It turns out that light is among the most complicated topics in physics to talk about. It is utterly unique in so many ways, and it defies many of our expectations for how we think the universe ought to work. If there is a single lesson to take away from this section, it’s that all of the intuitive ideas that we will develop about light are wrong at some level. But unless you’re a physicist doing research on light, the ways that our basic models are wrong aren’t important enough to worry about.

Light is electromagnetic radiation. Before the term radiation scares you, it’s important to understand that everything in the universe that has a temperature above aboslute zero emits some form of electromagnetic radiation. The sun emits electromagnetic radiation, some of it that we can see and some of it we can’t, such as ultraviolet radiation, which causes sun burns and skin cancer. Cell phones emit electromagnetic radiation, which is how they communicate with each other. You emit electromagnetic radiation, which is how night vision goggles can see you. And all light bulbs, regardless of the type (incandescent, fluorescent, or LED) emit electromagnetic radiation.

It might seem strange to say that you emit light, and that is because what we normally call "light" is electromagnetic radiation that our eyes can detect. However, in physics, we try to be more precise and call that "visible light" to avoid confusion. The reason for this is that the only difference between the radiation that our eyes can detect and the radiation that we need technology to detect is the amount of energy contained in that radiation. If we were to organize all of the types of electromagnetic radiation and order them by how much energy they each have, we would have a diagram known as the electromagnetic spectrum. Below is a diagram that labels the different energy levels.

There are two main ways to think about what light is. The first is that light is a photon, or a packet of energy. In this perspective, light is a particle. That means that it has a specific location that is traveling in a specific direction at a certain speed. In this framework, the energy of a photon is an intrinsic property that defines what type of light it is. Alternatively, we can think of light as a wave. In this perspective, we think of light traveling through the universe the way that a wave travels across water. The wave does not have a specific location, but is kind of spread out over a region. The wave may be moving in a specific direction and speed, but it’s less clear what, exactly, is the thing that’s moving.

This mixed identity of light is often called wave-particle duality, which is a catchy-but-imprecise name that tries to capture the idea that a wave is somehow both of these things, while also being neither one. There is some history behind this, which is why the term sticks around. It turns out that physicists debated about whether light was a wave or a particle, with different scientists proposing different experiments to try to prove it one way or the other. Eventually, it was discovered that both sides could be right, depending on how the experiment was set up.

The importance of wave-particle duality is that it allows us to mix the two concepts together. For example, we often think of a photon as a ball of light that can bounce off of surfaces (reflect) in particle-like ways. At the same time, the way we perceive the color of light is based on the "wavelength" of light (the distance between consecutive peaks), which is a wave-like language to use. And we often don’t think twice about it when we go back and forth like that.