IGCSE Physics 0625

3.2.2 Refraction of light

Objective: Describe the use of optical fibres, particularly in telecommunications

Optical fibres are thin strands of glass or plastic that transmit light. They work on the principle of total internal reflection, which is a key concept in the refraction of light. This section will explore how optical fibres are used in telecommunications to transmit information efficiently and over long distances.

How Optical Fibres Work

An optical fibre consists of two main parts:

• Core: The central part of the fibre, made of high-refractive index glass or plastic.
• Cladding: A layer of glass or plastic surrounding the core, with a slightly lower refractive index.

When light enters the core at a sufficiently shallow angle (greater than the critical angle), it undergoes total internal reflection and is guided along the fibre. This means the light is continuously reflected off the core-cladding interface, preventing it from escaping.

Optical Fibres in Telecommunications

Optical fibres have revolutionized telecommunications because they offer several advantages over traditional copper wires:

• High Bandwidth: Optical fibres can transmit much more data than copper wires. This is because light has a much higher frequency than electrical signals.
• Low Signal Loss: Light signals in optical fibres experience significantly less signal loss over long distances compared to electrical signals in copper wires.
• Immunity to Electromagnetic Interference: Optical fibres are not affected by electromagnetic interference, which can disrupt the transmission of signals in copper wires.
• Security: It is much more difficult to tap into an optical fibre cable without being detected compared to a copper wire.

The Process of Data Transmission

1. Signal Conversion: Information (voice, data, video) is first converted into electrical signals.
2. Light Emission: These electrical signals are then used to modulate a light source (e.g., a laser diode or LED) within the fibre. The modulation changes the intensity of the light.
3. Light Propagation: The modulated light pulses travel down the optical fibre via total internal reflection.
4. Signal Detection: At the receiving end, a photodiode detects the light pulses and converts them back into electrical signals.
5. Signal Conversion (Reverse): The electrical signals are then converted back into the original information.

Types of Optical Fibres

Type	Description	Advantages	Disadvantages
Single-mode fibre	Has a small core diameter, allowing only one mode of light to travel through it.	High bandwidth, suitable for long distances.	More expensive, requires more precise alignment.
Multi-mode fibre	Has a larger core diameter, allowing multiple modes of light to travel through it.	Less expensive, easier to align.	Lower bandwidth, suitable for shorter distances.

Applications of Optical Fibres

Optical fibres are used in a wide range of applications, including:

• Telecommunications networks: The backbone of modern internet and telephone systems.
• Cable television: Transmitting TV signals to homes.
• Medical imaging: Endoscopes use optical fibres to transmit light and images within the body.
• Industrial sensors: Monitoring temperature, pressure, and other parameters in industrial processes.
• Lighting: Used in some lighting applications for efficient and long-distance light transmission.