At the start of the 19th century, there were scientific debates on the substance of light. Many argued that light behaved as individual particles moving through space, whereas others believed it to be composed of waves (much like sound). In 1803, scholar Thomas Young proposed an experiment to the Royal Society. He planned to end the argument in scientific discovery.
To understand his experiment (which would become known as the famous Double-Slit Experiment), we must first look into a phenomenon known as wave interference.
The image above details a quick demonstration done by the YouTuber Veritasium. He pushes two identical bobbers up and down in a lake to create two sets of waves. As the waves collide, a pattern emerges. In the places where two crests hit, they constructively interfere and a larger crest is born. Similarly, when two troughs hit, they create a larger trough. However, when a crest meets a trough, the two waves “cancel out” or destructively interfere and form a flat spot on the water. You can clearly see these places of destructive interference as flat lines among the wave patterns in the image.