Explain what happens to the speed and wavelength of blue light and red light as they pass from air into glass, and explain why this causes white light to split into a spectrum of colours.

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).

Model answer (5 marks)

Both blue and red light slow down when they enter glass, a denser medium. Consequently their wavelengths decrease in the glass. Blue light slows down more than red light because the refractive index of glass is higher for shorter wavelengths. The greater reduction in speed causes blue light to bend more at the air–glass interface than red light. The different amounts of bending for each wavelength spread the colours apart, producing a continuous spectrum.

Examiner tips

  • Use the term ‘refractive index’ and state that it is wavelength‑dependent.
  • Show the relationship v = c/n and explain that a larger n gives a smaller v.
  • Mention that the larger change in v for blue light gives a larger change in angle (Snell’s law).
  • Explain that dispersion is the separation of wavelengths due to different refraction.

Common mistakes

  • Confusing the direction of change – claiming wavelengths increase instead of decrease.
  • Forgetting that blue light slows down more than red light.
  • Using vague terms like ‘different colours’ instead of specifying blue and red.

Mark scheme (5 marks)

  1. Both colours (all wavelengths of light) slow down when entering glass (a denser medium)
  2. The wavelength of both colours decreases in glass
  3. Blue light slows down more than red light in glass
  4. Because blue light slows down more, it refracts (bends) more than red light
  5. The different wavelengths therefore spread out / separate, forming a continuous spectrum (dispersion)

Key terms in this question

wavelength

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