Q.Explain the working principle of an optical fibre, and describe the role played by the relative refractive indices of its core and cladding.

An optical fibre has two coaxial parts: an inner core of refractive index $n_1$ and a surrounding cladding of a deliberately lower refractive index $n_2<n_1$. This is exactly the denser-to-rarer arrangement TIR requires. Light launched into the core at one end strikes the core-cladding boundary at an angle of incidence greater than the critical angle for that particular pair of refractive indices, so instead of refracting out into the cladding it undergoes total internal reflection back into the core; it then travels across the core and strikes the opposite core-cladding boundary, undergoes TIR again, and so on, bouncing repeatedly down the whole length of the fibre. Because TIR reflects (ideally) 100% of the light with no absorption loss at each bounce, and because this guiding works regardless of how the fibre itself is bent or coiled (as long as the bend is not sharp enough to reduce the angle of incidence below the critical angle), a signal can be carried with very …

Identify the core (higher n) / cladding (lower n) step as the denser-rarer TIR boundary, then expl …

• Thinking the fibre's outer plastic protective jacket is what guides the light (it is only mechanical protection).
• Forgetting that the core must have the HIGHER refractive index, not the cladding. …