🧵 Total Internal Reflection — how an optical fibre pipes light

Light strikes the core wall from inside. Beyond the critical angle it is trapped and zig-zags down the fibre; below it, it leaks out. Change the wall angle i and the core index n.

Walkthrough 1 / 4

Step 1 — a ray inside the core strikes the wall. i is its angle from the wall normal.

Angle at wall i 60°

Core index n 1.50

i = 60° · critical i_c = 41.8° · n = 1.50
i > i_c → trapped: total internal reflection

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