Fibre optic cable
A fibre optic cable is a long stretch of thin glass.
It’s used to transmit information - e.g. in the internet.
How it works
- The angle of incidence of each transmitted pulse of light must be greater than the critical angle.
- When light hits the edge of the fibre, it reflects. That’s because the angle is more than the critical angle, and so total internal reflection occurs.
- It’s not a ‘mirror’, but does still reflect (almost) all light.
Parts of a fibre-optic cable
- The core (the middle bit) has the highest refractive index
- The cladding (outside) has a slightly lower refractive index than the core (but still higher than air)
- The air around the cable has the lowest refractive index, around
1 .
Problems with fibre-optic cables:
flashcards
| Question | Answer |
|---|---|
| What is a fibre optic cable made of? | It is a long stretch of thin glass used to transmit information, e.g. in the internet. |
| What condition must the angle of incidence of a light pulse in a fibre optic cable satisfy? | The angle of incidence must be greater than the critical angle. |
| What physical phenomenon occurs when light hits the edge of a fibre optic cable? | Total internal reflection (TIR) occurs because the angle of incidence is greater than the critical angle. |
| Does a fibre optic cable use a mirror to reflect light? | No, it is not a mirror but it still reflects (almost) all light via total internal reflection. |
| What is the name of the central part of a fibre optic cable, and what is its refractive index like? | The core is the middle bit and has the highest refractive index. |
| What is the cladding of a fibre optic cable, and how does its refractive index compare to the core? | The cladding is the outside part that has a slightly lower refractive index than the core (but still higher than air). |
| What is the approximate refractive index of the air around a fibre optic cable? | Approximately |
| What problem occurs in fibre optic cables caused by light rays taking different paths (lengths) through the cable? | Fibre optic modal dispersion. |
| What problem occurs in fibre optic cables caused by light of different wavelengths travelling at different speeds? | Fibre optic material dispersion. |