Two separate light bulbs are used as the two sources in an interference experiment. Which of the following is correct?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → They cannot produce an interference pattern
A steady interference pattern is seen only when the two sources are coherent. Coherent sources keep a constant phase difference between them all the time.
Option (1) They can produce an interference pattern — Incorrect. The light from a bulb changes its phase suddenly, in about \( 10^{-10}\ \mathrm{s} \). The two bulbs do this on their own, so any fringes shift far too fast to be seen. The screen only shows the sum of the two intensities.
Option (2) They cannot produce an interference pattern — Correct. Two separate bulbs are independent sources. Their light has no fixed phase relationship, so the sources are incoherent and give no steady pattern.
Option (3) They can produce an interference pattern if they give light of the same frequency — Incorrect. Even at the same frequency, the phase of each bulb still jumps at random. The phase difference keeps changing, so the fringes do not stay in one place.
Option (4) They can produce an interference pattern if they give light of the same phase — Incorrect. Two separate bulbs can never keep the same phase, because the phase of each one changes randomly and on its own. This condition is never met by such sources.
NCERT (2026-27):
Interference of Light Waves and Young’s Experiment – “… 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. Thus the light waves coming out from two independent sources of light will not have any fixed phase relationship and would be incoherent …”