# A ray of light travels from glass into air and undergoes total internal reflection at the glass–air boundary. State and explain the two conditions that must both be satisfied for total internal reflection to occur.

> Cambridge International IGCSE Physics (0625) — 3.2 Light · Explain · 4 marks

## 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 into air).
2. The angle of incidence must be greater than the critical angle.
3. When the angle of incidence equals the critical angle, the refracted ray travels along the boundary; beyond this angle no refraction into the second medium occurs.
4. The light is reflected back into the glass (denser medium), obeying the law of reflection at the boundary.

## Key terms

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

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](https://www.gradenine.co.uk/learn)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-ray-of-light-travels-from-41501def) · Published by Druglandscape Ltd.