Optical Fibre:
Optical fibre works on the principle of total internal reflection. Light rays can be used to transmit a huge amount of data, but there is a problem here - the light rays travel in straight lines. So unless we have a long straight wire without any bends at all, harnessing this advantage will be very tedious. Instead, the optical cables are designed such that they bend all the light rays' inwards (using TIR). Light rays travel continuously, bouncing off the optical fibre walls and transmitting end to end data. It is usually made of plastic or glass.
Modes of transmission: Single-mode fibre is used for long-distance transmission, while multi mode fiber is used for shorter distances. The outer cladding of these fibres needs better protection than metal wires. Although light signals do degrade over progressing distances due to absorption and scattering. Then, optical Regenerator system is necessary to boost the signal.
Types of Optical Fibres: The types of optical fibers depend on the refractive index, materials used, and mode of propagation of light. The classification based on the refractive index is as follows:
• Step Index Fibres: It consists of a core surrounded by the cladding, which has a single uniform index of refraction.
• Graded Index Fibres: The refractive index of the optical fibre decreases as the radial distance from the fibre axis increases.
Optical fibre is made of:

Options

A. copper
B. semiconductor
C. plastic or glass
D. superconductors

Explanation

### Step-by-Step Solution:

1. **Understanding the Principle of Optical Fibres**:
Optical fibres operate on the principle of Total Internal Reflection (TIR). This principle states that when light travels from a denser medium to a rarer medium at an angle greater than the critical angle, it reflects back into the denser medium instead of refracting out.

2. **Materials Used in Optical Fibres**:
Optical fibres are primarily made of two materials: plastic and glass. These materials are chosen for their optical properties, which allow for efficient transmission of light signals.

3. **Modes of Transmission**:
There are two main modes of transmission in optical fibres:
- **Single-mode fibre**: This type is used for long-distance transmission, allowing only one mode of light to propagate.
- **Multi-mode fibre**: This type is used for shorter distances, allowing multiple modes of light to propagate.

4. **Need for Cladding**:
The outer cladding of optical fibres provides protection and helps maintain the total internal reflection of light within the core. The cladding must be made of a material that has a lower refractive index than the core.

5. **Signal Degradation and Regeneration**:
Over long distances, light signals can degrade due to absorption and scattering. To counteract this degradation, an optical regenerator system is necessary to boost the signal strength.

6. **Types of Optical Fibres**:
Optical fibres can be classified based on their refractive index:
- **Step Index Fibres**: These have a core surrounded by cladding with a uniform refractive index.
- **Graded Index Fibres**: In these fibres, the refractive index decreases with increasing radial distance from the fibre axis.

### Final Answer:
Optical fibre is usually made of **plastic and glass**.

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