The interference pattern is said to be sustained if the position and intensity of fringes remain same throughout on the screen. For sustained interference:
A. The size of slits must be large.
B. Two sources must be coherent.
C. Screen must be very close to slits.
D. Separation between the slits must be small.
E. The screen must be placed close to the plane of slits.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → B and D only
A sustained interference pattern needs a constant phase difference between the two waves, and the fringes must be wide enough to be seen as separate. The fringe width is
\( \displaystyle \beta = \frac{\lambda D}{d} \)
where \( D \) is the slit-screen distance and \( d \) the slit separation.
(A) The size of slits must be large. — Incorrect. The slits must be narrow; wide slits act as many sources and the fringes blur.
(B) Two sources must be coherent. — Correct. Only coherent sources keep a constant phase difference, so the fringes stay fixed.
(C) Screen must be very close to slits. — Incorrect. A small \( D \) makes \( \beta \) very small, so the fringes crowd together.
(D) Separation between the slits must be small. — Correct. A small \( d \) gives a large \( \beta \), so the fringes are wide and distinct.
(E) The screen must be placed close to the plane of slits. — Incorrect. This is statement C again; the screen must be far from the slits.
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.”