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(I) Draw a ray diagram… (Class 12 Physics SQP 2024-25, Q24)

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PhysicsCBSE Class 12 · Sample paper, session 2024-25 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2024-25: question 24

From the official CBSE sample question paper for 2024-25, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerRay Optics and Optical Instruments

(I) Draw a ray diagram for the formation of image by a Cassegrain telescope.
(II) Why these types of telescopes are preferred over refracting type telescopes. (Write 2 points)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

A Cassegrain telescope uses two mirrors; one is concave and the other convex.

Hint 2

Think about which defects lenses suffer from that mirrors don't, and about the size of the objective.

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) Ray diagram (described): parallel rays from a distant object fall on a large concave (paraboloidal) primary mirror with a small hole at its centre. The reflected rays converge towards the primary's focus.

    1 mark

  2. Before reaching that focus, the rays are intercepted by a small convex (hyperbolic) secondary mirror placed coaxially. It reflects them back through the central hole of the primary, forming the image just behind the primary, where it is viewed through an eyepiece. Arrows show the direction of light; primary, secondary, hole, image and eyepiece are labelled.

    1 mark

  3. (II) Advantage 1: the objective is a mirror, so there is no chromatic aberration, and a paraboloidal mirror also removes spherical aberration.

    ½ mark

  4. Advantage 2: a mirror is much lighter and easier to support than a lens of the same size, so very large apertures can be made. This gathers more light, giving brighter images and higher resolving power. (The folded path also makes the telescope compact.)

    ½ mark

Answer: (I) Concave primary → convex secondary → through hole in primary → image viewed by eyepiece; (II) free from chromatic/spherical aberration, large aperture gives brighter image and better resolution

Where the marks are

  • Correct ray diagram: primary concave mirror, secondary convex mirror, image behind the primary through the hole, with labels (2 marks)
  • Any two valid advantages over refracting telescopes (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 paperSample paper 2024-25: 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.