Explain why white light splits into a spectrum of colours when it passes through a glass prism.

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).

When a narrow beam of white light enters a glass prism, a rainbow-like band of colours emerges from the other side.

Model answer (5 marks)

White light consists of many wavelengths (colours). When it enters glass it slows because the medium is denser. Shorter wavelengths are slowed more than longer wavelengths. The different slowing causes each wavelength to refract by a different amount. This separation of angles spreads the colours into a spectrum.

Examiner tips

  • Use the term ‘wavelength’ and ‘frequency’ for white light.
  • Explain the slowing in a denser medium before the angle change.
  • Show the relationship ‘shorter wavelengths bend more’. Mention the final result – a spectrum.
  • Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Saying the light is ‘split’ without mentioning wavelengths.
  • Confusing refraction with reflection.
  • Using vague phrases like ‘different colours’ without linking to wavelength or speed.

Mark scheme (5 marks)

  1. White light is made up of different wavelengths (colours / frequencies)
  2. When light enters glass it slows down (entering a denser medium)
  3. Shorter wavelengths slow down more than longer wavelengths (blue slows more than red)
  4. Because they slow by different amounts, the different wavelengths refract (bend) by different amounts
  5. This causes the colours to spread out / separate, forming a spectrum

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