In optical fibers, what causes total internal reflection?

Total internal reflection in optical fibers occurs when light travels from a medium of higher refractive index to a medium of lower refractive index and hits the boundary between these two media at a specific angle known as the critical angle. When the angle of incidence exceeds this critical angle, the light cannot pass into the second medium; instead, it reflects entirely back into the original medium. This principle is fundamental for the efficient transmission of light along optical fibers, allowing signals to carry over long distances with minimal loss.

The phenomenon relies on the interaction of light with the refractive indices of the materials involved. The optical fiber typically consists of a core surrounded by a cladding. The core's refractive index is higher than that of the cladding, which enables the total internal reflection necessary for guiding light effectively through the fiber. Hence, the condition of the light hitting the boundary at the critical angle is essential for achieving the total internal reflection that keeps the light confined within the core of the fiber.