Explain why astronomers began building radio telescopes in addition to optical telescopes, and describe one advantage and one disadvantage of using a radio telescope compared to an optical telescope.

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)

Astronomers built radio telescopes because they detect radio waves, which are a different part of the electromagnetic spectrum to optical telescopes. Some objects, such as pulsars, radio galaxies and the cosmic microwave background, emit little or no visible light but are strong radio emitters, so optical telescopes cannot see them.

Advantage: Radio waves can pass through dust and gas clouds and through most of Earth’s atmosphere without being absorbed or scattered, allowing observations of regions hidden from optical light.

Disadvantage: To achieve a useful angular resolution, radio telescopes must be very large – often kilometres in diameter – far larger than optical telescopes, making them expensive and difficult to construct.

Examiner tips

  • Use the word ‘radio waves’ and ‘optical light’ to show understanding of the spectrum. Mention a specific radio‑emitting object. State the advantage as ‘penetration of dust/gas’. State the disadvantage as ‘need for very large apertures’. Keep the answer within 5 marks, no extra words.

Common mistakes

  • Saying the disadvantage is ‘low resolution’ without linking it to the large size requirement. Confusing the disadvantage of all telescopes with that specific to radio. Using vague terms like ‘big’ instead of ‘very large’ or ‘kilometre‑scale’.

Mark scheme (5 marks)

  1. Radio telescopes detect radio waves (from space) / different part of the electromagnetic spectrum to optical telescopes
  2. Some astronomical objects / phenomena emit radio waves but little or no visible light, so optical telescopes cannot detect them
  3. Advantage: radio waves pass through dust clouds / gas clouds / Earth's atmosphere without being absorbed / scattered, so observations are not blocked
  4. Disadvantage: radio telescopes need to be very large (much larger than optical telescopes) to produce a detailed / high-resolution image
  5. Disadvantage must be correctly identified as a limitation of radio telescopes (not a general disadvantage of any telescope) — mark is awarded only if the disadvantage stated is specific and scientifically valid

Key terms in this question

radio telescope · optical telescope

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