This is because, after all, the eyes of human beings are simply photon detectors. Light consists of particles called photons, and these photons register with our consciousness as they enter our eyes.

In fact, strictly speaking, we don’t actually see things with our eyes; really, what we are seeing is light as it bounces off the things and enters our eyes. But for now the only thing that matters to our discussion is that for your girlfriend under the tree the two lightning strikes happen at the same time as each other.