In a Young double slit experiment, if one of the slit is covered with a black paper then:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Diffraction pattern is observed on the screen
Interference needs light from two coherent sources overlapping on the screen. Once one slit is covered, light comes through only one slit, so there is nothing for it to interfere with. The single open slit still bends the light, and the screen shows the single-slit diffraction pattern: a broad central maximum with weaker maxima on either side.
Option (1) Fringe width of central maxima decreases — Incorrect. The narrow interference fringes disappear; the central maximum of the diffraction pattern, of width \( 2\lambda D/a \), is much wider than an interference fringe, not narrower.
Option (2) Intensity of the central maxima increases — Incorrect. Only one slit now sends light, and the two-beam reinforcement at the centre is lost, so the central intensity falls.
Option (3) Width of maxima decreases — Incorrect. The maxima of a single-slit pattern are broader than the interference fringes, so the width does not decrease.
Option (4) Diffraction pattern is observed on the screen — Correct. Light from one slit cannot interfere with itself in the two-slit way, so the screen shows the diffraction pattern of a single slit.