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 · View as Markdown

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

Model answer (5 marks)

When the angle of incidence is less than the critical angle the ray refracts away from the normal as it passes into the air – it bends away because the light is moving from a denser medium (glass) to a less dense medium (air). When the angle of incidence equals the critical angle the refracted ray travels along the glass–air boundary. When the angle of incidence is greater than the critical angle total internal reflection occurs; the ray is reflected back into the glass with the angle of reflection equal to the angle of incidence, so all the light remains within the glass.

Examiner tips

  • Use the key terms ‘refracts’, ‘critical angle’, ‘total internal reflection’, ‘angle of incidence = angle of reflection’.
  • Show the sequence of behaviours as the angle increases – less than, equal to, greater than the critical angle.

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 in this question

critical angle

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