PhysicsCBSE Class 12 · Board exam 2026, set 55/1/1 · 2 marks · Very short answer

CBSE Class 12 Physics 2026: question 18(b)

Set in the CBSE Class 12 board exam of 2026, paper 55/1/1, section B, question 18(b), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q18(b) · 2 marks · Very short answerWave Optics

In a Young’s double-slit experimental set-up with slit separation 0.6 mm a beam of light consisting of two wavelengths 440 nm and 660 nm is used to obtain interference pattern on a screen kept 1.5 m in front of the slits. Find the least distance of the point from the central maximum where the bright fringes due to both the wavelengths coincide.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/1/1.

Hints

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

Hint 1

The bright fringes of both wavelengths coincide where their distances from the centre are equal.

Hint 2

Solve $n_{1} λ_{1} = n_{2} λ_{2}$ for the smallest integers $n_{1}$ and $n_{2}$ .

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.

In YDSE the $n$ th bright fringe lies at $y_{n} = \frac{nλ D}{d}$ .
½ mark
For the bright fringes to coincide: $n_{1} (440) = n_{2} (660) \Rightarrow \frac{n _{1}}{n _{2}} = \frac{3}{2}$ . Take the least values $n_{1} = 3$ , $n_{2} = 2$ .
½ mark
$y = \frac{3 \times 440 \times 1 0 ^{- 9} \times 1.5}{0.6 \times 1 0 ^{- 3}} = 3.3 \times 1 0^{- 3}$ m $= 3.3$ mm
1 mark

Answer: 3.3 mm from the central maximum

Where the marks are

Writes the bright fringe position $y = nλ D / d$ (½ mark)
Coincidence condition $3 \times 440 = 2 \times 660$ (½ mark)
Least distance of 3.3 mm (1 mark)

Revise the chapter

Chapter: Wave Optics (notes, formulas, where marks are lost and every past question)
Topic: Young's Double Slit Experiment

Practise this chapterBoard-style questions from Wave Optics, one at a time, marked against the answer key and marking points.Back to the full paper2026 board paper, set 55/1/1: every question, by section.

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CBSE Class 12 Physics 2026: question 18(b)

Hints

Step-by-step solution

Revise the chapter

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