Explain why a light ray changes direction when it passes from air into glass.

Edexcel GCSE Physics (1PH0) — 4.1 Waves · Explain · 4 marks · View as Markdown

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

A ray of light travels through air and then enters a glass block. The ray changes direction at the boundary between the air and the glass.

Model answer (4 marks)

The change of direction is called refraction.
When the ray enters the glass the speed of light decreases.
Because the speed decreases, the wavelength also decreases.
The reduction in wavelength causes the ray to bend towards the normal, so the angle of refraction is smaller than the angle of incidence.

Examiner tips

  • Use the word ‘refraction’ early. Mention the decrease in speed and wavelength. Explain that the ray bends towards the normal. Keep the answer concise and use correct terminology.

Common mistakes

  • Forgetting to name the process as refraction. Not stating that the speed of light decreases. Failing to mention the wavelength change or the bending towards the normal.

Mark scheme (4 marks)

  1. Names the process as refraction
  2. States that the speed of light decreases as it enters the glass
  3. States that the wavelength decreases as the light enters the glass
  4. States that the ray bends towards the normal / the angle of refraction is smaller than the angle of incidence

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