A student shines a beam of white light onto a rough white surface and onto a smooth mirror. Explain the difference in the way the two surfaces reflect the light, and why only one surface produces a clear image.

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 contains all the colours of the visible spectrum. When light strikes a surface, it can be reflected in different ways depending on the texture of that surface.

Model answer (5 marks)

A smooth mirror reflects light by specular reflection: all incident rays strike the surface at the same angle to the normal and are reflected at the same angle, so the reflected rays leave in the same direction and are parallel. A rough white surface reflects light by diffuse reflection: the surface normal varies across the texture, so the reflected rays are scattered in many different directions. Only the smooth mirror produces a clear image because the reflected rays leave in a consistent direction, preserving the pattern of the light and allowing the eye to trace the rays back to a single point; the rough surface scatters the rays so no consistent image is formed.

Examiner tips

  • Use the terms "specular" and "diffuse" reflection
  • Show the law of reflection for the mirror
  • Explain how the normal varies on the rough surface
  • Link the direction of reflected rays to image formation

Common mistakes

  • Confusing diffuse with specular reflection
  • Forgetting to mention the law of reflection
  • Saying the rough surface reflects light but not that it scatters it

Mark scheme (5 marks)

  1. The smooth mirror undergoes specular reflection
  2. All incident rays hit the smooth surface at the same angle and are reflected at the same angle (law of reflection), so rays leave in the same direction / are parallel
  3. The rough white surface undergoes diffuse reflection
  4. The rough surface scatters the reflected light in many different directions because the normal varies across the surface
  5. Only the smooth mirror produces a clear image because the reflected rays leave in a consistent direction, preserving the pattern of the light / allowing the eye to trace rays back to a single point; the rough surface scatters rays so no consistent image is formed

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