Explain why white light separates into different colours when it passes through a glass prism.

Cambridge International IGCSE Physics (0625) — 3.2 Light · Explain · 4 marks · View as Markdown

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

A glass prism is used to produce a spectrum from a beam of white light. The colours emerge from the prism spread out, with violet light deviated the most and red light deviated the least.

Model answer (4 marks)

White light is a mixture of all visible colours, each with a different wavelength.

When it enters the glass prism the speed of each colour changes because the refractive index of glass depends on wavelength.

The change in speed causes each wavelength to bend by a different amount – this is dispersion.

Violet light, having the shortest wavelength, has the highest refractive index and therefore bends the most, while red light, with the longest wavelength, bends the least.

Examiner tips

  • Use the term ‘dispersion’ to link speed change and bending.
  • Show the order of bending (violet > … > red).
  • Mention refractive index varies with wavelength.
  • Keep the answer concise – 4 points only.

Common mistakes

  • Saying only the prism material causes separation, ignoring wavelength dependence.
  • Confusing refraction with reflection.
  • Using vague terms like ‘different colours’ without linking to wavelength or speed.

Mark scheme (4 marks)

  1. White light is made up of different colours / frequencies / wavelengths
  2. Different colours travel at different speeds in glass
  3. Each colour refracts by a different amount because of its different speed in glass
  4. Violet light refracts the most because it travels slowest in glass / has the greatest refractive index; red light refracts the least

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