Fibre-Optic Cable

Twisted pair and co-axial cables send electricity down copper. A fibre-optic cable sends light down glass, a strand thinner than a hair. It is what carries the Internet under the sea between continents, and it is the “fiber” in a home fibre broadband connection.

1What a fibre-optic cable is

Fibre-optic cable

A fibre-optic cable (optical fibre) carries data as light, travelling inside a thin strand of glass. The glass at the centre, the core, is covered by a layer of less dense glass called the cladding, and the whole is covered by an outer jacket of PVC or Teflon.

Data becomes light: a flash for 1 and no flash for 0, millions of times every second. A light source (an LED or a laser) makes the flashes at one end; a light detector reads them at the other. (You will see the word spelt fibre or fiber; they are the same thing.)

2How the light stays inside

Light normally travels in a straight line, so why doesn’t it leave the glass? Because every time it reaches the edge of the core, it meets the less dense glass of the cladding at a very slanting angle, and at such an angle glass acts like a mirror. The light is reflected back into the core, again and again, all the way along. This is called total internal reflection.

Refraction or reflection?

Your NCERT book says optical fibre “uses refraction to direct the light”. What actually keeps light inside is total internal reflection at the core’s boundary with the cladding — the same idea you meet in Physics. It happens because the core is denser glass than the cladding.

Light inside a strand of glass

Bend the fibre, lay a power cable next to it, or add the fibre for the return direction. Drag sideways to turn.

Pulses of light — a flash for 1, no flash for 0 — bounce off the cladding all the way to the detector.

The zigzag is drawn much steeper than real light, which runs within about 9° of the fibre’s centre. The angles in the caption are worked out for a core of refractive index 1.48 inside cladding of 1.46; the bends are 400 and 60 times the core’s radius.

Bend the fibre gently and the light still stays in. Bend it too sharply and the light meets the wall too steeply to be reflected, and it escapes — which is why fibre cables must never be kinked. And with the power cable beside it, nothing changes at all: light is not affected by electrical noise.

Light in the glass travels at about 2,03,000 km per second, around two-thirds of its speed in air.

Think of a slide with high walls

Roll a ball down a long water slide with curved walls. It keeps hitting the walls and bouncing back into the channel, all the way to the pool. Tip the slide into a sharp corner, though, and the ball flies over the wall.

3One fibre, one direction

A fibre carries light in one direction, from the source to the detector. NCERT calls optic fibres unidirectional, so to send and receive at the same time, a link normally uses two fibres, one each way. Switch on Both directions in the lab to see the second one.

4Uses, advantages and disadvantages

Fibre-optic cables are used in backbone networks — the main links that join networks together — between cities, under the sea between countries, between buildings of a large campus, and more and more right into homes.

✅ Advantages

The highest bandwidth, so the highest data transfer rate.
Signals travel much longer distances before they need a repeater.
Not affected by electrical noise.
Light in weight.

❌ Disadvantages

The most expensive wired medium.
Glass is fragile: it cannot be bent sharply.
Hard to install and to join; it needs special tools.
One fibre carries light one way, so links need two.

5From the board papers

Which fast and effective wired transmission medium should be used to connect the prime office at Bengaluru with the Chennai division (2000 km away)?CBSE board exam 2024 (part of a case study) · asked the same way in the CBSE compartment exams 2023 and 2025 and the CBSE sample paper 2025-26

1 mark

▶Show the answer

Optical fibre cable. Fast, high bandwidth and able to cover long distances. CBSE’s sample-paper marking scheme gives “Optical Fibre” for this kind of question.

Which of the following transmission media has the highest bandwidth? (A) Co-axial cable (B) Fibre optic cable (C) Twisted pair cable (D) None of theseKV pre-board 2024-25, Ahmedabad region

1 mark

▶Show the answer

(B) Fibre optic cable.

6Check yourself

Quick Check

What does a fibre-optic cable use to carry data?

Quick Check

Why is a fibre-optic cable not affected by a power cable running next to it?

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