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Estimate the distance for which ray optics is good approximation for an aperture of 4 mm and wavelength 400 nm.

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Given a = 4  mm = 4 × 10-3 m.

λ = 400 nm = 4 × 10-7m

ZF = ? 

∴ The minimum distance a beam of light has to travel before its deviation from straight line path becomes significant is called Fresnel distance ZF

∴ ZF = \(\frac{a^2}{λ}\) \(\frac{16\times10^{-6}}{4\times10^{-7}}\) = 40 m.

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