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Physics · 2025 · 3 marks
CBSE 2025 · Region 5 · Set 3 · Q23
(a) "You cannot see a person standing on the other side of a boundary wall but can hear him." Explain with reason. (b) Light of wavelength 750 nm is…
(a)
"You cannot see a person standing on the other side of a boundary wall but can hear him." Explain with reason.
(b)
Light of wavelength $\displaystyle 750$ nm is incident normally on a slit of width $\displaystyle 1.5$ mm . Diffraction pattern is obtained on a screen $\displaystyle 1.0$ m away from the slit. Find the distance of the nearest point from the central maxima at which the intensity is zero.
Marking-scheme solution
(a)
The wavelength of light is very small as compared to the size of obstacles, so diffraction of light is not seen easily. But sound waves have large wavelength, so they get diffracted easily by obstacles.
(b)
Position of first minimum:
$\displaystyle x=\frac{\lambda D}{a}$
$\displaystyle =\frac{750 \times 10^{-9} \times 1}{1.5 \times 10^{-3}}$
$\displaystyle =0.5 \mathrm{~mm}$
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