# Q10.10 Estimate the distance for which ray optics is good approximation for an aperture of 4 mm and wavelength 400 nm.

Given

Aperture $a=4mm=4*10^{-3}m$

Wavelength of light  $\lambda =400nm=400*10^{-9}m$

Now,

Distance for which ray optics is a good approximation also called Fresnel's distance:

$Z_f=\frac{a^2}{\lambda }=\frac{(4*10^{-3})^2}{400*10^{-9}}=40m$

Hence distance for which ray optics is a good approximation is 40m.

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