Optical fibres work by guiding light along a core made of a material with a higher refractive index, surrounded by a cladding material with a lower refractive index. Light rays entering the core at appropriate angles strike the interface between the core and the cladding at an angle greater than the critical angle. When this condition is met, the light is completely reflected back into the core, a phenomenon called total internal reflection (TIR). This process repeats along the length of the fibre, allowing the light signal to propagate with minimal loss.