Light is something that we tend to take for granted in everyday life. It’s not light itself that we are usually interested in, but the things that light illuminates: the shapes of objects, colors, faces, and cherry blossoms that seemingly spring out of nowhere and inspire us with their beauty.
Light is also something I have been dealing with in the lab for several years now. Low levels of light that carry information, quantum information, that one day, I hope, may find an application in low-energy, high-speed optical communications. But certain things about light have always puzzled me. Like the way it just appears, or is emitted, when electrons relax to lower energy levels. Where does it come from? Where does it go when it gets absorbed, like in solar cells? Does it just disappear? After travelling approximately 100 million miles from the sun to the earth, it just up and disappears? We know it travels at a very high speed, but how? With all the advances in modern physics, the true nature of light is still a mystery. So what is light, exactly?
Philosophers and scientists alike have grappled with this question for centuries, and their views seem to generally place the nature of light into two vastly different categories: 1- Light is composed of tiny particles traveling in straight lines and bouncing off objects, or 2- Light consists of oscillating fields propagating through the fabric of the universe as waves. Quantum mechanics makes things even more confusing, stating that light behaves sometimes like a particle and sometimes like a wave.
This duality has fueled numerous debates over the years. Some of the earliest Greek philosophers, including Empedocles (5th century BCE) and Plato (4th century BCE), held that light originates from our own eyes, and bounces off objects and returns to our eyes, allowing us to see them. Empedocles believed that Aphrodite, the Greek goddess of love, lit a fire in our eyes, fire being one of the four elements, serving as a source of light. To address the problem of not being able to see at night, he postulated a necessary reaction between the rays from the eyes and the rays from a source, such as the sun. Aristotle’s view was radical at the time, in thinking of light as a disturbance in the element air created by a source such as the sun and only detected, not created, by the eye. Meanwhile, Democritus, the father of atomistic thinking, considered light to be no different from matter in the sense that it is ultimately composed of indivisible, unbreakable units, which places him in the particle camp and makes him an early visionary of the modern concept of the photon.
Much later, Dutch physicist Christiaan Huygens and French physicist Augustin-Jean Fresnel were able to explain many properties of light, such as reflection, refraction, and later diffraction and interference, by treating light as a wave where each point on the wave front can itself be considered to be a point source of a secondary spherical wave. Meanwhile, Newton was convinced of the particle, or ‘corpuscule’ nature of light, due to the power of Euclidean ray optics in describing how reflection occurs in mirrors and how prisms produce rainbows from sunlight.