How does light travel hundreds of kilometers down a glass thread without leaking out? Total internal reflection traps it in the core, bounce after bounce.
Fiber OpticsLive
total internal reflection
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An optical fiber traps light in a high-index core surrounded by lower-index cladding: rays hitting the boundary above the critical angle undergo total internal reflection and bounce down the fiber for kilometers. The numerical aperture sets the acceptance cone, and the core size decides whether it carries one mode (single-mode, for long-haul) or many (multimode).
Reading this result: Rays steeper than the 80.6° critical angle reflect and stay trapped; here V = 29.1 allows ~423 modes, so this behaves as multimode fiber — cheap and easy to launch, but modal dispersion caps the bandwidth.
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How it works
An optical fiber has a high-index core inside a lower-index cladding. Light striking the boundary above the critical angle is completely reflected, guiding it along the fiber. The numerical aperture sets the acceptance cone of usable input angles, and the core diameter decides whether the fiber is single-mode for long-haul telecom or multimode for short links.
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