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Answer the following. (Class 10 Science SQP 2023-24, Q31)

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ScienceCBSE Class 10 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 10 Science sample paper 2023-24: question 31

From the official CBSE sample question paper for 2023-24, section C, question 31, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q31 · 3 marks · Short answerLight – Reflection and Refraction

Answer the following.

  1. (i)

    Explain why the refractive index of any material with respect to air is always greater 1.

    1 mark
  2. (ii)

    In the figure below a light ray travels from air into the semi-circular plastic block. Give a reason why the ray does not deviate at the semi-circular boundary of the plastic block.

    1 mark
  3. (iii)

    Complete the ray diagram of the above scenario when the light ray comes out of the plastic block from the top flat end.

    1 mark

Figure in the paper: A semi-circular plastic block with its flat top face (labelled 'plastic block') and a dashed 'normal' drawn perpendicular to the flat top at the point of incidence. A light ray (labelled 'light ray') enters from below-left into the curved bottom surface, travelling toward the centre, making an angle of 40° with the normal. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Write the definition of refractive index in terms of speeds of light.

Hint 2

A line from the centre of a circle to its edge is perpendicular to the curve there.

Hint 3

Going from plastic to air means going from denser to rarer: which way does the ray bend?

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) Refractive index of a medium with respect to air:

    ½ mark

  2. Light travels slower in any material medium than in air, so the denominator is always smaller than the numerator. Hence .

    ½ mark

  3. (ii) The ray travels towards the centre of the semicircle, i.e. along a radius. A radius is normal to the curved surface, so the ray strikes the curved boundary along the normal (angle of incidence ). A ray falling normally on a surface passes through undeviated.

    1 mark

  4. (iii) Ray diagram: at the flat top face, the ray meets the surface at the centre with angle of incidence inside the plastic. It goes from the denser medium (plastic) into the rarer medium (air), so it bends away from the normal. Draw the emergent ray in air on the opposite side of the normal (upper right), with angle of refraction .

    ½ mark

  5. Mark arrows on the incident, internal and emergent rays to show the direction of travel, and label the normal, and ().

    ½ mark

Answer: (i) and light is slower in any medium, so n > 1; (ii) the ray moves along a radius, so it strikes the curved surface normally and is undeviated; (iii) at the flat face the ray emerges into air bending away from the normal ().

Where the marks are

  • Writes n as speed in air / speed in medium (0.5 mark)
  • Speed in medium is less, so n > 1 (0.5 mark)
  • Ray is along the radius, i.e. normal incidence on the curved surface, so no deviation (1 mark)
  • Emergent ray drawn bending away from the normal (0.5 mark)
  • Correct arrows on the rays (0.5 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 paperSample paper 2023-24: 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.