Describe the differences between how a refracting telescope and a reflecting telescope form an image of a distant star.

AQA A-Level Physics (7408) — 3.9.1 Telescopes · Describe · 5 marks · View as Markdown

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

Model answer (5 marks)

A refracting telescope uses a convex objective lens to refract and focus light from a distant star.
A reflecting telescope uses a concave primary mirror to reflect and focus light from a distant star.
In a refractor the objective lens collects the light and brings it to a focal point.
In a reflector the primary mirror collects the light and brings it to a focal point.
Because mirrors are easier and cheaper to produce in large size than lenses, reflecting telescopes can be built larger and collect more light.

Examiner tips

  • Use the terms "refracting" and "reflecting" to show you know the two types. Mention the key optical element (lens vs mirror) and the action (refract vs reflect). Include the point about size and light‑gathering advantage for reflectors.
  • common_mistakes
  • :
  • Confusing the action of the objective (refract vs reflect). Forgetting to mention the size/light‑gathering advantage of mirrors.

Mark scheme (5 marks)

  1. A refracting telescope uses lenses (to refract/bend light)
  2. A reflecting telescope uses a curved/concave mirror (to reflect light)
  3. In a refracting telescope the objective lens collects and focuses the incoming light
  4. In a reflecting telescope the primary/curved mirror collects and focuses the incoming light (to a focal point)
  5. Large mirrors are easier/cheaper to make than large lenses, so reflecting telescopes can be made larger / collect more light

Key terms in this question

refracting telescope · reflecting telescope

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