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A convex lens of focal… (Class 12 Physics SQP 2026-27, Q14)

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PhysicsCBSE Class 12 · Sample paper, session 2026-27 · 1 mark · Assertion-reason

CBSE Class 12 Physics sample paper 2026-27: question 14

From the official CBSE sample question paper for 2026-27, section A, question 14, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q14 · 1 mark · Assertion-reasonCompetency-basedRay Optics and Optical Instruments

A convex lens of focal length 20 cm is used to form a real image of a point object placed 30 cm from the lens. The lens is then immersed in water of refractive index 1.33 (the lens material has refractive index 1.5).

Assertion (A): The focal length of the lens increases when it is immersed in water.

Reason (R): The focal length of a lens depends on the relative refractive index of the lens material with respect to the surrounding medium, as given by the lens maker’s formula.

  1. (A)both Assertion and Reason are true and Reason is the correct explanation of Assertion.
  2. (B)both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
  3. (C)Assertion is true but Reason is false.
  4. (D)both Assertion and Reason are false.

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

Write the lens maker's formula with the surrounding medium included.

Hint 2

Compare in air and in water.

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. By the lens maker's formula, .

    ½ mark

  2. In air, . In water, . This factor is smaller, so increases. Both statements are true and R explains A. Option (A).

    ½ mark

Answer: Option (A)

Where the marks are

  • Applies the lens maker's formula with relative refractive index to show increases (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.