^Resnel’s distance (DF)
The distance of the screen from the slit at which the diffraction spread of a beam is equal to the size of the aperture of the slit is called Fresnel’s distance. i.e., when y = d, D = DF, thus
For a given value of d the quantity is called the size of Fresnel zone and is denoted by dF.
Geometrical optics or ray optics is based upon the rectilinear propagation of light. If D < DF, then there will not be too much broadening by diffraction i.e., the light will travel along straight lines and the concepts of ray optics will be valid.