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An astronomical… (Class 12 Physics SQP 2026-27, Q32B)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 5 marks · Long answer

CBSE Class 12 Physics sample paper 2026-27: question 32(B)

From the official CBSE sample question paper for 2026-27, section E, question 32(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(B) · 5 marks · Long answerRay Optics and Optical Instruments

An astronomical telescope uses an objective lens of focal length 100 cm and an eyepiece of focal length 5 cm.

I. Calculate the magnifying power of the telescope when the final image is formed at infinity.
II. If the least distance of distinct vision of the observer is 25 cm, calculate the magnifying power when the final image is formed at this distance.
III. Explain why the objective lens of an astronomical telescope is made with a large focal length and aperture, while the eyepiece has a small focal length.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

In normal adjustment the telescope's magnifying power depends only on the two focal lengths.

Hint 2

When the final image is at , the eyepiece's contribution gains an extra factor .

Hint 3

For part III, ask what controls , and what controls how much light enters the telescope.

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. I. Normal adjustment (final image at infinity). Given cm and cm:

    1 mark

  2. 1 mark

  3. **II. Final image at the least distance of distinct vision ( cm):**

    1 mark

  4. 1 mark

  5. III. Design of the lenses.

    • The objective has a large focal length, because the magnifying power is . It also has a large aperture, which gathers more light from faint, distant objects, so the image is brighter and the resolving power is higher.
    • The eyepiece has a small focal length, so it magnifies the intermediate image strongly. A small also makes large.
    1 mark

Answer: I. ; II. ; III. A large and small give a high ; a large aperture gives a brighter image and better resolution.

Where the marks are

  • Writes for normal adjustment (1 mark)
  • Gets (1 mark)
  • Writes (1 mark)
  • Gets (1 mark)
  • Reasons for a large and aperture (magnification, light gathering, resolution) and a small (1 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 paperSample paper 2026-27: 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.