Fibre Optics
Internet signals travel under the oceans as flashes of light inside glass threads. Aim a laser into a fibre and see how the light gets trapped inside.
- Critical angle
- 76.7°
- Hits the wall at
- 80.1°
- Light lost
- 0%
- Bounces
- 7
The maths, live
Critical angle sin θc = n₂ ÷ n₁sin θc = 1.46 ÷ 1.50 = 0.973, θc = 76.7°Into the end 1.00 × sin θ = 1.50 × sin θ₂sin 15° ÷ 1.50 → θ₂ = 9.9°Straight fibre: wall angle = 90° − θ₂80.1° > 76.7° → trapped (TIR)With cladding, find the biggest launch angle that still keeps all the light in. Then bend the fibre and watch light escape on the outside of the curve. Does bare glass in air cope better with corners?
How it works
Light going from glass into something with a lower refractive index bends away from the normal. If it hits the side at a big enough angle (more than the critical angle), it can't get out at all and reflects back. This is total internal reflection, and it bounces the light all the way along the fibre.
The critical angle comes from sin θc = n₂ ÷ n₁. Real fibres have a glass core wrapped in cladding made of slightly different glass, so the critical angle is very large (about 77° here). Light that goes in too steeply, or meets a sharp bend, hits the wall below the critical angle and leaks out.
Try this
- Find the biggest launch angle that still gets all the light to the end of the fibre with cladding.
- Bend the fibre more and more. Where does the light escape?
- Swap the cladding for air. Is the critical angle bigger or smaller now?