What is light? Seems like a silly question at first, but after some thought most of us will agree we don't know much. It comes out of the sun, lets us see things, and makes us warm. We can't grab it, can't outrun it, and can't seem to define it. Yet, physics has made many successful attempts at clarifying what light is and what it can do.
Much of the difficulty in defining light results from its seemingly contradictory behavior. In defining the behavior of a phenomenon scientists try to create a model, a system of rules explaining what is observed. In the early part of the 20th century, there were two different models for light. One looked at light as a particle and the other looked at light as a wave. Just thinking about the two ideas seems a bit strange. How can waves, vibrations in things, and particles both describe light when they are so different? In the following pages we will try to explain light from both perspectives.
With the development of quantum mechanics in the 20th century came the decision that it's both! Light can act like a particle and a wave and the same time. We just see different aspects of its nature at different times. If this at all seems strange, don't worry. Quantum physics is confusing to most people, even to the people who invented it!