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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A ray of light travels through a glass block towards its flat outer surface, where it meets the glass-air boundary.
Model answer (4 marks)
The light must be travelling from a more optically dense medium into a less optically dense medium (e.g. glass to air). The angle of incidence must be greater than the critical angle for the glass–air interface. When these conditions are satisfied, all of the light is reflected back into the glass – none is transmitted into the air. The reflected ray obeys the law of reflection, so the angle of reflection equals the angle of incidence.
Examiner tips
- State the two conditions (denser to rarer medium, incidence > critical angle) first. Use the phrase "critical angle". Mention that all light is reflected and none is transmitted. Include the law of reflection. Keep the answer concise and use correct terminology.
- Use the exact wording from the mark scheme – "more optically dense", "less optically dense", "critical angle", "all of the light is reflected".
Mark scheme (4 marks)
- The light must be travelling from a more optically dense medium into a less optically dense medium (e.g. glass to air)
- The angle of incidence must be greater than the critical angle
- All of the light is reflected back into the glass (none passes into the air)
- The angle of reflection equals the angle of incidence (law of reflection applies)
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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