A study in light
Four chapters · interactive
A beam of white light is not one thing. It is a crowd of wavelengths travelling together, agreeing on a direction — until a piece of glass asks each of them a slightly different question.
Chapter I
The beam arrives at the boundary and, for one clean instant, nothing has happened yet. A surface is only a promise: here, the rules change.
θᵢ = θᵣ The part that bounces back leaves at exactly the angle it came in — the only part of this story that keeps its shape.
Chapter II
Glass is slow. Light crossing into it has to bend, because the near edge of the wave arrives — and is slowed — before the far edge does. The beam pivots on its own wavefront.
n₁ sin θ₁ = n₂ sin θ₂ In crown glass light drops to about 0.66 c: roughly 198,000 km every second, and still late.
Chapter III
The glass does not slow every colour equally. Red hurries through; violet drags. What entered as one line leaves as a fan — the crowd, finally, disagreeing.
n = 1.514 at 700 nm · 1.532 at 400 nm A difference of eighteen thousandths is the entire rainbow.
Chapter IV
Send the same light down two paths and let them meet again. Where crest lands on crest: brightness. Where crest lands on trough: nothing at all. Light plus light, equalling dark.
Δ = mλ → bright · Δ = (m + ½)λ → dark Proof, in 1801, that light travels as a wave.
Drag to turn the stone. Press it to break the light open.