A convex lens can be used to form a real image of a distant object. Explain how a convex lens brings parallel rays of light to a focus, and state what is meant by the term 'focal length'.

WJEC GCSE Physics (Wales) — 2.1 Light, lenses and the universe · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A convex lens refracts parallel rays of light as they enter and leave the lens.
The refraction bends the rays towards the principal axis so that they converge after passing through the lens.
The converging rays meet at a single point called the focal point (principal focus).
The focal length is the distance from the centre of the lens to this focal point.

Examiner tips

  • Use the word ‘refract’ and mention both entry and exit of the lens.
  • Show the sequence: parallel → refracted → converge → focal point.
  • Define focal length as the distance from lens centre to focal point.
  • Keep the answer concise to fit the 4‑mark limit.

Common mistakes

  • Confusing a convex lens with a concave lens; describing diverging rays.
  • Failing to mention that the rays are refracted on both sides of the lens.
  • Giving a vague definition of focal length such as ‘the distance to the image’ instead of the centre to focal point.

Mark scheme (4 marks)

  1. Parallel rays of light are refracted (bent) as they enter and leave the convex lens
  2. The rays are bent towards the principal axis / converge after passing through the lens
  3. The rays meet at a single point called the focal point (principal focus)
  4. The focal length is the distance from the centre of the lens to the focal point (principal focus)

Key terms in this question

convex lens · focal length · parallel rays

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