Read the following passage and answer the questions.
A student studies diffraction at a single slit. She uses a laser that gives light of wavelength 600 nm. The beam falls normally on a narrow slit. The width of the slit can be changed with a screw. A white screen is placed 1.0 m behind the slit. With the slit width set at 0.2 mm, she sees a broad bright band at the centre of the screen. On both sides of it there are dark and bright bands. She marks the centre of each band with a pencil. Later she changes the slit width, the wavelength and the screen distance, and notes how the bands move.
At home she tries a simpler version. She holds two razor blades with their edges parallel, so that a narrow slit is left between them. Through this slit she looks at a clear glass bulb with a straight filament. She keeps the slit parallel to the filament and close to her eye.
To explain the pattern, the slit is divided into many small parts. Each part acts as a secondary source, and their effects at a point on the screen are added. These secondary sources are in phase because
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the incoming wavefront is parallel to the plane of the slit
Each part of the slit is a point on the incoming wavefront. All points of one wavefront have the same phase.
Option (1) the laser gives light of a single wavelength — Incorrect. Light of one wavelength can still reach different parts of the slit at different times. This happens when the beam falls at a slant, and then those parts are not in phase.
Option (2) the incoming wavefront is parallel to the plane of the slit — Correct. The beam falls normally, so the plane wavefront reaches every part of the slit at the same instant. All the secondary sources start in phase.
Option (3) the parts are at equal distances from the point on the screen — Incorrect. The parts are at different distances from a point on the screen. These different paths give the phase differences that are added at that point.
Option (4) the slit is very narrow compared with the slit-screen distance — Incorrect. This lets the lines from the slit to a point on the screen be treated as parallel. It says nothing about the phase at the slit.
NCERT (2026-27):
The single slit – “The basic idea is to divide the slit into much smaller parts, and add their contributions at P with the proper phase differences. We are treating different parts of the wavefront at the slit as secondary sources. Because the incoming wavefront is parallel to the plane of the slit, these sources are in phase.”