The correct option is **B. Total Internal Reflection**. ### Detailed Explanation **1. Understanding Optical Fibres:** Optical fibres are thin strands of glass or plastic that transmit light signals over long distances. They are widely used in telecommunications, medical instruments, and various other applications due to their ability to carry large amounts of data with minimal loss. **2. The Principle of Total Internal Reflection:** The primary principle that allows light to be transmitted through optical fibres with minimal loss is **total internal reflection**. This phenomenon occurs when light travels from a medium with a higher refractive index (like glass) to a medium with a lower refractive index (like air) at an angle greater than the critical angle. - **Refractive Index:** This is a measure of how much light slows down in a medium compared to its speed in a vacuum. Glass has a higher refractive index than air. - **Critical Angle:** This is the minimum angle of incidence at which total internal reflection occurs. If the angle of incidence is greater than this critical angle, all the light is reflected back into the denser medium instead of refracting out into the less dense medium. **3. How Total Internal Reflection Works in Optical Fibres:** - When light enters the optical fibre, it strikes the boundary between the glass (or plastic) and air at an angle greater than the critical angle. - Instead of passing through the boundary, the light reflects back into the fibre, allowing it to travel along the length of the fibre with minimal loss. - This process can occur multiple times as the light travels through the fibre, maintaining the signal strength over long distances. ### Why Other Options Are Incorrect **A. Diffraction:** - **Explanation:** Diffraction refers to the bending of waves around obstacles and the spreading of waves when they pass through small openings. While diffraction can affect light, it is not the primary mechanism for light transmission in optical fibres. - **Weakness:** Diffraction leads to spreading and loss of intensity, which is contrary to the goal of minimizing loss in optical fibres. **C. Refraction:** - **Explanation:** Refraction is the bending of light as it passes from one medium to another due to a change in speed. While refraction is involved in the initial entry of light into the fibre, it does not account for the efficient transmission of light within the fibre. - **Weakness:** Refraction alone does not ensure that light remains within the fibre; it is total internal reflection that keeps the light contained. **D. Dispersion:** - **Explanation:** Dispersion is the phenomenon where different wavelengths of light travel at different speeds in a medium, leading to the spreading of light pulses over time. This can cause signal distortion in optical fibres. - **Weakness:** Dispersion is generally a challenge in optical fibre communication rather than a principle that aids in light transmission. ### Summary of Key Points - **Total Internal Reflection** is the primary principle that allows light to be transmitted through optical fibres with minimal loss. - Light reflects back into the fibre when it hits the boundary at an angle greater than the critical angle. - Other options like diffraction, refraction, and dispersion do not effectively explain the efficient transmission of light in optical fibres. - Understanding the critical angle and refractive indices is crucial for grasping how optical fibres work. This thorough understanding of total internal reflection and its role in optical fibres will help you grasp the fundamental principles of light transmission in this context.