 The result is the Fraunhofer approximation, which is only valid very far away from the object 
Depending on the size of the diffraction object, the distance to the object and the wavelength of the wave, the Fresnel approximation, the Fraunhofer approximation or neither approximation may be valid.As the distance between the measured point of diffraction and the obstruction point increases, the diffraction patterns or results predicted converge towards those of Fraunhofer diffraction, which is more often observed in nature due to the extremely small wavelength of visible light.Multiple-slit arrangements can be mathematically considered as multiple simple wave sources, if the slits are narrow enough.