# For an aperture of size 'a' illuminated by a parallel beam of light having wavelength $\lambda$ , the Fresnel distance is
$\begin{array}{1 1} \large\frac{a}{\lambda} \\ \large\frac{a^2}{\lambda} \\ a^2 \lambda \\ \large\frac{a}{\lambda^2} \end{array}$