A student investigates the refraction of light as it passes from air into a glass block. The student observes that the light ray bends towards the normal as it enters the glass. Explain why the light ray changes direction, and describe what happens to the speed and wavelength of the light as it enters the glass.

Edexcel A-Level Physics (9PH0) — 4.1 Stress, strain and the Young modulus · Explain · 5 marks · View as Markdown

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

Light travelling through air enters a rectangular glass block at an angle to the normal at the surface.

Model answer (5 marks)

The light changes from one medium (air) to another (glass). The speed of light in glass is lower than in air, so the light slows down. When the speed decreases, the ray bends towards the normal according to Snell’s law. The frequency of the light does not change when it enters the new medium. Because the speed decreases while the frequency stays the same, the wavelength must decrease in the glass.

Examiner tips

  • Use the phrase ‘changes medium’ to show understanding of refraction. Mention ‘speed decreases’ and ‘frequency unchanged’ – these are key points. Explain that the wavelength decreases to keep the frequency constant. Use the word ‘bends towards the normal’ to link speed change to direction change.

Common mistakes

  • Saying the frequency changes or ignoring the frequency point. Forgetting to state that the ray bends towards the normal. Using ‘speed increases’ or ‘wavelength increases’ – the opposite of what happens in glass.

Mark scheme (5 marks)

  1. The light changes medium (from air to glass)
  2. The speed of light decreases as it enters the glass
  3. Because the speed decreases, the light bends towards the normal
  4. The frequency of the light remains constant / unchanged
  5. The wavelength of the light decreases

Key terms in this question

refraction · normal · wavelength

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