Full Idea
Thomas Young in 1803 confirmed the idea of Huyghens that light is waves, showing how light passing through two slits produces an interference pattern that resembles water waves sluicing through two slits.
Gist of Idea
The interference of light through two slits confirmed that it is waves
Source
Peter Watson (Convergence [2016], 04 'Conception')
Book Reference
Watson,Peter: 'Convergence' [Simon and Schuster 2016], p.109
A Reaction
The great puzzle emerges when it also turns out to be quantised particles.