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Answer the following. (Science 2025 31/1/1 Q36b)

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ScienceCBSE Class 10 · Board exam 2025, set 31/1/1 · 5 marks · Long answer

CBSE Class 10 Science 2025: question 36(b)

Set in the CBSE Class 10 board exam of 2025, paper 31/1/1, section D, question 36(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q36(b) · 5 marks · Long answerLight – Reflection and Refraction

Answer the following.

  1. (i)

    Draw a ray diagram to show the refraction of a ray of light through a rectangular glass slab when it falls obliquely from air into glass.

    0 marks
  2. (ii)

    State Snell's law of refraction of light.

    0 marks
  3. (iii)

    Differentiate between the virtual images formed by a convex lens and a concave lens on the basis of: (I) object distance, and (II) magnification.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/1/1.

Hints

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

Hint 1

Going from rarer to denser medium, which way does the ray bend relative to the normal?

Hint 2

Compare the angle of incidence at the first face with the angle of emergence at the second face.

Hint 3

A convex lens gives a virtual image in only one object region; a concave lens always does.

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 (describe and draw): Draw a rectangular glass slab ABCD with air on both sides. Draw an incident ray EO striking face AB obliquely at O, and the normal NN′ at O. In glass the ray bends towards the normal along OO′, so angle is less than angle .

    1 mark

  2. At face CD, draw the normal MM′ at O′. The ray goes from glass to air and bends away from the normal along O′H. The emergent ray O′H is parallel to the incident ray EO but laterally displaced; show the extended incident ray as a dotted line. Mark , , (with ) and put arrows on all rays.

    1 mark

  3. (ii) Snell's law: for light of a given colour and a given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant: , the refractive index of the second medium with respect to the first.

    1 mark

  4. (iii)(I) Object distance: a convex lens forms a virtual image only when the object is placed between the optical centre O and the focus F. A concave lens forms a virtual image for the object placed anywhere in front of it.

    1 mark

  5. (iii)(II) Magnification: the virtual image formed by a convex lens is enlarged (). The virtual image formed by a concave lens is diminished ().

    1 mark

Answer: (i) Ray bends towards normal entering glass, away from normal on leaving; emergent ray parallel to incident ray (laterally displaced). (ii) sin i / sin r = constant for a given colour and pair of media. (iii) Convex: object between O and F, image magnified; Concave: object anywhere, image diminished.

Where the marks are

  • Correct ray diagram with normals, refraction towards and away from normal, emergent ray parallel to incident ray, angles and arrows (2 marks; ½ deducted if arrows missing)
  • Correct statement of Snell's law (1 mark)
  • Object position difference for virtual images (1 mark)
  • Magnification difference: enlarged vs diminished (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Light – Reflection and Refraction, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/1/1: 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.