An aperture of size a illluminated by a parallel beam sends diffracted light into a angle of approximately ~y/a. This is hte angular size of the bright central maximum. In trevelling a distance z, the diffracted beam therefore acquires a width zy/a due to diffraction. this gives distance beyond which divergence of the beam of width a becomes significant. Therefore, z ~ a2/y we dfien a quantity ZF called the Fresenls distance by the following equation ZF= a2/y
For distance greater than ZF the spreading due to diffraction over that due to ray optics. The above equation shows that ray optics is valid in the limit of wavelength tending to zero.