Question

what is fresnel distance?

Answer 1

Fresnel's distance is that distance from the slit at which the spreading of light due to diffraction becomes equal to the size of the slit.

Answer 2

Answer:

Fresnel distance describes the distance at which spread due to diffraction becomes comparable to the width of the slit or not. This is the boundary of ray optics and wave optics. For distances << zf : Spreading due to diffraction is smaller compared to size of the beam then the ray optics is valid.