# Light travels from glass into air and undergoes total internal reflection at the glass-air boundary. Explain the conditions that must be met for total internal reflection to occur, and describe what happens to the light ray at the boundary when these conditions are satisfied.

> Pearson Edexcel International GCSE Physics (4PH1) — 3.3 Light and sound · Explain · 4 marks

> A ray of light travels through a glass block towards its flat outer surface, where it meets the glass-air boundary.

## Mark scheme (4 marks)

1. The light must be travelling from a more optically dense medium into a less optically dense medium (e.g. glass to air)
2. The angle of incidence must be greater than the critical angle
3. All of the light is reflected back into the glass (none passes into the air)
4. The angle of reflection equals the angle of incidence (law of reflection applies)

## Key terms

- [total internal reflection](https://www.gradenine.co.uk/glossary/total-internal-reflection)

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](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/light-travels-from-glass-into-air-2edbeda1) · Published by Druglandscape Ltd.