Explain why optical telescopes are often built on top of mountains or in space, rather than at sea level in populated areas.

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

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

Model answer (5 marks)

Optical telescopes are placed on mountains or in space because the atmosphere above them is thin or absent. This reduces absorption and refraction of light, giving a clearer, sharper image of astronomical objects. Additionally, light pollution from towns and cities at sea level makes it difficult to detect faint objects. Remote, high‑altitude sites minimise this background light, allowing fainter stars or galaxies to be observed.

Examiner tips

  • Use the word "absorption" and "refraction" to show understanding of atmospheric effects.
  • Mention both clearer images and reduced light pollution as separate reasons.
  • Keep the answer concise – 5 marks can be earned with 5 short points.
  • Use UK spelling (e.g., "separate").

Common mistakes

  • Failing to mention light pollution as a separate point.
  • Using vague terms like "better conditions" without specifying atmosphere or light pollution.
  • Writing in a narrative style rather than concise points.

Mark scheme (5 marks)

  1. The atmosphere absorbs or distorts light (from space)
  2. At high altitude / in space there is less atmosphere (above the telescope)
  3. This gives a clearer / sharper image (of astronomical objects)
  4. Light pollution from towns / cities makes it harder to observe faint objects
  5. Remote / high-altitude sites reduce the effect of light pollution (so fainter stars or objects can be detected)

Key terms in this question

optical telescope

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