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NCERT Class XII Chapter
Wave Optics
Questions With Solutions

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Question : 10
Total: 21
Estimate the distance for which ray optics is good approximation for an aperture of 4 mm and wavelength 400 nm.
Solution:  
Fresnel distance required for a sufficient spreading of central bright fringe, so that diffraction is appreciable
DF =
a2
λ
 =
(4×10−3)2
400×10−9
 = 40 m
So, for distance less than 40 m between slit and screen, ray optics is a good approximation as within this distance, the spreading is negligible.
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