The phenomenon by which light travels in an optical fibres is

Options

A. a) total internal reflection
B. b) scattering
C. c) diffraction
D. d) refraction

Answer

To solve the question regarding the phenomenon by which light travels in optical fibers, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Optical Fibers**:
- Optical fibers consist of two main parts: the core and the cladding. The core is the central part through which light travels, while the cladding surrounds the core and has a lower refractive index.

2. **Refractive Indices**:
- Let’s denote the refractive index of the core as \( n_1 \) and that of the cladding as \( n_2 \). For light to be guided through the optical fiber, it is essential that \( n_1 > n_2 \). This means the core is denser than the cladding.

3. **Light Propagation**:
- When light enters the optical fiber, it strikes the boundary between the core and the cladding. If the angle of incidence is greater than the critical angle, total internal reflection occurs. This means that the light does not pass into the cladding but reflects back into the core.

4. **Total Internal Reflection**:
- The phenomenon that allows light to be continuously reflected within the core of the optical fiber is known as total internal reflection. This process enables the light to travel long distances with minimal loss.

5. **Conclusion**:
- Therefore, the correct answer to the question is that the phenomenon by which light travels in optical fibers is **total internal reflection**.

### Final Answer:
The phenomenon by which light travels in optical fibers is **total internal reflection**.

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