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Draw the reflected… (Class 12 Physics SQP 2025-26, Q26)

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PhysicsCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Physics sample paper 2025-26: question 26

From the official CBSE sample question paper for 2025-26, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerWave Optics

Draw the reflected wave front for a plane wave front incident on a plane reflecting surface. Hence verify the laws of reflection using Huygen’s principle.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Draw the incident wavefront just touching the mirror at one end.

Hint 2

In the time the far end reaches the mirror, how far does the wavelet from the first point spread?

Hint 3

Compare the two right triangles sharing the hypotenuse on the mirror.

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. Diagram: plane reflecting surface MN; incident plane wavefront AB meets it at A at angle ; point B travels to C on MN. With A as centre, a secondary wavelet (hemisphere) of radius is drawn; the tangent CE from C is the reflected wavefront, making angle with MN. Rays ⟂ to the wavefronts are shown.

    1 mark

  2. Construction: if is the speed of light and the time for the disturbance at B to reach C, then . In the same time the wavelet from A spreads to radius (same medium). By Huygens' principle CE, the common tangent, is the reflected wavefront.

    1 mark

  3. In right triangles ABC and AEC: [wavefront ⟂ ray], AC is common, . So the triangles are congruent (RHS), giving , i.e. . Also incident ray, reflected ray and normal lie in the same plane (plane of the figure). Hence the laws of reflection are verified.

    1 mark

Answer: Reflected wavefront CE drawn; congruent triangles ABC and AEC give i = r

Where the marks are

  • Correct diagram of incident and reflected wavefronts (1 mark)
  • Huygens construction with (1 mark)
  • Proof of congruence giving (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Wave Optics, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2025-26: 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.