How does the refractive index of the core compare to the cladding in an optical fibre?

The core must have a higher refractive index than the cladding. This difference ensures that light traveling through the core is reflected back when it hits the boundary at a shallow angle, enabling total internal reflection.

What is the primary advantage of optical fibres over copper wires regarding electromagnetic interference (EMI)?

Optical fibres are immune to EMI because they transmit signals as light rather than electricity. This allows them to be placed near high-voltage equipment or in areas with heavy radio interference without signal degradation.

In terms of signal security, why is optical fibre preferred over traditional metallic cables?

Optical fibres do not emit electromagnetic fields, so they cannot be 'tapped' by external sensors. Any physical attempt to tap the cable usually breaks the light path, which is immediately detectable as a signal loss.

What is a common misconception about the path light takes inside an optical fibre?

A common error is assuming light travels in a perfectly straight line through the center. In reality, light travels in a zig-zag pattern, constantly reflecting off the internal walls of the core via total internal reflection.

Why is it incorrect to say that optical fibres have zero signal loss?

While loss is extremely low, it is not zero. Signals are gradually weakened by 'attenuation,' which is caused by the scattering of light from microscopic density changes in the glass and absorption by residual water molecules.

What happens to the light signal if the fibre is bent too sharply?

If the bend radius is too small, the angle at which light hits the core-cladding boundary may become smaller than the critical angle. This causes light to leak out into the cladding, resulting in significant signal loss.

Define 'Total Internal Reflection' in the context of fibre optics.

Total Internal Reflection is the phenomenon where 100% of light is reflected back into a denser medium (the core) when it strikes the boundary of a less dense medium (the cladding) at an angle of incidence greater than the critical angle.

What is an 'Endoscope' and how does it utilize optical fibres?

An endoscope is a medical instrument used to look inside the body. It uses two bundles of fibres: one to carry light from an external source to illuminate the internal organ, and another to carry the reflected image back to the observer.

What is the 'Cladding' of an optical fibre?

The cladding is the outer layer of optical material that surrounds the core. It has a lower refractive index than the core to facilitate total internal reflection and protects the core from surface contaminants.

How does an optical fibre sensor detect changes in temperature or pressure?

Physical changes like heat or pressure slightly deform the fibre or change its refractive index. These changes alter the phase, intensity, or wavelength of the light passing through, which can be measured with high precision.

Uses of Optical Fibres | WJEC GCSE Physics

1. Electricity, Energy & Waves

Uses of Optical Fibres

Summary

Optical fibres are thin strands of high-purity glass or plastic that transmit information as light pulses using the principle of total internal reflection. Their unique properties, such as high bandwidth, low signal attenuation, and immunity to electromagnetic interference, make them indispensable in modern telecommunications, medicine, and industrial sensing.

1. Definition & Core Concepts

Optical Fibre: A flexible, transparent waveguide made of silica or plastic, consisting of a central core surrounded by a cladding layer with a lower refractive index.
Total Internal Reflection (TIR): The fundamental physical principle where light hitting the boundary between the core and cladding at an angle greater than the critical angle is reflected entirely back into the core.
Signal Transmission: Data is encoded into light pulses (photons) rather than electrical currents (electrons), allowing for significantly faster data transfer rates over much longer distances.

Diagram showing a light ray undergoing total internal reflection inside the core of an optical fiber, bounded by the cladding.

2. Telecommunications Applications

High Bandwidth: Optical fibres can carry vast amounts of data simultaneously by using different wavelengths of light, a technique known as wavelength-division multiplexing.
Low Attenuation: Unlike copper wires, light signals in high-quality glass fibres lose very little energy over long distances, reducing the need for signal repeaters or amplifiers.
EMI Immunity: Because the signals are optical rather than electrical, they are completely unaffected by electromagnetic interference from power lines, lightning, or radio signals.

3. Medical Applications

Endoscopy: Bundles of optical fibres are used in endoscopes to view internal organs without major surgery. One bundle transmits light to illuminate the area, while another carries the reflected image back to the surgeon.
Laser Surgery: High-power laser light can be delivered through flexible fibres to precise locations inside the body to cauterize tissue, break up kidney stones, or perform delicate eye surgeries.
Non-Invasive Diagnostics: Fibre-optic sensors can be inserted into blood vessels to monitor parameters like blood pressure, temperature, and oxygen saturation in real-time.

4. Industrial & Sensing Applications

5. Key Distinctions: Fibre vs. Copper

6. Exam Strategy & Tips