Refraction occurs when a wave changes speed as it moves from one medium to another. Explain what happens to a light wave when it travels from air into glass, referring to its speed, wavelength, frequency, and direction of travel.

Eduqas A-Level Physics — 5.3 Electric and magnetic fields (A-Level only) · 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 a light wave enters glass from air the speed of the wave decreases because the refractive index of glass is higher than that of air.
The wavelength shortens in proportion to the decrease in speed (λ = v/f) while the frequency remains unchanged as it is determined by the source.
The change in speed causes the wavefronts to bend; the ray is refracted towards the normal, i.e. it travels at a smaller angle to the normal in the glass.

Examiner notes: speed ↓, wavelength ↓, frequency =, direction changes towards normal.

Examiner tips

  • Use the formula λ = v/f to link speed and wavelength.
  • State that frequency is constant because it is set by the source.
  • Show the direction change as bending towards the normal.
  • Mention the refractive index concept briefly if space allows.

Common mistakes

  • Saying the frequency increases or decreases.
  • Forgetting that the direction change is towards the normal.
  • Confusing the direction of bending with the direction of the wavefronts.

Mark scheme (5 marks)

  1. The speed of the light wave decreases as it enters the glass
  2. The wavelength of the light wave decreases
  3. The frequency of the light wave remains constant / does not change
  4. The light wave changes direction / bends
  5. The light bends towards the normal (because it is moving into a denser medium / slowing down)

Key terms in this question

refraction · wavelength · frequency

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