The bent straw
Put a straw in a glass of water. Look from the side. The straw seems to bend where it meets the water.
The straw is straight. The light coming from it changes direction at the water. That one idea, light changing direction, is how a thin glass strand carries data over long distances.
Key words
| Word | What it means |
|---|---|
| Refraction | the bending of light as it passes from one material to another |
| Total internal reflection | light bouncing fully back inside a material at its edge, short name TIR |
| Core | the centre of a fibre, which carries the light |
| Cladding | the glass layer round the core that bounces the light back in |
| Single mode | a fibre with a very thin core that lets one path of light through |
| Multi-mode | a fibre with a wider core that lets many paths of light through |
| Micrometre | one thousandth of a millimetre |
| Laser | a light source that makes a narrow, strong beam |
| LED | a light source that makes a wider, weaker beam |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can explain refraction and total internal reflection, and compare single mode with multi-mode fibre.
Starter: fibre
Have a go
Starter: fibre
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Refraction: light changes direction
Refraction happens when light goes from one material into another, such as air into water or glass. It slows down and its path bends.
How much it bends depends on the two materials. Glass of one kind bends light more than glass of another kind. Fibre makers use this on purpose.
Total internal reflection
A fibre has a core in the middle and cladding around it. The glass of the core bends light more than the cladding does.
Light travelling along the core hits the edge at a shallow angle. It does not escape. It bounces back in. This is total internal reflection, or TIR. The cladding does not soak the light up. It bounces it back, so the signal can travel a long way.
Put the light path in order
Have a go
Put the light path in order
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.