Young's double slit experiment is performed using a source of white light.
A. The center fringe will be white.
B. The fringe next to the center fringe will be violet.
C. The fringe next to the center fringe will be red.
D. There will be no completely dark fringe.
E. The center fringe will be of no colour.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, B, D only
Note: The given answer is as per the official NTA Answer Key. However, according to the textbook the correct answer should be Option (3), as explained below.
In white light each colour forms its own fringe pattern, with fringe width \( \beta = \lambda D/d \); violet has the narrowest fringes and red the widest.
Working as per the NTA key
Since violet has the smallest fringe width, NTA seems to have taken the fringe next to the central white fringe as violet (B). With (A) and (D) true, this gives A, B and D, which is Option (2).
Working as per the standard method
Moving away from the centre, the path difference first reaches \( \lambda_{\mathrm{blue}}/2 \), the first minimum for blue, while red is still bright, so the fringe closest to the central white fringe on either side is red.
(A) The center fringe will be white. — Correct. The path difference is zero for every wavelength, so all colours are bright together at the centre.
(B) The fringe next to the center fringe will be violet. — Incorrect. Blue and violet are the first colours to be cut out away from the centre, so the next fringe looks red.
(C) The fringe next to the center fringe will be red. — Correct. Where blue reaches its first minimum, red is still near its maximum.
(D) There will be no completely dark fringe. — Correct. The minima of the different wavelengths fall at different places, so some colour is present at every point.
(E) The center fringe will be of no colour. — Incorrect. The central fringe is white, the mixture of all the colours.
The correct statements are A, C and D. Hence, according to the textbook, the correct answer is Option (3).
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), but it is still in the CUET 2026 syllabus, so do not skip it.
NCERT (Pre-rationalised edition, 2022-23):
Interference of Light Waves and Young’s Experiment – “… The central bright fringes for different colours are at the same position. Therefore, the central fringe is white. For a point P for which S2P − S1P = λb/2, where λb (≈ 4000 Å) represents the wavelength for the blue colour, the blue component will be absent and the fringe will appear red in colour. … Thus, the fringe closest on either side of the central white fringe is red and the farthest will appear blue. After a few fringes, no clear fringe pattern is seen.”