In modern communication systems, an optical fibre cable guides light signals over long distances. On which physical principle does an optical fibre mainly work?

Introduction / Context:
Optical fibre technology is at the heart of modern high speed communication networks, from internet backbones to cable television and medical endoscopes. This question tests whether the learner knows the basic physical principle that allows light to be guided through long, flexible glass or plastic fibres, even when the fibre is bent. Understanding this concept requires familiarity with how light behaves when it tries to pass from one medium to another and under what conditions it is reflected back rather than refracted out.

Given Data / Assumptions:

• Light travels through an optical fibre core made of glass or plastic with a higher refractive index.
• The core is surrounded by cladding with a lower refractive index.
• The question asks for the main principle responsible for guiding light along the fibre.
• Options include scattering, interference, total internal reflection and simple refraction.

Concept / Approach:
When a ray of light travels from a denser medium (higher refractive index) to a rarer medium (lower refractive index), and if the angle of incidence at the boundary is greater than a certain critical angle, all the light is reflected back into the denser medium. This phenomenon is called total internal reflection. Optical fibres are specially designed so that light entering the core at suitable angles always hits the core cladding boundary at angles greater than the critical angle. As a result, the light is repeatedly totally internally reflected and remains trapped inside the core, even if the fibre is bent gently. This repeated total internal reflection is the working principle of optical fibre communication.

Step-by-Step Solution:
Step 1: Identify the core and cladding structure of the optical fibre, where the core has higher refractive index than the surrounding cladding. Step 2: Recall the condition for total internal reflection: light must travel from a denser to a rarer medium and must strike the boundary at an angle of incidence greater than the critical angle. Step 3: Understand that in an optical fibre the geometry ensures that rays entering within a certain acceptance angle always meet the core cladding interface at angles above the critical angle. Step 4: Because this condition is satisfied, light rays are not refracted out into the cladding but instead are reflected back into the core again and again. Step 5: Conclude that the basic principle enabling long distance light guidance in optical fibres is total internal reflection, and therefore that option must be chosen.

Verification / Alternative check:
If one visualises an optical fibre, it behaves like a long, thin light pipe. For such a pipe to guide light, the light must be reflected internally rather than escaping. Simple scattering or interference phenomena cannot explain the tight guidance of rays along the length, especially through curves. Refraction at a single surface would bend the path, but that alone would allow light eventually to leak out. Only total internal reflection, occurring repeatedly at the core cladding boundary, naturally explains why light remains trapped inside the fibre until it exits at the far end, which matches the observed behaviour of optical fibre systems.

Why Other Options Are Wrong:
Scattering of light involves light being deflected in many directions due to particles or irregularities and would cause signal loss rather than controlled guidance. Interference of light is the phenomenon of overlapping waves creating bright and dark patterns; it is important in many optical instruments but does not explain the guiding mechanism in fibres. Refraction at a single surface describes bending of light when it passes from one medium to another, but does not by itself keep light confined inside a long fibre.

Common Pitfalls:
Students sometimes confuse simple refraction with total internal reflection and may choose refraction because they remember that light changes direction at interfaces. Others might think of scattering because of loss mechanisms in real fibres. The key for exam success is to remember the precise name of the phenomenon that traps light inside the fibre: total internal reflection. Whenever a question mentions optical fibres, light pipes, periscopes or mirage like phenomena, one should immediately think of total internal reflection as the central concept.

Final Answer:
An optical fibre works mainly on the principle of total internal reflection of light at the core cladding interface.