3.3.2.3
Optical fibres use total internal reflection to transmit light signals
Refraction & Total Internal Reflection — AQA A-Level Physics
- Light enters one end of the fibre, undergoes repeated total internal reflectionThe complete reflection of a wave at a boundary when the angle of incidence exceeds the critical angleThe angle of incidence at which the refracted ray travels along the boundary (angle of refractionThe change in direction of a wave as it passes from one medium to another, caused by a change in wave speed. = 90 degrees). For angles greater than this, total internal reflection occurs. and the wave travels from a denser to a less dense medium. against the sides, and exits the other end.
- Signals travel long distances without significant loss of information or speed.
- Uses: telecommunications (telephone and internet), medical imaging (endoscopes).
Structure of a step-index optical fibre
- Core: optically dense glass or plastic tube through which light travels.
- Cladding: a layer of lower refractive indexThe ratio of the speed of light in a vacuum to the speed of light in a medium. Determines how much light bends on entering the medium. material surrounding the core.
- Outer sheath: protective covering.
- Called 'step-index' because the refractive indexThe ratio of the speed of light in a vacuum to the speed of light in a medium. Determines how much light bends on entering the medium. changes abruptly at the core-cladding boundary.
Role of cladding
- Protects the core from scratches and damage.
- Ensures TIR occurs at the core-cladding boundary (n_core > n_cladding).
- Prevents signal degradation from light escaping the core.
- Keeps fibres separate to prevent information crossover between adjacent fibres.
Related:Diffraction