In a Young's double-slit experiment, the two slits behave as coherent sources. When coherent light waves superpose over each other they create an interference pattern of successive bright and dark regions due to constructive and destructive interference. Two slits 2 mm apart are illuminated by a source of monochromatic light and the interference pattern is observed on a screen 5·0 m away from the slits as shown in the figure. (i) What property of light does this interference experiment demonstrate ? (ii)(a) The wavelength of light used in this experiment is : (ii)(b) The fringe width in the interference pattern formed on the screen is : (iii) The path difference between the two waves meeting at point P, where there is a minimum in the interference pattern is : (iv) When the experiment is performed in a liquid of refractive index greater than 1, then fringe pattern will :

PhysicsCBSE Class 12 · Board exam 2026, set 55/5/1 · 4 marks · Case-based

CBSE Class 12 Physics 2026: question 30

Set in the CBSE Class 12 board exam of 2026, paper 55/5/1, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedWave Optics

In a Young's double-slit experiment, the two slits behave as coherent sources. When coherent light waves superpose over each other they create an interference pattern of successive bright and dark regions due to constructive and destructive interference.

Two slits 2 mm apart are illuminated by a source of monochromatic light and the interference pattern is observed on a screen 5·0 m away from the slits as shown in the figure.

(i)
What property of light does this interference experiment demonstrate ?
1. (A)Wave nature of light
2. (B)Particle nature of light
3. (C)Transverse nature of light
4. (D)Both wave nature and transverse nature of light
1 mark
(ii)(a)
The wavelength of light used in this experiment is :
1. (A)720 nm
2. (B)590 nm
3. (C)480 nm
4. (D)364 nm
1 mark
OR
(ii)(b)
The fringe width in the interference pattern formed on the screen is :
1. (A)1·2 mm
2. (B)0·2 mm
3. (C)4·2 mm
4. (D)6·8 mm
1 mark
(iii)
The path difference between the two waves meeting at point P, where there is a minimum in the interference pattern is :
1. (A) $8 \cdot 1 \times 1 0^{- 7}$ m
2. (B) $7 \cdot 2 \times 1 0^{- 7}$ m
3. (C) $6 \cdot 5 \times 1 0^{- 7}$ m
4. (D) $6 \cdot 0 \times 1 0^{- 7}$ m
1 mark
(iv)
When the experiment is performed in a liquid of refractive index greater than 1, then fringe pattern will :
1. (A)disappear
2. (B)become blurred
3. (C)be widened
4. (D)be compressed
1 mark

Figure in the paper: Light of wavelength λ falls on two slits separated by 2·0 mm; screen is 5·0 m away. On the screen, a position axis y (mm) is marked from −3·0 to 3·0 in steps of 0·5 with 0 at the central axis, and an intensity pattern with bright fringes (maxima) is drawn; the central maximum is at 0. Point P is marked at about 1·5 mm on the screen (a minimum). See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 55/5/1.

Hints

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

Hint 1

Read the fringe width as the gap between neighbouring bright fringes on the scale.

Hint 2

Use β = λD/d; at a dark fringe the path difference is an odd multiple of λ/2.

Hint 3

In a medium the wavelength becomes λ/μ.

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.

(i) Interference (superposition of waves giving bright and dark fringes) shows the wave nature of light; it does not show transverse nature (polarisation does). Option (A).
1 mark
(ii)(a) From the figure, successive bright fringes are 1.2 mm apart, so $β = 1.2$ mm. $λ = \frac{β d}{D} = \frac{1.2 \times 1 0 ^{- 3} \times 2 \times 1 0 ^{- 3}}{5.0} = 4.8 \times 1 0^{- 7}$ m $= 480$ nm — option (C).
1 mark
(ii)(b) [Alternative to (ii)(a), 1 mark] Fringe width read from the pattern (distance between consecutive maxima), consistent with $β = λ D / d$ : $β = 1.2$ mm — option (A).
(iii) P is the second minimum, at $y = 1.5 β = 1.8$ mm. Path difference $= \frac{y d}{D} = \frac{1.8 \times 1 0 ^{- 3} \times 2 \times 1 0 ^{- 3}}{5} = 7.2 \times 1 0^{- 7}$ m $(= \frac{3}{2} λ)$ — option (B).
1 mark
(iv) In a liquid of refractive index μ, wavelength becomes $λ / μ$ , so $β^{'} = \frac{λ D}{μ d} < β$ : fringes become narrower — the pattern is compressed. Option (D).
1 mark

Answer: (i) A; (ii)(a) C (480 nm); (ii)(b) A (1.2 mm); (iii) B (7.2 × 10^-7 m); (iv) D

Where the marks are

(i) Wave nature of light, option A (1 mark)
(ii)(a) λ = βd/D = 480 nm, option C; or (ii)(b) β = 1.2 mm, option A (1 mark)
(iii) Path difference at minimum = 3λ/2 = 7.2 × 10⁻⁷ m, option B (1 mark)
(iv) β decreases by factor μ — compressed, option D (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 55/5/1: 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.

CBSE Class 12 Physics 2026: question 30

Hints

Step-by-step solution

Revise the chapter

More questions from Wave Optics