# What is the principal cause of diffraction?

I have found another question similar to mine here. But I want to know why does diffraction of light happen in the first place. I have found other resources on google which explain the topic partially on the basis of Huygens principle. But

I'd like to know if the phenomenon of diffraction could be explained at the atomic level in terms of photons, electrons and atoms and how they interact to create this effect?

I've found this video. Is this explanation given here correct?

Lastly, I'd like to know if refraction could be explained at the atomic level.

Please explain in detail. Thank you very much!!

• Explaining the interactions of light with matter 'in detail' would take a book. And there are a variety of books that do just fine. Explaining diffraction is easier - an infinite plane wave is not a solution to a wave propagating through inhomogeneous space. Jan 18, 2019 at 15:57
• No, the video is false along with other videos of this "researcher". A good way to learn physics is by reading a good textbook, not by watching junk videos on YouTube. Jan 18, 2019 at 16:05
• en.wikipedia.org/wiki/Huygens-Fresnel_principle Jan 18, 2019 at 16:07

So the question is, given a certain input optical field, why does it diffract the way it does? How does it work? It is simply a wave effect. There are certain special waves called plane waves that do not change while they propagate through free space apart from a change in phase. Considering propagation in a specific direction that we can call the $$z$$-direction, one finds that these plane waves pick up different phases, depending on the angle between their directions of propagation and the $$z$$-direction.