How a group of nucleus can act as a circular aperture

[Alvan]

So I have been learning about measuring nuclei radius using electron diffraction, using sin(theta)=1.22lambda/d, doing some research I found out that is the equation for circular aperture diffraction but I don’t really understand how a group of nucleus can act as a circular aperture. Also is there anyone can show me the derivation of the diffraction equation, the only one example that I could find was way out of my league, hope someone can enlighten me on that, thank you

 

[Charles Link]

This discussion in wikipedia is quite good: https://en.wikipedia.org/wiki/Fraunhofer_diffraction