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NCERT Class XII Chapter
Wave Optics
Questions With Solutions

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Question : 19
Total: 21
A parallel beam of light of wavelength 500 nm falls on a narrow slit and the resulting diffraction pattern is observed on a screen 1 m away. It is observed that the first minimum is at a distance of 2.5 mm from the centre of the screen. Find the width of the slit
Solution:  
First minimum is observed at a distance 2.5 mm from centre of the screen.
So, x = 2.5 mm = 2.5 × 10–3 m
l = 500 nm = 500 × 10–9 m = 5 × 10–7 m
D = 1 m, n = 1

So, condition for minima is
dx
D
 = nλ
∴ Slit width =
nλD
x
∴ d =
5×10−7×1×1
2.5×10−3
 = 0.2 mm
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