There's something very peculiar about light. That's what Professor Mark Kuzyk told me the other day. He should know, because he studies optics here at Washington State University.
Optics is the study of light.
Professor Kuzyk says that light is a wave, like waves on the water or the waves that curl along a long rope when you snap it. Well, it's one of those things that just is. In some scientific experiments, light acts like a wave. In others, it acts like a particle. But for talking about mirrors, we can just think about light as particles.