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.
Consider the following statements about diffraction:
(A) Diffraction is shown by sound waves and water waves as well as by light waves.
(B) The colours seen when a CD is viewed are due to diffraction effects.
(C) In a double-slit experiment, the pattern on the screen is the double-slit interference pattern alone.
(D) The position of the central bright band changes when the wavelength of the light is changed.
Which of the above statements are correct?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (B) only
Diffraction is the spreading of a wave around the edges of an obstacle or an opening. Light has a very short wavelength, so we do not notice its diffraction in everyday life.
(A) Diffraction is shown by sound waves and water waves as well as by light waves. — True. Diffraction is a general property of all types of waves.
(B) The colours seen when a CD is viewed are due to diffraction effects. — True. The colours seen on the surface of a CD come from the diffraction of light.
(C) In a double-slit experiment, the pattern on the screen is the double-slit interference pattern alone. — False. Each slit also gives its own single-slit diffraction. The pattern is a superposition of this diffraction and the double-slit interference pattern.
(D) The position of the central bright band changes when the wavelength of the light is changed. — False. The central band lies at \( \theta=0 \) for every wavelength. Only the other bands shift with wavelength. So they show colours in white light.
NCERT (2026-27):
Diffraction – “… Diffraction is a general characteristic exhibited by all types of waves, be it sound waves, light waves, water waves or matter waves. …”
Diffraction – “… Indeed the colours that you see when a CD is viewed is due to diffraction effects. …”
The single slit – “In the double-slit experiment, we must note that the pattern on the screen is actually a superposition of single-slit diffraction from each slit or hole, and the double-slit interference pattern.”
Seeing the single slit diffraction pattern – “… Since the position of all the bands (except the central one) depends on wavelength, they will show some colours. …”