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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. The light must travel 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 for that interface.
3. At the critical angle the refracted ray runs along the boundary; for larger angles no refraction occurs.
4. The light is then reflected back into the denser medium, obeying the law of reflection.

Examiner tips

  • Use the exact phrase "more optically dense" and "less optically dense". Show the two conditions separately. Mention the critical angle and the behaviour beyond it. Include that the reflected ray stays in the denser medium.

Common mistakes

  • Confusing the direction of travel (e.g. saying light goes from air to glass). Forgetting to state that the reflected ray remains in the denser medium. Mixing up the critical angle with the angle of refraction.

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 in this question

total internal reflection

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