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.
In graded index optical fibre:

Options

A. the refractive index of the optical fibre increases as the radial distance from the fibre axis increases.
B. the refractive index of the optical fibre decreases as the radial distance from the fibre axis increases.
C. the refractive index of the optical fibre remains same throughout.
D. inner side of cladding is mirrored to ensure reflection

Explanation

**Step-by-Step Solution:**

1. **Understanding the Principle of Total Internal Reflection:**
Optical fibers operate on the principle of total internal reflection (TIR). This occurs when light travels from a denser medium (with a higher refractive index, N1) to a rarer medium (with a lower refractive index, N2) at an angle greater than the critical angle. Under these conditions, the light is reflected back into the denser medium rather than refracting out.

2. **Functionality of Optical Fibers:**
Optical fibers are designed to transmit data by allowing light rays to bounce off the walls of the fiber. This design enables the transmission of data even when the fiber is bent, as the light rays continue to reflect internally due to TIR.

3. **Materials Used:**
Optical fibers are typically made from materials like plastic or glass, which are suitable for guiding light effectively.

4. **Modes of Transmission:**
There are two primary modes of transmission in optical fibers:
- **Single-mode fiber:** Used for long-distance transmission.
- **Multi-mode fiber:** Used for shorter distances.

5. **Types of Optical Fibers Based on Refractive Index:**
Optical fibers can be classified based on their refractive index:
- **Step Index Fibers:** These consist of a core surrounded by cladding with a uniform refractive index.
- **Graded Index Fibers:** In these fibers, the refractive index decreases as the radial distance from the fiber axis increases.

6. **Specifics of Graded Index Optical Fibers:**
In graded index optical fibers, the refractive index is not uniform. Instead, it decreases gradually from the center of the fiber (the core) to the outer edge (the cladding). This design helps in reducing modal dispersion, allowing for better signal quality over distance.

7. **Conclusion:**
The correct statement regarding graded index optical fibers is that the refractive index of the optical fiber decreases as the radial distance from the fiber axis increases.

**Final Answer:**
The refractive index of the optical fibre decreases as the radial distance from the fibre axis increases.

---