In an optical fibre communication system, the light signal is able to travel along the curved fibre over long distances mainly due to which physical phenomenon?

Introduction / Context:
Optical fibres are widely used to transmit data as light signals over long distances in telecommunication and internet networks. They are thin strands of glass or plastic that can guide light even when the fibre is bent. The question asks which physical principle allows light to remain confined inside the fibre core instead of leaking out. Understanding this principle is central in optics and modern communication technology.

Given Data / Assumptions:

• Light is transmitted through an optical fibre, which may be curved.
• The fibre has a core surrounded by cladding with lower refractive index.
• The signal must stay inside the fibre without significant loss.
• Options mention straight line motion, refraction, total internal reflection, and absorption related processes.

Concept / Approach:
In an optical fibre, the core has a slightly higher refractive index than the cladding. When light enters the core at a suitable angle, it strikes the core cladding boundary from the denser side. If the angle of incidence is greater than the critical angle for that boundary, the light is totally internally reflected. Because this happens repeatedly along the length of the fibre, the light is effectively trapped and guided along the fibre, even if the fibre bends gently. This repeated total internal reflection is the key guiding mechanism. Simple straight line travel cannot explain guidance along curves, and ordinary refraction alone would cause the light to escape into the cladding instead of staying confined.

Step-by-Step Solution:
Step 1: Recognise that the core of the optical fibre has a higher refractive index than the surrounding cladding.Step 2: Light entering the core at a suitable angle strikes the core cladding interface from the denser medium toward the rarer medium.Step 3: If the incidence angle exceeds the critical angle, total internal reflection occurs, and the light is reflected back into the core.Step 4: As the fibre extends and even bends, the light continues to strike the boundary at angles above the critical angle, undergoing repeated total internal reflections.Step 5: This process allows the light signal to stay within the core and travel long distances with low loss, so repeated total internal reflection is the correct phenomenon.

Verification / Alternative check:
Applications like endoscopes in medicine and undersea fibre optic cables in communication all rely on optical fibres. Textbooks and technical references clearly describe total internal reflection as the operating principle of these fibres. If light simply travelled in a straight line, it would escape whenever the fibre curves. If refraction were dominant, light would gradually leak out through the cladding. Experimental demonstrations with glass rods and water filled tanks also show that light can be guided around bends when conditions for total internal reflection are met. These observations confirm the explanation based on total internal reflection.

Why Other Options Are Wrong:
Option A, straight line motion, is only valid in a homogeneous medium with no boundaries and cannot account for guiding around bends. Option B, guidance by refraction, would cause light to bend but ultimately escape rather than being perfectly confined. Option D, absorption and re emission due to heating, would greatly reduce signal strength and speed and is not how optical fibres operate. Only option C, repeated total internal reflection of light within the core, accurately describes the mechanism that keeps light trapped inside an optical fibre.

Common Pitfalls:
Some learners think of refraction whenever light crosses boundaries with different refractive indices and may incorrectly select it in this context. Others assume that light must always travel in straight lines and forget that internal reflections can redirect it. To avoid confusion, remember the specific conditions for total internal reflection: the ray must travel from denser to rarer medium and the angle of incidence must exceed the critical angle. Optical fibres are carefully designed to satisfy these conditions, making total internal reflection the central phenomenon in their operation.

Final Answer:
Repeated total internal reflection of light within the core