Don't know if it's just me, but whenever I read or asked about quantum physics, I'd immediately get stuck on what it means that a photon is a particle and a wave, usually that exact phrasing. A wave of what? Some explanations even make it look like the particles move through space in a sine wave. And then they keep going on the math around waves, introducing the Schrödinger equation, and I still don't get what these waves physically are.

This doc is the first one I've seen that explains upfront what a wave means here: "The quantity which travels in this way is known as a 'disturbance'. Precisely what this disturbance is depends on the wave," and for EMR, "it is an electric field and a magnetic field which are perpendicular to each other and also perpendicular to the direction of the wave." So now I'm going to read the rest.