# A student is investigating how light behaves when it travels from glass into air. When the angle of incidence inside the glass is exactly equal to the critical angle, the refracted ray travels along the boundary. Explain what happens to the light ray when the angle of incidence is increased beyond the critical angle, and explain why optical fibres rely on this process to transmit data over long distances.

> Edexcel A-Level Physics (9PH0) — 5.1 Wave properties · Explain · 5 marks

> Optical fibres are thin strands of glass used to carry light signals over long distances. Light enters one end of the fibre and must reach the other end without escaping through the sides of the glass.

## Mark scheme (5 marks)

1. When the angle of incidence is greater than the critical angle, total internal reflection occurs
2. All of the light is reflected back into the glass / denser medium (none escapes into the air)
3. The angle of reflection equals the angle of incidence (law of reflection applies)
4. In an optical fibre, light travels from a denser medium (glass) to a less dense medium (air/cladding), so the angle of incidence inside the fibre exceeds the critical angle at the glass–air boundary
5. Total internal reflection keeps the light bouncing along the inside of the fibre, allowing it to travel the full length without escaping / losing signal

## Key terms

- [critical angle](https://www.gradenine.co.uk/glossary/critical-angle)
- [optical fibre](https://www.gradenine.co.uk/glossary/optical-fibre)

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-how-light-34102117) · Published by Druglandscape Ltd.