow those same rules; in fact, light was shown to travel in no medium whatsoever.
So how did light behave? Thus came the need for an experiment to show everyone how.
The setup for the double-slit experiment is not complicated. One simply needs to shine a laser through an object which has two slits, side by side, finely cut into it. Then one only needs to watch the interference pattern on a flat surface. Waves, when traveling through a slit, disperse on the other side. When the light disperses through two slits, the light on the other side interferes with each other. The parts where two crests meet double in amplitude (height). The parts where a trough and a crest meet cancel each other out. When this hits a flat surface, it leaves an interference pattern. In the interference pattern, the parts where the waves add to each other are lit up while the parts where the waves cancel each other out are dark. This clearly shows light to be a wave, albeit a media-less wave.
The one aspect that made this particular experiment special was decreasing the intensity of the light leaving the laser. When the intensity is decreased, the “amount” of light is less as well. Scientists called this packet of light a photon. The intensity was decreased until only one packet of light, photon, left the laser at a particular moment in time. Using no slits, scientists saw that at one point there was a bright “blip” of light and then nothing. This continued to happen. Scientists then added the double slit. However, upon looking at the interference pattern, they saw that the tiny “blips” of light only showed on areas that were lit when the laser was running on normal. The photons were following the exact same interference pattern despite the fact that there was nothing to interfere with!
Quantum mechanics claims that wave-particle duality is an inherent aspect of all matter. The double-slit experiment showed photons, light particles, to behave in a wave-like manner. Light was behaving as both particles and waves. What did this mean? It meant the photon was neither a wave nor a particle; it was a quantum object that can manifest either waves or particles, depending on the physical situation. Quantum mechanics allows us to calculate the probability of where a quantum object can be. The interference pattern of a laser shows exactly where the regions of high probability are; those regions being where the flat surface is lit.
This was a groundbreaking experiment in its legacy. The double-slit experiment is easy to set up with modern technology and has been done with electrons instead of light. Sure enough, electrons followed an interference pattern. In the coming of age of quantum mechanics, this experiment was a large boost for the veracity of quantum theory, the currently accepted theory for how matter behaves.