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

AQA A-Level Physics (7408) — 3.3.2 Refraction, Diffraction and Interference · Explain · 5 marks · View as Markdown

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

A ray of light travelling through air enters a glass block. The ray bends towards the normal as it crosses the boundary.

Model answer (5 marks)

Light slows down when it enters glass, a denser medium.
The wavelength of the light decreases as it enters the glass.
The frequency of the light stays the same.
Because the speed and wavelength decrease, the ray bends towards the normal.
This change in direction is called refraction.

Examiner tips

  • Use the exact terms: ‘slows down’, ‘decreases wavelength’, ‘frequency unchanged’, ‘bends towards the normal’, ‘refraction’.
  • Show the logical link: speed↓ → λ↓ → bend towards normal → refraction.

Common mistakes

  • Saying the frequency changes, or that the ray bends away from the normal.
  • Using vague phrases like ‘changes direction’ without naming refraction.

Mark scheme (5 marks)

  1. Light slows down when it enters glass (a denser medium)
  2. The wavelength of light decreases as it enters the glass
  3. The frequency of the light stays the same / does not change
  4. The ray bends towards the normal because it is entering a denser medium
  5. This change in direction is called refraction

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