# Background: properties of light

## What is light?

You may have heard that light has both wave and particle properties (wave/particle duality), which is true.  Sometimes it’s useful to describe light as a ray, which makes its behavior easier to analyze; but this is an approximation that’s only valid when the wavelength of light (the distance between wave peaks) is much smaller than the physical features of whatever system you’re analyzing (a lens, a prism, etc.).  For now I will discuss the wave properties of light.  Light is an electromagnetic wave, which means that light waves are composed of electric and magnetic fields.  Mathematically, light waves must be solutions of Maxwell’s equations, which describe the behavior of electric and magnetic fields.  It’s pretty extraordinary that the static electric fields that make your hair stand up, and the static magnetic fields that hold decorations onto your refrigerator, are described by the same equations (Maxwell’s equations) that also describe light – it’s not immediately obvious that these disparate phenomena are related to one another, but they are.  Physical phenomena that are governed by Maxwell’s equations ranges from the mundane (contact forces) to the bizarre (metamaterial-based ‘invisibility cloaks’).