Fraunhofer Diffraction (diffraction by a double slit)

Fraunhofer Diffraction inoptics is used to model the diffraction of waves when the diffraction pattern is viewed at a long distance from the diffracting object, and also when it is viewed at the focal plane of an imaging lense. In the double-slit experiment, the two slits are illuminated by a single light beam. If the width of the slits is small enough (less than the wavelength of light), the slits diffract the light into cylindrical waves. These two cylindrical wavefronts are superimposed, and the amplitude, and therefore the intensity, at any point in the combined wavefronts depends on both the magnitude and the phase of the two wavefronts.  The following formula gives the spacing of the fringes at the distance z:
`w_f=(zlambda)/d`

References

Wikipedia (https://en.wikipedia.org/wiki/Fraunhofer_diffraction)