Question

Why do approximations work with diffraction?

Answer 1

We usualy consider that the distance between the slit and the screen is much larger than the slit width but why would that not matter? I mean sure if the wavelength was long enough yes, but visible light has a wavelength in the nano meter range, which means that even those approximations should affect the end result quite a bit, but then why do we neglect it?
The Fraunhofer approximation is used when the diffraction pattern is viewed at a long distance from the diffracting object (or when viewed at the focal plane of an imaging lens). But when the diffraction pattern observed near the slits (in the near field) one should use Fresnel diffraction. Zone plates are an example of when this is necessary.

Answer 2

Hey !

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• Approximations are to be done on the basis of diffraction because they are quite as a whole in nature.

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Thanks !