# A scientist is designing an optical fibre made from a dense glass material. Light travels along the fibre by entering the glass from air, passing through the glass, and then striking the inside surface of the glass at a very shallow angle. Explain what happens to the light at each stage of this journey, including why the light stays inside the glass when it strikes the inside surface at a very shallow angle.

> WJEC A-Level Physics (Wales) — 2.6 Refraction of light · Explain · 5 marks

> Optical fibres are used in medicine and telecommunications to carry information as pulses of light over long distances without the light escaping through the sides of the fibre.

## Mark scheme (5 marks)

1. When light passes from air into the glass it changes speed (slows down)
2. The light bends towards the normal as it enters the glass (refracts towards the normal)
3. When striking the inside surface at a very shallow angle, the angle of incidence (measured from the normal) is large / greater than the critical angle
4. Total internal reflection occurs — all the light reflects back into the glass
5. Total internal reflection can only occur when light travels from a denser medium to a less dense medium (glass to air)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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-scientist-is-designing-an-optical-c346df12) · Published by Druglandscape Ltd.