| 10 min read
What is light?
Everywhere we look, we’re surrounded by a mysterious phenomenon. Just like the air we draw to our lungs allows us to breathe, this ubiquitous force bestows upon us the gift of perception.
For those of us fortunate to have a sense of sight, being able to see things is something we take for granted. Be it the sun, road luminaires we pass while driving, or panels suspended above our heads in the office, there is always something that takes care of providing our eyes with stimuli. Without it, we would be sentenced to a lifetime in darkness.
What's the matter
As you probably know, we cannot look at light the same way as we look at substances. Unlike it’s the case with water, we cannot pour light into a bucket or catch it and seal it in a container so we can use it later. That is because light is not a fluid, gas, or solid – it is a form of radiated energy.
Historically, the physical nature of light was a subject of fiery debate since the dawn of scientific inquiry. Researchers and scholars proposed various theories that shaped the understanding of the matter throughout the millennia. Our path towards the comprehension of light was meandering through more or less sound ways of reasoning, all of which however ultimately failed to offer convincing explanations for all of the phenomena observed. It was the advent of quantum physics that finally, excuse the pun, shed some light on the topic.
Light is propagated by elementary particles called photons. They are the smallest, massless carriers of energy.
We cannot see the flow of photons itself. What we can see is its distribution in a medium and interaction with various objects. Even laser beams are visible only due to interaction with particles which make up the medium through which the beam travels – most likely, air.
A fog machine is used to make laser beams clearly visible in a club [1]
Once it reaches an object, light can be reflected off it, pass through it, or be absorbed by it. In real-life scenarios, it is usually a combination of the above. Various physical phenomena can be observed in this process, but for now, we will focus on the basic principles and save that for further investigation in the future.
Both the properties of an object and those of the light itself take part in shaping the resulting visual stimuli. The surface of the object, its color, shape, and many other factors influence how it interacts with light. For example, directing a flashlight at a rough surface of a wall will produce a non-directional reflection in the form of a spot with soft edges. Pointing the same flashlight to a finely polished mirror will result in a nearly perfect reflection which causes a virtual image to be projected.
Let's get physical
It all begins with elementary physics. It’s years before we think about chasing a degree in physics that we instinctively start to recognize what energy is. We often conclude that it can be stored, it can have many forms, and we learn that it can be both dangerous and incredibly useful.
We watch out not to burn our fingers while ironing clothes. With a charger in our hands, we look around for a power socket to save our phone’s battery when it’s close to dying. A beginner archer quickly comes to understand, that if one pulls harder on the string, an arrow released from a bow will travel over a longer distance. Energy, the shapeshifting entity behind this, manifests its presence in our everyday lives, and throughout history, we learned how to intuitively harness much of its power.
Waves of electromagnetic radiation propagated in space carry energy. Over the years, as science progressed towards understanding the nature of this phenomenon, we learned that these waves possess various distinctive qualities that change along with their length. What was perhaps the most important observation was that the shorter it is, the more energy an electromagnetic wave carries.
From deadly, ionizing Gamma rays that can easily penetrate matter and X-rays that brought revolution once introduced to the medical world, we finally arrive at our destination – the range of wavelengths that are visible to the human eye. This is light.
Swipe left to see the basic order of electromagnetic radiation types:
As you can see, we could go further to meet invisible infrared, microwaves, and radio waves, all of which are infinitely interesting, but for now let’s focus on what allows us to see.
Even with such fierce competition, the contest for the most influential type of electromagnetic radiation is easily won by the relatively narrow range found somewhere between 380 nm and 780 nm. Visible radiation or simply light (the term visible light being somewhat redundant – more on that later) constitutes the ample supply of energy that provides our eyes with stimuli.
Each wavelength of light can make our visual system respond with a sensation of a certain color. It is however uncommon that we have a chance to observe light that consists of a single wavelength in nature. Such monochromatic (Latin: mono – single, chroma – color) radiation was rarely seen before the advent of complex lighting devices. In most cases, the light that we see is made up of radiation in many wavelengths, a composite creating a variety of resulting colors. All this is possible thanks to one of the most important properties of light – the fact that it can be added up to produce different colors, up to the point at which our visual system projects the sensation of white light. Such is the main principle of additive mixing.
Three primary colors: red, green and blue produce white when combined
If we were to examine the contents of light produced by a given light source and analyze how much energy there is for each wavelength (each color), we would eventually end up with a handy analytical device called spectrum.
The most widespread form in which spectral data is presented takes shape of a chart, where the wavelength is presented on the x axis while values of power are assigned to the y axis. Wavelengths are usually given in nanometers, while values of power can be relative or absolute, given in any unit related to the measured quantity.
Spectral power distribution curve provides information on how much energy is carried by all spectral compontents of light
The resulting curve (often called spectral power distribution curve or SPD) proves to be a very useful tool that allows for a brief assessment of a given source of radiation. A quick glimpse at a spectrum lets one conclude that the light will be cool or warm, that it will have a certain colour dominant, or even how good it will be at rendering certain colours of objects.
Spectra (plural of spectrum) are all over the place in lighting, and for a good reason. Simple and easy to interpret, they are possibly the most succinct and informative form of displaying basic information whenever light needs to be characterized.
A brief description
As far as definitions of light go, the International Commission on Illumination (CIE) provides three explanations related to the term [2]:
"radiation that is considered from the point of view of its ability to excite the visual system"
light, noun < psychophysical >
CIE 17-21-012
"radiation within the spectral range of visible radiation"
light, noun < photometric >
CIE 17-21-013
"optical radiation capable of causing a visual sensation directly
"
visible radiation
CIE 17-21-003
The exact limits of the wavelength range of radiation defined as light are often debated. As much as the lack of consensus seems to cause confusion, one does have to consider that it might just not be that important outside of the academic world.
Supplementing the terminology suggested by the CIE is a short comment stating that the lower limit is expected to be found between 360 nm and 400 nm, while the upper limit is usually set between 760 nm and 830 nm. The ability of radiation to excite the human visual system is largely dependent on the conditions, the irradiance (power per unit area) on the surface of the eye, and the specific characteristics of the observer. One of the frequently discussed factors that impact our ability to see is the gradual decrease in sensitivity related to aging. For example, an experiment [3] carried out in a controlled environment showed how one person could see down to 310 nm at age 24. That ability was reduced to 400 nm at age 74.
Only the beginning
Each light source, whether it be of natural or artificial origin, will emit light that possesses a set of defining characteristics, such as spectral content, luminous intensity distribution, polarization, and many more. Knowing about them provides us with incredible opportunities to shape the world around us, but that’s something we will discuss in the future. If we want to find out what humans can do with light, we must first understand what light does to humans…
Part 2 coming soon
Summary
- – Light is a form of radiated energy, not a substance
- – Photons are elementary particles which are quanta of electromagnetic radiation, including light
- – We cannot see the flow of photons itself, we only see how it affects the world around us
- – Light belongs to the very same category of energy as radio waves, gamma rays or microwaves
- – The way we see light depends on its properties, the medium it’s propagated in and the objects it interacts with
Related articles
- Coming soon
Sources
[1] photo by Alexander Popov on Unsplash
[3] D H Sliney (2016): What is light? The visible spectrum and beyond, Eye 2016 Feb; 30(2): 222–229.
Borys Skrzypiński
Engineer, Poznan University of Technology graduate, MSc, lighting enthusiast, author of Luminforum