What is light? This perhaps may be the most simple question with the most complex answer in photography. Light enables us to see, create, and most importantly convey meaning through our art. Light also gives us the oxygen we breathe (through plant photosynthesis).
The Physics of Light
Light is energy. Simple enough, right? But let’s dive deeper.
The exact physics definition of light exists in many forms and over the past century, Nobel prizes were awarded for various additions to what we simply call light. The type of energy light is is called electromagnetic radiation. Modern physics agrees that light exists as photons, small particles of light that are pure energy.
Although school physics may be a little rusty in some readers’ heads, you may remember that energy creates an electromagnetic field around the photon. This field is not constant, and it changes as the photon propels forward. As such, we can picture light as a wave for the simplicity of further explanation. The particle nature of light will be useful when discussing how it interacts with and reflects off surfaces.
The electromagnetic light wave has two components: electric and magnetic. The electric component fluctuates out of phase with the magnetic one. As such, when the electric field strength around a photon is at its maximum, the magnetic one is at its minimum. Since the two fields are perpendicular to each other, the total field around a photon is constant.
The reason I am detailing this is to build a foundation for describing where color comes from and how basic light effects work. Although photographic lighting isn’t all about physics, knowing the science behind light will allow you to solve difficult problems on set, which pays a good dividend in the invoice.
All photons travel through space, but they have very different electromagnetic fields (waves) around them. This depends on just how much energy any given photon has. High energy photons are known as gamma radiation, and ones that have slightly less energy are X-rays. Photons with the lowest energy are radio waves. If you’re interested in this, look up the electromagnetic spectrum.
We humans can see a small fraction of this spectrum, the portion is known as visible light.
Red light has much less energy than blue light. Hence the energy fluctuates at a higher frequency around blue light. The rate of fluctuation is known as frequency and is measured in Hertz (Hz).
The speed at which light travels through a vacuum is fixed. Likewise, no matter the energy of light, it will travel through some materials too. For example, visible light can travel through glass, while gamma-radiation can travel through concrete and metal.
The equation commonly used to talk about waves unifies three quantities: frequency, speed, and wavelength.
The wavelength for each color is measured in nanometers. For example, the blue light will be 450 nm long while the green will be 550 nm.
The brightness of the light will be controlled by how high the wave peaks are. As such, an electromagnetic field with a large amplitude will suggest that the light brightness is high.
The Art of Light
Now what we know where light, color, and brightness come from, let’s look at how we can most practically apply this to the beloved art form of photography.
Most flashes are balanced to produce a daylight output, which is measured around 5500 Kelvin (K). This light would be seen as white. Human eyes have learned to recognize an even 33% mix of three primary colors: red green blue as white. Importantly, our eyes can adjust and perceive very uneven mixtures of the three primary colors as, still, white.
Modern cameras are not nearly as smart and easygoing as our eyes. They need to know where exactly white is. A scale with temperature measures of Kelvin comes in handy: most light is within the 2000K-10000K scale.
The reason this borrowed-from-physics scale is so useful to photographers is because it describes the colors at which an object heated to that temperature will glow. Contrary to popular belief, an object at 10000K will not be orange but blue. Human intuition breaks when someone says that a blue object is much hotter than an orange one.
There are two types of Kelvin-balanced lights: Tungsten and daylight. Tungsten is around 3200K and is fairly yellow while daylight is 5500K and looks white.
Post-production offers great flexibility for making your image cool or warm, all with the turn of a mouse wheel. Generally, a cool image will have a more detached feeling while a warm one will calm the viewer.
The second most important quality of light is how bright it is. Photographers often feel that a bright pixel is a good pixel. Somehow, even portrait/fashion photographers working with 4800W of power complain that there’s not enough brightness. The concept of brightness is closely linked to the third component of answering: “what is light?”
A bright highlight and extremely dark shadow will convey strong contrast and generally increase drama in the picture. In order to achieve this, you need to make sure that light rays strike the subject from the same angle. For example, adding a grid on your softbox will increase the contrast as now you’ve limited the number of angles the light rays can strike your subject.
On the other hand, diffusing light will scatter the light, increase the number of angles, and produce a softer light with less contrast.
Light is the raw ingredient of image-making. Learning to shape light is like learning to cut tomatoes in order to become a chef. Photographic lighting is both physics and art. Learning physics is generally a slog for most people as they don’t imagine a photographer’s job to ever be anything remotely science-based. Yet, the physics of light is like driving theory: it enables you to understand what is possible and how it works. The artistic side is made easier by going through the slog of learning, mastering, and practicing light shaping.
P.S. For an in-depth understanding of light, I highly recommend the book Light Science & Magic: An Introduction to Photographic Lighting, which covers these concepts and much more.