# A student shines a ray of light from glass into air. Explain what happens to the ray of light as the angle of incidence is increased from less than the critical angle, through the critical angle, to greater than the critical angle.

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

## Mark scheme (5 marks)

1. When the angle of incidence is less than the critical angle, the light refracts away from the normal as it passes into the air
2. This is because the light is moving from a denser medium (glass) to a less dense medium (air)
3. When the angle of incidence equals the critical angle, the refracted ray travels along the boundary between the glass and air
4. When the angle of incidence is greater than the critical angle, total internal reflection occurs
5. During total internal reflection, the angle of reflection equals the angle of incidence (the law of reflection is obeyed) and all the light remains within the glass

## Key terms

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

## 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-shines-a-ray-of-e17fb817) · Published by Druglandscape Ltd.