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Question
Estimate the distance for which ray optics is good approximation for an aperture of 4 mm and wavelength 400 nm.
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Solution
Fresnel’s distance (ZF) is the distance for which the ray optics is a good approximation. It is given by the relation,
`"Z"_"F" = "a"^2/lambda`
Where,
Aperture width, a = 4 mm = 4 ×10−3 m
Wavelength of light, λ = 400 nm = 400 × 10−9 m
`"Z"_"F" = (4 xx 10^(-3))^2/(400 xx 10^(-9))`
= 40 m
Therefore, the distance for which the ray optics is a good approximation is 40 m.
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