Which of the following is used in optical fibres ?
(a) Total internal reflection
(b) Scattering
(c)

Q: Why doesn't the light just pass through the walls of the optical fibre?

Because of Total Internal Reflection. The fibre is designed with a core (denser medium) and a cladding (rarer medium). When light hits the boundary at an angle greater than the critical angle, it acts like a perfect mirror, reflecting the light back inside.

Q: What happens if the angle of incidence is less than the critical angle?

If the angle is less than the critical angle, the light will refract (bend) and escape into the cladding, leading to a loss of signal. That's why light must be injected into the fibre at specific angles.

Q: Why do we need the cladding? Can't we just use a glass core in the air?

Technically, a glass core in air would work because air has a lower refractive index than glass. However, any scratch, dirt, or moisture on the glass surface would disrupt the reflection and cause light to leak. The cladding protects the core and ensures a consistent, controlled boundary for TIR.

Q: Does the light lose energy as it bounces back and forth?

In ideal Total Internal Reflection, 100% of the light is reflected, so no energy is lost at the boundaries. However, in reality, the glass itself absorbs a tiny fraction of the light over long distances, which is why signal amplifiers are used in long underwater cables.

Q: Can optical fibres bend at any angle?

No. If you bend an optical fibre too sharply, the angle at which the light hits the boundary inside will change. If it becomes less than the critical angle, the light will escape. This is known as 'bending loss'.

Question

Which of the following is used in optical fibres ?

Options:

(A) Total internal reflection

(B) Scattering

(C) Diffraction

(D) Refraction

Sigma Percentile · Accepted Answer

### The Magic of Optical Fibres

Have you ever wondered how high-speed internet reaches your home across oceans and continents in the blink of an eye? The secret lies in a hair-thin strand of glass called an **optical fibre**. But how does light, which usually travels in straight lines, bend and twist along these cables without escaping? Let's dive into the fascinating physics behind it.

### The Anatomy of an Optical Fibre

An optical fibre is not just a simple glass tube. It is meticulously engineered with two distinct layers:
1. **The Core:** This is the central part of the fibre where the light actually travels. It is made of high-quality glass or plastic and has a relatively high refractive index, let's call it $n_1$.
2. **The Cladding:** Surrounding the core is another layer of glass or plastic called the cladding. The crucial design feature here is that the cladding has a lower refractive index, $n_2$, compared to the core. So, $n_1 > n_2$.

This difference in refractive indices sets the stage for a beautiful optical phenomenon.

### The Principle: Total Internal Reflection

When a light ray enters the core, it travels until it hits the boundary between the core and the cladding. Because it is trying to move from a **denser medium** (the core) to a **rarer medium** (the cladding), it bends away from the normal.

However, if the light ray hits this boundary at a shallow enough angle—specifically, an angle of incidence $i$ that is greater than a certain threshold known as the **critical angle** $	heta_c$—something magical happens. The light does not refract into the cladding at all. Instead, it acts as if the boundary is a perfect mirror and reflects $100\%$ back into the core.

This phenomenon is called **Total Internal Reflection (TIR)**.

### The Journey of Light

Because the optical fibre is cylindrical, once the light undergoes TIR at the first boundary, it bounces across to the opposite side and hits that boundary at the exact same angle. Since the angle is still greater than the critical angle, it undergoes TIR again.

This continuous series of total internal reflections allows the light to zigzag its way down the entire length of the fibre, even if the fibre is bent or twisted. Because the reflection is 'total', there is virtually no loss of light energy, allowing signals to travel for kilometers without degrading.

Therefore, the fundamental principle that makes optical fibres work is **Total Internal Reflection**.

Animated Solution for Physics - Optics: Which of the following is used in optical fibres ?

Select Answer:

Visualized Solution

Structure of an Optical Fibre

Total Internal Reflection (TIR)

Conclusion

The Sigma Insight: Refraction and Total Internal Reflection

The Magic of Optical Fibres

The Anatomy of an Optical Fibre

The Principle

The Journey of Light

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