Fibre Optic
Why does light stay inside a glass fibre, even around corners? Aim a laser into the fibre and see whether total internal reflection traps it — then find the acceptance angle, the numerical aperture and how fast a signal crosses a kilometre of cable.
How Does a Fibre Guide Light?
An optical fibre is a thin glass core wrapped in a cladding of slightly lower refractive index. Light travelling along the core keeps hitting the core–cladding wall at a grazing angle — beyond the critical angle — so it is reflected completely, bounce after bounce, for kilometres.
Only light that enters within the acceptance cone is trapped; steeper rays leak into the cladding. The sine of the cone’s half-angle is the numerical aperture. Steeper rays also take a longer zig-zag path, so a short pulse smears out over a long cable — this modal dispersion limits how fast you can send data.
Inside the Fibre
guidedFibre Formulas
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Worked Techniques
Follow the steepest ray that is still trapped: it meets the wall exactly at the critical angle. Snell’s law at the entry face, plus cos² = 1 − sin², gives the numerical aperture.
The straight ray and the steepest guided ray arrive at different times. A pulse spreads by Δt, so pulses must be at least that far apart.
Make the core thin enough and only the straight-through path survives (a wave effect). Single-mode fibres have no modal dispersion and carry terabits per second across oceans.
Sources & Verification
- E. Hecht, Optics, 5th ed., Pearson, 2017 — Ch. 4 (The Propagation of Light: total internal reflection) and Ch. 5 (Geometrical Optics: fiberoptics).