How light stays trapped in an optical fiber

An optical fiber is a thin glass core wrapped in cladding glass with a slightly lower refractive index. Light that hits the core wall at a shallow enough angle cannot escape and reflects completely back inward, a process called total internal reflection. Bouncing again and again, the light can travel many kilometers down the fiber.

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Core refractive index

Cladding refractive index

Laser entry angle

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Core and claddingThe core glass bends light slightly more than the cladding around it. That small difference in refractive index is what makes the fiber work.
Total internal reflectionWhen light inside the core strikes the wall at a grazing angle, beyond the critical angle, none of it passes into the cladding. All of it reflects back into the core.
The acceptance coneOnly light entering within a certain cone of angles gets trapped. Tilt the laser past that cone and the light hits the wall too steeply and leaks out.
A bigger index stepRaise the core index and the acceptance cone widens, so the same steep beam is trapped again.
Signal arrivesAt the far end a photodiode turns the light pulses back into electrical signals. Telecom fibers carry data this way across oceans.

The two angles

Snell's law gives the critical angle at the core wall, measured from the perpendicular: sin(critical angle) = n2 / n1. Light striking the wall at a larger angle than that is totally reflected.

Working back to the fiber's end face gives the numerical aperture, NA = square root of (n1 squared minus n2 squared). The largest entry angle from air that will still be trapped is asin(NA). The zigzag path in the scene is a ray picture; very thin single mode fibers really carry light as a wave pattern.

Try it: set the core to 1.48 and the cladding to 1.46. The acceptance cone is only about 14 degrees, so an entry angle of 20 degrees leaks. Now drop the cladding to 1.40 and watch the cone open past 28 degrees.

Why the index step is small

Telecom fibers use a tiny index difference, often under 1 percent, because rays at very different angles travel different distances and spread a pulse out in time. A small step keeps pulses sharp, which allows faster data rates.

Sources

Fiber Optics (MAE 438 lecture notes)University at Buffalo
Principles and Characteristics of Optical FibersNASA Technical Reports Server
Numerical Aperture of Optical Fiber: TheoryVirtual Labs (Ministry of Education, India)

How to cite this model

Avid Solutions International. (2026, November 11). How light stays trapped in an optical fiber [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/fiber-optic

Last updated November 11, 2026

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