Determine the Fresnel distance if the aperture is of the size 1cm and the wavelength is 500nm

Wavelength, λ = 500nm

Aperture size, a = 1cm

$z= frac{a^{2}}{lambda }$

$z=frac{1times 10^{-2}1times 10^{-2}}{500times 10^{-9}}$

$z=frac{1000}{5}$

z = 200m