Two sources of light are said to be coherent when they emit light of
A. same wavelength
B. same amplitude
C. same intensity
D. constant phase difference
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A and D only
Two sources are coherent when the phase difference between the waves they emit stays constant with time. For this the two waves must have the same frequency (and so the same wavelength in a given medium), because waves of different frequencies drift in phase.
A. same wavelength — Correct. Equal frequency is needed to keep the phase difference fixed.
B. same amplitude — Incorrect. Coherence does not require equal amplitudes. Sources of unequal amplitudes still give a steady pattern, with less perfect dark fringes.
C. same intensity — Incorrect. Intensity follows the amplitude, so this is not required either.
D. constant phase difference — Correct. This is the defining condition of coherence.
NCERT (2026-27):
Coherent and Incoherent Addition of Waves – “… Now if the two sources are coherent (i.e., if the two needles are going up and down regularly) 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.”
Summary – “… 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.”