Optical fibre communication cables guide light over long distances based mainly on which fundamental principle of light?

Introduction / Context:
Optical fibres are thin strands of glass or plastic that can transmit light signals over long distances with very low loss. They are the backbone of modern high speed communication networks. The ability of optical fibres to confine light within their cores even when bent is based on a specific principle of geometrical optics. This question asks you to identify that principle.

Given Data / Assumptions:

• An optical fibre has a higher refractive index core surrounded by a lower refractive index cladding.
• Light is launched into the fibre within a certain range of angles.
• We assume clear, lossless fibre and neglect absorption and scattering for conceptual understanding.
• Options include reflection, refraction, diffraction, and total internal reflection.

Concept / Approach:
When light travelling in a denser medium reaches the boundary with a rarer medium at an angle greater than the critical angle, it is completely reflected back into the denser medium. This phenomenon is called total internal reflection. Optical fibres are designed so that light rays inside the core always meet the core cladding boundary at angles greater than the critical angle, causing repeated total internal reflection. As a result, the light is trapped and guided along the fibre, even if the fibre is curved. Simple reflection at a mirror or ordinary refraction are not sufficient to explain this efficient light guiding mechanism.

Step-by-Step Solution:
Step 1: Recall that total internal reflection occurs when light travels from a medium with higher refractive index to one with lower refractive index and the angle of incidence exceeds the critical angle. Step 2: In an optical fibre, the core has a slightly higher refractive index than the cladding, satisfying the condition for potential total internal reflection at the core cladding interface. Step 3: Light is injected into the core at suitable angles so that when it reaches the boundary, the angle of incidence is greater than the critical angle. Step 4: At each such encounter, light is totally reflected back into the core, with virtually no loss into the cladding, allowing it to travel long distances through multiple reflections. Step 5: This repeated total internal reflection is the key mechanism that keeps light confined within the optical fibre.

Verification / Alternative check:
Diagrams of optical fibres in physics and communication textbooks show zigzag paths of light bouncing inside the core, with reflection angles above the critical angle. Mathematical analysis using Snell law and critical angle formulas further confirms that for typical core and cladding indices, total internal reflection occurs for a range of entry angles. Practical fibre optic systems rely on this principle to maintain signal strength over many kilometres, while simple reflection or refraction at a single interface would not provide the same level of confinement.

Why Other Options Are Wrong:
Reflection at a plane mirror surface: Mirrors reflect light, but optical fibres do not contain mirror coatings inside; they rely on internal reflections at glass interfaces instead. Simple refraction at a single interface: Refraction changes the direction of light as it enters a new medium, but by itself does not trap light in a waveguide. Diffraction through narrow slits: Diffraction describes spreading of waves around obstacles and apertures; it is important in optics but is not the main guiding principle in standard step index fibres.

Common Pitfalls:
Students sometimes confuse total internal reflection with ordinary reflection or think that refraction is enough to explain light guidance. While there is initial refraction as light enters the fibre, the crucial ongoing mechanism is repeated total internal reflection. Another confusion is to associate total internal reflection only with prisms and diamonds; in fact, optical fibres are a major practical application of the same phenomenon.

Final Answer:
Optical fibres work primarily on the principle of total internal reflection inside the higher refractive index core.