What is the limit of classical electrodynamics?

[KristallIsa]

Hello everyone,

I was wondering about the following question:

Under what condition(s) could one say that the interaction between an electron and an electromagnetic wave is not governed anymore by the laws of classical electrodynamics?

I would also be glad if someone could point out any good references on this topic. I've been searching for some time already but with not much luck...!

Thank you very much.

 

[mfb]

If the interaction of single photons with electrons becomes significant (for example at the photoelectric effect), classical ED will give wrong predictions. In the limit of many photons, it should be fine. This is usually the case with (nearly) static fields or radiation with long wavelengths, and sometimes with visible light of extreme intensity.