For centuries, light has been a mysterious phenomenon, daily taken for granted by people who would otherwise not survive without the benefits that light provides. From a more primitive perspective, light generates heat, provides a way for our eyes to see objects, avoid dangerous situations, and distinguish a wide variety of colors and textures. Today, light offers even greater benefits, such as therapies that accelerate the healing of wounds, the ability to slice through numerous impenetrable materials, and the ability to transmit data and communicate all over the world in high-definition color.
We know how to use it, but what exactly is it? In my course on Optics, the answer is more complicated than it might seem. Is it a ray, wave, or particle? Prior to classical physics, light rays followed a geometric behavior that reflected off of surfaces or bent slightly through surfaces. Observe a window at night. Given the proper ambient lighting inside the room, you can see faintly through the glass but also see your reflection looking back at you. This effect is defined in Snell’s Law, which states that the sine of the angle of incidence of light is proportional to the sine of the angle of refraction through the medium. This law is further observed when looking at a straw inside a glass of water. To an observer, the straw may seem to bend slightly and widen when it is inside the water, even though the straw is physically straight and has a constant radius.
Using terms of Geometry is a great way to describe some factors of light, but it doesn’t elude to the possible interference light may have with itself. This interference can be heard audibly when listening to white noise on a radio because radio waves are light waves. Sunlight can interfere with the transmission of radio waves. Light is best described in classical physics as: “A transverse electromagnetic wave, meaning it consists of oscillating electric and magnetic fields propagating through space at a constant speed, with characteristics like wavelength and frequency, and its behavior can be explained using Maxwell’s equations of electromagnetism.”
The speed of light is derived from the constants of permittivity and permeability of free space, a fact found in electromagnetic theory. In short, light can be defined as an electromagnetic wave. This foundation is where interference can come in because electromagnetic waves crossing each other behave in a state of superposition, where the highs and lows of the waves are added and subtracted from each other. This is seen in the famous double-slit experiment.
One of the lesser-known aspects of Einstein’s work is that he did not receive his Nobel prize for his famous equation, E=mc2, but for his discovery of the photoelectric effect. This discovery fundamentally altered our understanding of light, demonstrating that it can behave as a particle as well as a wave. This duality is at the heart of modern physics, with the concept of light as a quantum particle known as a photon. By challenging the ideas of classical large-body physics, we can now detect light on a very small scale and understand that it has energy levels that are discrete rather than continuous.
So, to answer the question “What is light,” the answer requires another question. “How are you viewing light?” The use and application of light are totally dependent on what you are applying light in. Are you trying to detect a laser? Try to use a geometric approach first. Is there noise in your radio system that you are trying to mitigate? Try using a classical physics approach. What about detecting the losses of an LED system or solar panel system? Try to consider the losses when measuring a photon, as the position and velocity of a photon cannot both be measured (Heisenburg’s uncertainty principle).