Light Trapped in Glass
When light travels from a denser medium (glass core) to a less dense medium (glass cladding), Snell's law dictates that rays hitting the interface at shallow angles refract outward. Below a specific critical angle, the refracted ray would need to bend past 90 degrees, which is physically impossible — the light is instead perfectly reflected back into the core. This total internal reflection is the foundation of every optical fiber ever drawn.
Numerical Aperture
The numerical aperture quantifies how much light a fiber can capture. It depends only on the refractive index difference between core and cladding: NA = sqrt(n1^2 - n2^2). A higher NA means a wider acceptance cone and easier coupling, but also more guided modes and potentially more dispersion. Fiber designers balance NA against bandwidth requirements.
Critical Angle and Acceptance Cone
The critical angle at the core-cladding boundary sets the maximum bounce angle for guided rays. Translating this through Snell's law at the fiber entrance face gives the acceptance half-angle — the cone within which launched light will be guided. Rays outside this cone refract through the cladding and are lost within centimetres.
Single-Mode vs Multimode
When the core is small enough (typically under 9 um at 1550 nm), only the fundamental mode propagates, eliminating modal dispersion entirely. Larger cores support hundreds of modes, each travelling at a slightly different group velocity. This simulation visualises ray paths inside the fiber, showing how changing the indices and core size affects acceptance, guiding, and the transition from multimode to single-mode operation.