Why does an optical fibre have a core and cladding with different refractive indexes?

An optical fibre consists of a central core surrounded by a cladding with a lower refractive index than the core. This specific arrangement is essential because the abrupt change in the refractive index at the core-cladding interface allows for total internal reflection.12

When light rays travel through the core at small angles relative to the fibre axis, they strike the interface between the core and the cladding. Because the cladding has a lower refractive index, the light is reflected back into the core rather than leaking out. This process acts like a tubular mirror, confining the light to the interior of the core and enabling it to be transmitted over long distances with minimal loss. Without this difference in refractive indexes, the light would not be effectively guided through the fibre, rendering it unable to function as an efficient medium for optical communication.213