The effect on the interference fringes in Young’s double slit experiment if the width of the source slit is increased:
(A) The interference pattern gets less and less sharp
(B) The interference pattern gets brighter
(C) The interference pattern may disappear, if, condition \( s/S \le \lambda/d \) is not satisfied. (s is size of source and S is its distance from two slits)
(D) The interference fringes becomes broad
(E) No effect on the interference pattern
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (C) Only
A wider source slit acts as many incoherent point sources side by side. Each produces its own fringe pattern, shifted slightly from the others, and these patterns overlap and wash out the dark and bright bands.
(A) The interference pattern gets less and less sharp — Correct. The overlapping, shifted patterns reduce the contrast between bright and dark fringes.
(B) The interference pattern gets brighter — Incorrect. More light enters, but the contrast of the fringes falls rather than the pattern becoming brighter.
(C) The interference pattern may disappear, if, condition \( s/S \le \lambda/d \) is not satisfied — Correct. This is the condition for seeing fringes; when the source is so wide that it fails, the patterns of its different parts fill in the dark fringes completely.
(D) The interference fringes becomes broad — Incorrect. The fringe width \( \beta = \lambda D/d \) depends on the wavelength, the slit separation and the screen distance, not on the width of the slit.
(E) No effect on the interference pattern — Incorrect. The sharpness of the fringes clearly changes, as in (A) and (C).