A beam of white light passes through a red colour filter and then strikes a rough white surface. Explain what the observer sees after the light has interacted with both the filter and the rough surface, and explain why each effect occurs.

Edexcel A-Level Physics (9PH0) — 5.1 Wave properties · Explain · 5 marks · View as Markdown

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

White light is made up of all the colours of the visible spectrum. Colour filters and surface texture both affect how light behaves.

Model answer (5 marks)

The red filter absorbs all wavelengths except red, so only red light reaches the rough surface.
The rough surface produces diffuse reflection, scattering the red light in many directions.
Thus the observer sees red light coming from the surface, visible from many angles.

Examiner tips

  • State that the filter transmits only red and absorbs the rest.
  • Explain that a rough surface causes diffuse reflection by scattering light.
  • Show that the scattered light is still red, so the observer sees red.
  • Use correct terminology: ‘diffuse reflection’, ‘wavelengths’.”]

Mark scheme (5 marks)

  1. The colour filter absorbs all colours except red (or: only red wavelengths are transmitted through the filter).
  2. So only red light reaches the rough surface (correct consequence of filtering).
  3. The rough surface causes diffuse reflection (naming the process).
  4. Diffuse reflection scatters the red light in many different directions (because the surface has many different angles / orientations of tiny surfaces).
  5. The observer sees red light coming from the surface (scattered / spread in many directions so visible from different positions).

Key terms in this question

colour filter · white light

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