As far as I'm aware, there are two different (and almost contradictory) models that describe the behavior of light: light as a wave (EM), and light as a particle (QM). From what I've heard, depending on the context one may choose which model to use, and it'll work.
For example, in Young's double slit experiment, we look at light as if it were a wave. If we use a photomultiplier, light behaves like a particle.
Now, what I'm wondering is this- do we have a single model/theory whose equations accurately describe the behavior of light in all scenarios? (Is this what QED does?) Essentially, do we have a single model to describe light (but with multiple interpretations regarding how to "think of" or "imagine" light), or do we use two completely different models to describe its behavior depending on the context?
Best Answer
Yes, for all intents and purposes: QED. Clearly, we can not say that QED works at all scenarios (e.g., very tiny length scales and very high energy scales), since we can not test it in all scenarios. But, as far as we know, QED does the trick.
Yes, given the above caveats.
Yes and No. We apply full QED when the circumstances require it, but if we can ge away with using classical electrodynamics (which is a limiting form of QED) we usually do because it is often computationally simpler.
We also describe light differently when the different descriptions are easier to understand intuitively. For example, a gamma ray it often better described as a "corpuscle of light" (a "photon") than as a wave. I.e., it is "hard". On the other hand if you are designing a radio antenna for your car stereo, there is no need to invoke full QED. I.e., you can treat those soft radio wave completely classically and ignore quantum effects.