Light stays inside a fiber because the core and cladding have different refractive indexes. The core has the higher index. When light reaches the boundary at a suitable angle, it reflects inward instead of escaping. This process is called total internal reflection. The boundary matters.

A practical ray model makes the idea easy to see. Imagine a narrow beam bouncing along a glass tunnel, touching the inner wall repeatedly. Each reflection redirects the beam toward the far end. However, the light does not bounce like a ball. In reality, it travels as guided electromagnetic modes through the core. These modes are shaped by the fiber’s diameter, materials, and wavelength.

The cladding keeps the optical field concentrated near the core, even though a small portion extends into it. That field is essential for guidance. Light launched outside the allowed acceptance angle may leak away quickly. Sharp bends can also push light through the cladding, causing measurable loss. Absorption, microscopic scattering, and tiny manufacturing variations reduce the signal over distance.

The simple reflection picture is useful, but incomplete. I find it tempting to call the core a perfect pipe. It is not. A real fiber preserves light efficiently, not perfectly, and careful installation still matters. Clean end faces, controlled bends, and accurate alignment help maintain the guided signal. Even a small curve can change how several modes travel.