Reason for using X-ray to determine internuclear distance

[littleboy]

From the book,i know that X-ray is used to determine internuclear distance between atoms in a compound(or element) because wavelength of X-ray is similar to the internuclear distance between 2 atoms.But my question is that:what is the relationship between the nearness and its use to determine the internuclear distance?

[swansont]

For diffraction and interference to be helpful, you need something with a wavelength of the same order as the spacing of the structure you are investigating.