Picture light as a ball bouncing down a long, narrow, mirrored pipe.
- The pipe here is the fibre's core, and the mirror effect at its walls comes not from a real coating of silver, but from the physics of total internal reflection at the core-cladding boundary.
- For this mirror effect to work, two conditions must hold: the core must have a higher refractive index than the cladding, and the light must strike the boundary at an angle steeper than the critical angle.
- As long as both conditions are met, every single ray keeps reflecting internally at the boundary and none of it leaks out through the sides, so the light signal travels the full length of the cable with minimal loss.
Refraction, interference and scattering are all real optical phenomena, but none of them explains how light is confined and guided along the fibre without escaping. Total internal reflection is the one principle that specifically traps and channels the light down the fibre, so option A is correct.