over complex variables has to be solved and in many cases, an analytic solution is not available. The Fraunhofer diffraction equation is a simplified version of Kirchhoff's diffraction formula and it can be used to model light diffraction when both a light source and a viewing plane (a plane of observation where the diffracted wave is observed) are effectively infinitely distant from a diffracting aperture. With a sufficiently distant light source from a diffracting aperture, the incident light to the aperture is effectively a plane wave so that the phase of the light at each point on the aperture is the same. At a sufficiently distant plane of observation from the aperture, the phase of the wave coming from each point on the aperture varies linearly with the point position on the aperture, making the calculation of the sum of the waves at an observation point on the plane of observation relatively straightforward in many cases. Even the amplitudes of the secondary waves coming from the aperture at the observation point can be treated as same or constant for a simple diffraction wave calculation in this case. Diffraction in such a geometrical requirement is called Fraunhofer diffraction, and the condition where Fraunhofer diffraction is valid is called Fraunhofer condition, as shown in the right box. A diffracted wave is often called Far field if it at least partially satisfies Fraunhofer condition such that the distance between the aperture and the observation plane is . For example, if a 0.5 mm diameter circular hole is illuminated by a laser light with 0.6 μm wavelength, then Fraunhofer diffraction occurs if the viewing distance is greater than 1000 mm. Derivation of Fraunhofer condition The derivation of Fraunhofer condition here is based on the geometry described in the right box. The diffracted wave path r2 can be expressed in terms of another diffracted wave path r1 and the distance b between two diffracting points by using the law of cosines; This can be expanded by using the binomial series for by letting , The phase difference between waves propagating along the paths r2 and r1 are, with the wavenumber where λ is the light wavelength, If so , then the phase difference is . The geometrical implication from this expression is that the paths r2 and r1 are approximately parallel with each other. Since there can be a diffraction plane - observation plane diffracted