Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): Two sources of light of different frequencies cannot produce a sustained interference pattern.
Reason (R): Light waves of different frequencies cannot be superposed at a point.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) is true but (R) is false.
Assertion (A) Two sources of light of different frequencies cannot produce a sustained interference pattern. — True. A steady pattern needs the phase difference at each point to stay the same with time. With different frequencies the phase difference keeps changing, so the maxima and minima keep shifting. The interference term then averages to zero and only the sum of the two intensities is seen.
Reason (R) Light waves of different frequencies cannot be superposed at a point. — False. The principle of superposition holds whenever two or more waves reach the same point, whatever their frequencies. The displacements still add at every instant. Only the stable pattern is lost, because the phase difference does not stay fixed.
NCERT (2026-27):
Summary – “3. The principle of superposition of waves applies whenever two or more sources of light illuminate the same point. When we consider the intensity of light due to these sources at the given point, there is an interference term in addition to the sum of the individual intensities. But this term is important only if it has a non-zero average, which occurs only if the sources have the same frequency and a stable phase difference.”