Skip to content

(a) Draw the ray diagram to… (Class 12 Physics 2026, Q27)

questionbuddy.in/q/cbse-12-physics-2026-55-1-1-q27-draw-ray-diagram-image-formation

PhysicsCBSE Class 12 · Board exam 2026, set 55/1/1 · 3 marks · Short answer

CBSE Class 12 Physics 2026: question 27

Set in the CBSE Class 12 board exam of 2026, paper 55/1/1, section C, question 27, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q27 · 3 marks · Short answerRay Optics and Optical Instruments

(a) Draw the ray diagram to show the image formation by a refracting telescope and write the expression for angular magnification for the telescope in normal adjustment.
(b) Give two reasons to explain why a reflecting telescope is preferred over a refracting telescope.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/1/1.

Hints

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

Hint 1

In normal adjustment, the image formed by the objective lies at the focus of the eyepiece.

Hint 2

Compare a lens and a mirror for aberrations and for how big and heavy they can be made.

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. (a) Ray diagram (normal adjustment):
    1. Parallel rays from a distant object fall on the objective (large focal length ), inclined at angle to the axis.
    2. They form a real, inverted, diminished image at the common focus of the objective and eyepiece.
    3. The eyepiece (small ) placed at distance from this image makes the rays emerge parallel at angle , giving the final image at infinity.
    4. Tube length .

    1½ marks

  2. Angular magnification in normal adjustment: (magnitude ).

    ½ mark

  3. (b) Reason 1: A mirror objective has no chromatic aberration. A paraboloidal mirror also removes spherical aberration.

    ½ mark

  4. Reason 2: A mirror is lighter than a lens of equal aperture and can be supported over its whole back surface, so very large apertures are possible. This gives higher resolving power and light-gathering power at lower cost.

    ½ mark

Answer: m = −f_o/f_e. A reflector is free from chromatic (and spherical) aberration, and large mirrors are easier and cheaper to make and support, giving higher resolving power.

Where the marks are

  • Correct labelled ray diagram with the image at infinity (1½ marks)
  • m = f_o/f_e (½ mark)
  • First reason for preferring a reflecting telescope (½ mark)
  • Second reason (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 paper2026 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.