What causes light to refract when it travels through a fiber-optic cable?

Light refracts in a fiber-optic cable primarily due to the interaction at the boundary between the core and the cladding. This phenomenon is governed by the principles of total internal reflection, which occurs when light passes from a medium with a higher refractive index (the core) to one with a lower refractive index (the cladding).

When light enters the fiber-optic cable, it does so at an angle that is greater than the critical angle specific to that core-cladding interface. As the light hits this boundary, it bends (or refracts) due to the difference in refractive indices. If the angle is appropriate, rather than passing into the cladding, the light reflects back into the core, allowing it to travel along the length of the fiber while maintaining its brightness and integrity.

This critical boundary is essential in guiding light through the fiber-optic cable and is what enables the transmission of data at high speeds over long distances. The core material does contribute to the overall refractive properties, but the specific refraction and the resulting light guidance are determined by the interaction at the core-cladding boundary.