Thomas Young · The double slit
If light is a particle, why do two slits make stripes?
That time, that place
Newton held that light is made of tiny particles: shadows have sharp edges, light travels straight.
His authority kept the question shut for a century. In Britain, suggesting light might be a wave was close to bad manners.
Why this question
Young's experiment is simple: send light through two narrow slits and look at the screen behind.
If light were particles you should see two bright bands, one per slit.
Instead there were many bright and dark stripes — and the brightest sat in the middle, between the slits. No arrangement of particles gives that.
What was found
A wave explains it. Where two ripples meet, crest on crest rises and crest on trough cancels. Those are the bright and dark stripes.
British reception was cold, with anonymous attacks for contradicting Newton. The wave theory took hold only after Fresnel's work in France.
But the story does not end there. A century later the photoelectric effect showed light is also a particle — and then the experiment was run with electrons, which also made stripes.
Stranger still: send them one at a time and the stripes still build up. Each one went through both slits. And if you watch which slit, the stripes vanish. Feynman said this single experiment contains the whole mystery of quantum mechanics.
Newton's particle view had been standard for a century
Light through two slits producing multiple interference fringes
The wave nature of light was established, and the experiment became quantum mechanics' central puzzle