A ray of light travels from glass into air and hits the boundary at a large angle to the normal. Explain why the ray does not pass into the air.

Eduqas GCSE Physics — 5.2 Waves at material interfaces · Explain · 4 marks · View as Markdown

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

Light can travel through both glass and air. When light reaches the boundary between the two materials, different things can happen depending on the angle at which it hits the boundary.

Model answer (4 marks)

The angle of incidence is greater than the critical angle for the glass–air interface.
Because the light is travelling from a denser medium (glass) into a less dense medium (air), the critical angle is reached.
When the incidence angle exceeds this critical value, total internal reflection occurs.
Thus the ray is reflected back into the glass and no light is transmitted into the air.

Examiner tips

  • Use the term "critical angle" and state it is exceeded. Mention "total internal reflection" as the mechanism. Explain the denser-to-less‑dense transition. Keep the answer to four concise points.

Common mistakes

  • Confusing refraction with reflection. Forgetting to mention the critical angle. Using the wrong order of media (air to glass).

Mark scheme (4 marks)

  1. The angle of incidence is greater than (or equal to) the critical angle
  2. Total internal reflection occurs
  3. The light is reflected back into the glass (not transmitted into the air)
  4. This occurs because light travels from a denser medium (glass) into a less dense medium (air)

Key terms in this question

boundary

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