Bouncing all the way
How Does Light Go Round a Bend?
Light in a glass fibre keeps bouncing off the inside walls, so it follows every curve without escaping.
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Step 1 of 5
Light really does only travel in straight lines.
Nothing bends a beam round a corner. Any apparent curve is actually a long series of short straight runs between reflections.
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Step 2 of 5
Light usually escapes when it meets a boundary.
Hitting glass at a steep angle, most light passes straight through and only a little reflects, which is no use for carrying a signal.
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Step 3 of 5
At a shallow enough angle, none escapes.
Below a certain angle the boundary reflects everything perfectly. No light leaks out at all, which is why the signal survives.
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Step 4 of 5
The fibre is built to guarantee that angle.
The core is wrapped in a second glass that bends light differently, and the fibre is thin enough that light can only ever strike shallowly.
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Step 5 of 5
So the light follows every curve.
Bend the fibre and the wall simply moves to meet the beam, which bounces again. Light ends up tracing the cable wherever it goes.
Light in a fibre can bounce thousands of times per metre yet still carry a signal thousands of kilometres.
The short version
Light follows a fibre by striking the inside wall so shallowly that it reflects perfectly every time, tracing the cable round any bend.
Try it yourself
Shine a torch into the end of a clear plastic rod bent into a curve. The light comes out the far end, not through the bend.
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