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.
In the activity at home, light from the filament passes through the gap between the blades and enters the eye. In this activity, the straight filament of the bulb plays the role of
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → the narrow source slit that sends light to the single slit
In the usual set-up, light from a narrow source slit falls on the single slit. The pattern is then formed on a screen.
Option (1) the narrow source slit that sends light to the single slit — Correct. The thin, straight filament is a narrow line source. It does the job of the source slit in the usual set-up.
Option (2) the single slit that spreads the light — Incorrect. The narrow gap between the two razor blades is the single slit that spreads the light.
Option (3) the lens that focuses the pattern — Incorrect. The lens of the eye focuses the pattern.
Option (4) the screen on which the pattern is formed — Incorrect. The retina of the eye acts as the screen.
NCERT (2026-27):
Seeing the single slit diffraction pattern – “In this experiment, the filament plays the role of the first slit S … The lens of the eye focuses the pattern on the screen (the retina of the eye).”