What is Fresnel distance? Write its formula.

(N/A) The Fresnel distance is defined as the distance from a slit or aperture at which the spreading of light due to diffraction becomes comparable to the size of the aperture itself.
Beyond this distance,the ray optics approximation (rectilinear propagation of light) is no longer valid,and diffraction effects become significant.
It acts as a limit to the validity of ray optics.
The formula for Fresnel distance $(z_F)$ is given by:
$z_F = \frac{a^2}{\lambda}$
Where:
$a$ = size of the aperture or slit width
$\lambda$ = wavelength of the light used