When exactly does light get refracted?

Question

Short question: When does a light ray know it has to bend/be refracted? I know it happens at the interface of two media. But when is there an interface of two media? When there is a single hydrogen atom and 500 km away a helium atom?

I've seen Why exactly does a material reflect/absorb/refract light? but I think, either there or here, there would be some more input helpful.

The idea of refraction at interfaces cannot be used right down at the molecular or atomic levels. "Refraction" in the conventional sense does not happen when there are just a few atoms. — Paddy, Jun 28 '21 at 17:29
makes sense, guess on the atomic level, there is only scattering? — Ben, Jun 29 '21 at 06:40
Scattering is certainly one of the microscopic phenomena that occurs. As usual, scattering is not as simple as high school level textbooks make it seem. I am however not experienced enough at physics to answer your question in any more detail. I think Hecht's book on optics would be a great place to learn more if you're interested. — Paddy, Jun 29 '21 at 16:31
I'm a particle physicist and for me, it's rather hard/uncommon to think at macroscopic levels. But yes, scattering is very complicated but optics and other stuff as well :) Wow, just googled your book advice and it shows me another book, for ~5 k ^^ — Ben, Jun 30 '21 at 06:40
Woah if you meant $5k, that sounds insane. Here's a link https://www.amazon.com/Optics-4th-Eugene-Hecht/dp/0805385665/ref=sr_1_3?crid=2H5FJ6XDX2S55&dchild=1&keywords=hecht+optics&qid=1625064064&sprefix=hecht+%2Caps%2C136&sr=8-3 . I got a copy when I was traveling for around 10USD. I am a photonics researcher and I think this book is an overall great entry to optics. Of course, I also thought that you might be a high school student so you might already be familiar with most of the stuff in the book. Most college libraries have a copy of this book. — Paddy, Jun 30 '21 at 14:42

When exactly does light get refracted?

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