Question

what is Fresnel distance show with diagram

Answer 1

It is a dimension less physical quantity used in ray optics, particularly in theory of diffraction. 
It is defined as - 
F = a^2/Lλ 
where:- 
a- is the characteristic size (e.g. radius) of the aperture 
L-is the distance of the screen from the aperture 
λ- is the incident wavelength
Geometrical optics holds true only if F is much greater than 1