Consider telecommunication through optical fibres. Which of the following statements is not true?

Options

A. optical fibers can be of graded refractive index
B. optical fibers are subject are subjected to electro- magnetic interference from outside
C. optical fibers have extremely low transmission loss
D. optical fibers may have homogeneous core with a suitable cladding

Answer

To solve the question regarding telecommunication through optical fibers and identify which statement is not true, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Optical Fibers**: Optical fibers are used for transmitting signals over long distances with minimal energy loss. They consist of a core and cladding, where the core has a higher refractive index than the cladding.

2. **Identify Key Concepts**:
- **Core and Cladding**: The core is the central part of the fiber through which light travels, and the cladding surrounds the core and has a lower refractive index.
- **Total Internal Reflection (TIR)**: The principle that allows light to be transmitted through the fiber without escaping is total internal reflection. This occurs when light travels from a medium with a higher refractive index to one with a lower refractive index at an angle greater than the critical angle.

3. **Evaluate the Statements**:
- **Statement A**: "Optical fibers can be made of graded refractive index material."
- This is true. Optical fibers can indeed be made with a graded refractive index to improve performance.

- **Statement B**: "Optical fibers are subjected to electromagnetic interference from outside."
- This is false. Optical fibers are immune to electromagnetic interference, which is one of their advantages over traditional copper cables.

- **Statement C**: "Optical fibers have extremely low transmission loss."
- This is true. Due to TIR, optical fibers experience very low transmission loss.

- **Statement D**: "Optical fibers may have a homogeneous core with suitable cladding."
- This is true. A homogeneous core can be used with appropriate cladding, provided the refractive index conditions are met.

4. **Identify the Incorrect Statement**: The only statement that is not true is Statement B, which claims that optical fibers are subjected to electromagnetic interference.

### Conclusion:
The statement that is not true is **B**: "Optical fibers are subjected to electromagnetic interference from outside."