When light goes through a single slit, you get an interesting diffraction pattern: the light spreads out but also has ripples. What happens if you now add a second slit? Thomas Young in 1803 demonstrated this to the Royal Society. The crowd was stunned to see that by adding a second slit, essentially adding more light, you can in fact remove light from certain sections on the distant screen. This firmly established the wave theory of light, and this classic experiment has been used to prove the "weirdness" of quantum. Namely, if you repeat this experiment with only one photon at a time, you still build up the exact same interference pattern. The single photon goes through both slits and interferes with itself on the other side. Say whaaaaaat?
Physics with Professor Matt Anderson