Total internal reflection in optical fibers occurs due to the difference in refractive indices between the core and the cladding of the fiber.
The core is made of a material with a higher refractive index, while the cladding has a lower refractive index. When light enters the fiber and strikes the boundary between the core and cladding at an angle greater than the critical angle, it does not pass into the cladding. Instead, the light is completely reflected back into the core. This is known as total internal reflection.
As the light continues to move forward, it repeatedly hits the core-cladding boundary at angles that satisfy the total internal reflection condition. This causes the light to be confined within the core and guided along the length of the fiber without escaping.
This process allows light signals to travel through the optical fiber with minimal loss, even over long distances. It is the principle that makes fiber-optic communication fast, efficient, and immune to electromagnetic interference.