Two waves are said to be coherent if:
A) They have the same amplitude
B) The phase difference varies randomly over time
C) Their phase difference remains constant over time
D) They have the same intensity
E) Their wavelength lies in the visible region
IMAT 2026 · Question 54
Q54 - Waves, interference and optics
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The correct answer is C) Their phase difference remains constant over time.
Two waves are coherent when they have the same frequency and a constant phase difference. Only then is the interference pattern stable, as in Young’s double-slit experiment, where light from one source passes through two slits.
A) Incorrect: Coherence does not require equal amplitudes.
B) Incorrect: A randomly changing phase difference is the definition of incoherent waves.
C) Correct: Constant phase difference is the definition of coherence.
D) Incorrect: Coherence does not require equal intensities.
E) Incorrect: Coherence applies to every kind of wave (sound, radio, water waves), not just visible light.
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