Explanation

An optical fibre guides light by total internal reflection: the core has a higher refractive index than the cladding, so rays striking the core-cladding boundary above the critical angle are reflected back and zigzag down the fibre with minimal loss. Refraction merely bends light at entry and does not confine it, while interference and scattering describe wave superposition and beam spreading, neither of which channels a signal. This confinement is what lets fibres carry signals over kilometres. Hence the responsible phenomenon is total internal reflection.