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Why is a reflecting… (Class 12 Physics 2023, Q21b)

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PhysicsCBSE Class 12 · Board exam 2023, set 55/1/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2023: question 21(b)

Set in the CBSE Class 12 board exam of 2023, paper 55/1/1, section B, question 21(b), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q21(b) · 2 marks · Very short answerRay Optics and Optical Instruments

Why is a reflecting telescope preferred over a refracting telescope ? Justify your answer giving two reasons.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 55/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Think about what goes wrong when white light passes through a lens.

Hint 2

Consider the weight and support of a very large lens compared to a mirror.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. Reason 1: No chromatic aberration — a mirror forms the image by reflection, not refraction, so there is no dispersion of colours.

    1 mark

  2. Reason 2: Easier mechanical support and larger aperture — a mirror weighs much less than a lens of equal optical quality and can be supported over its entire back surface, so very large diameters are possible, giving higher resolving power and brighter images (more light gathered). (Also: a parabolic mirror removes spherical aberration.)

    1 mark

Answer: No chromatic aberration; mirror lighter and supported over whole back, allowing large aperture (high resolving power, brighter image); parabolic mirror removes spherical aberration.

Where the marks are

  • First reason with justification, e.g. no chromatic aberration as no refraction (½ + ½ mark)
  • Second reason with justification, e.g. easy support/large aperture/no spherical aberration (½ + ½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Ray Optics and Optical Instruments, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 55/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.