Case: Read the passage and answer the question.
YDSE is the abbreviation used for Young’s double-slit experiment, which shows the interference of light. Interference of light is the phenomenon in which two light waves meet each other while travelling in the same medium. A light source is kept behind a wall in which two slits, S1 and S2, are made. On the screen there appears a pattern of alternate bright and dark fringes. The central fringe is always bright.
Consider the following statements about the interference of light:
(A) Light from an ordinary source, such as a sodium lamp, undergoes abrupt phase changes in times of the order of \(10^{-10}\) s.
(B) If the two sources are coherent, the positions of the maxima and minima change with time.
(C) If the phase difference between the two sources changes very rapidly with time, the intensities on the screen add up.
(D) In Young’s arrangement, the two pinholes S1 and S2 are made far apart from each other.
Which of the above statements are correct?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
A stable interference pattern needs two coherent sources. Coherent sources keep a constant phase difference between them.
(A) Light from an ordinary source, such as a sodium lamp, undergoes abrupt phase changes in times of the order of \(10^{-10}\) s. — True. Because of these sudden phase changes, two separate lamps have no fixed phase relation between them.
(B) If the two sources are coherent, the positions of the maxima and minima change with time. — False. For coherent sources the phase difference at a point stays constant. So the pattern is stable, and the maxima and minima stay in place.
(C) If the phase difference between the two sources changes very rapidly with time, the intensities on the screen add up. — True. Such sources are incoherent. The maxima and minima move about very fast, and the eye sees only the sum of the two intensities.
(D) In Young’s arrangement, the two pinholes S1 and S2 are made far apart from each other. — False. Young made the two pinholes very close to each other on an opaque screen.
NCERT (2026-27):
Interference of Light Waves and Young’s Experiment – “This is because of the fact that the light wave emitted from an ordinary source (like a sodium lamp) undergoes abrupt phase changes in times of the order of 10–10 seconds. …”
Coherent and Incoherent Addition of Waves – “Now if the two sources are coherent … then the phase difference φ at any point will not change with time and we will have a stable interference pattern; i.e., the positions of maxima and minima will not change with time. …”
Coherent and Incoherent Addition of Waves – “When the phase difference between the two vibrating sources changes rapidly with time, we say that the two sources are incoherent and when this happens the intensities just add up. …”
Interference of Light Waves and Young’s Experiment – “… He made two pinholes S1 and S2 (very close to each other) on an opaque screen …”