One experiment with two slits proved light behaves like a wave

In the early 1800s, Thomas Young shone light through two closely spaced slits and saw alternating bright and dark bands on a screen — the signature of waves overlapping and interfering with each other. It overturned Newton's dominant 'corpuscular' theory of light, and over a century later the same setup became the classic demonstration of quantum weirdness.

— Thomas Young, Experiments and Calculations Relative to Physical Optics — Philosophical Transactions of the Royal Society, 1804