Describe the use of optical fibres, particularly in telecommunications

Think of an optical fibre as a *light‑filled water slide*. Light enters the slide (the core), bounces off the walls (total internal reflection), and travels all the way to the other end without losing much energy. This is why we can send millions of internet packets across oceans in a flash!

An optical fibre is a thin strand of glass or plastic with a core surrounded by a cladding. Light is guided through the core by the principle of refraction.

• Core diameter: ~8–10 µm (single‑mode) or ~50–62.5 µm (multi‑mode)
• Core refractive index: ~1.48
• Cladding refractive index: ~1.46

Light bends when it passes from one material to another – this is described by Snell’s Law:

$n_1 \sin \theta_1 = n_2 \sin \theta_2$

In a fibre, the core has a higher refractive index than the cladding ($n_{\text{core}} > n_{\text{clad}}$). When light hits the core‑cladding interface at an angle greater than the critical angle ($\theta_c$), it is totally internally reflected and stays inside the core:

$$\theta_c = \sin^{-1}\!\left(\frac{n_{\text{clad}}}{n_{\text{core}}}\right)$$

Because the light never escapes, it can travel kilometres with very little loss.

• High bandwidth: Can carry thousands of channels simultaneously.
• Low attenuation: Only ~0.2 dB/km, far less than copper cables.
• Immunity to EMI: No interference from electrical signals.
• Lightweight & flexible: Easier to install and maintain.

These features make optical fibres the backbone of the internet, mobile networks, and cable TV.

1. Remember the key equation: $n_1 \sin \theta_1 = n_2 \sin \theta_2$.
2. Know the definition of the critical angle and how it leads to total internal reflection.
3. Be able to explain why $n_{\text{core}} > n_{\text{clad}}$ is essential.
4. Compare single‑mode and multi‑mode fibres – focus on core size and typical use cases.
5. When asked about telecom advantages, list bandwidth, attenuation, EMI immunity, and flexibility.