Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In Young’s double-slit experiment, the abrupt phase changes in the light emitted by the source S do not destroy the interference pattern.
Reason (R): The light from the slits S1 and S2 is derived from the same source S, so any abrupt phase change in S appears as the same phase change in the light from both slits.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
Assertion (A) In Young’s double-slit experiment, the abrupt phase changes in the light emitted by the source S do not destroy the interference pattern. — True. Light from an ordinary source undergoes abrupt phase changes in times of the order of \( 10^{-10}\,\mathrm{s} \). In Young’s arrangement the fringes still stay fixed on the screen. This is because the phase difference between the waves from S1 and S2 does not change.
Reason (R) The light from the slits S1 and S2 is derived from the same source S, so any abrupt phase change in S appears as the same phase change in the light from both slits. — True. Both slits are lit by the waves spreading out from S. So every phase jump of S reaches S1 and S2 together. The two slits are then locked in phase.
Why R explains A?
A stable pattern needs a constant phase difference between the two waves at every point of the screen. A phase jump that shifts both waves equally leaves this difference unchanged.
Two separate lamps would jump in phase at different random instants. Their fringes would then shift too fast to be seen. Hence (R) is the correct explanation of (A).
NCERT (2026-27):
Interference of Light Waves and Young’s Experiment – “We will now discuss interference using light waves. If we use two sodium lamps illuminating two pinholes … we will not observe any interference fringes. 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. …”
Interference of Light Waves and Young’s Experiment – “The British physicist Thomas Young used an ingenious technique to ‘lock’ the phases of the waves emanating from S1 and S2. He made two pinholes S1 and S2 (very close to each other) on an opaque screen …. These were illuminated by another pinholes that was in turn, lit by a bright source. Light waves spread out from S and fall on both S1 and S2. S1 and S2 then behave like two coherent sources because light waves coming out from S1 and S2 are derived from the same original source and any abrupt phase change in S will manifest in exactly similar phase changes in the light coming out from S1 and S2. Thus, the two sources S1 and S2 will be locked in phase; i.e., they will be coherent ….”