What is light?
Our Solar System is centered around a giant ball of plasma we call The Sun. It affects the Earth's seasons, ocean currents, weather and most importantly it provides energy in the form of light. In an essence, without this energy there will be no life on Earth. Not all light that comes from The Sun is good for us though.
If it was not for our planet’s atmosphere, the full electromagnetic spectrum constantly bombarding us would have made for a inhospitable environment like can be observed on many other planets with weak or non-existent atmosphere, e.g. Mars.
This brings us to the question, why do we see just a fraction of the full spectrum? As was just explained, because most of it is constantly blocked by our planet’s atmosphere. Our eyes have evolved for millions of years to process the reflected light that managed to reach the Earth’s surface and for better or worse, it is a very small fraction of the total existing light in the universe.
From the diagram of atmospheric electromagnetic opacity, we can see a visual representation of the light blocked by the Earth’s atmosphere. While some infrared, ultraviolet and radio waves still manage to break-thought, their energy is simply too low to be observed with a naked eye and appears transparent without a helping hand from technology.
To avoid feelings of inferiority, it is comforting to know that all life on Earth is born within a predefined biological “optical window” that limits the amount of visible light.
Wavelength is one of the key characteristics of any electromagnetic wave including light. Hence the common term for color vision - “the ability of living organisms to perceive differences between light composed of different wavelengths”.
Throughout history scientists have categorized wavelengths of different sizes into several groups. You have most likely heard or at least read about Radio, Microwaves, Infrared Light, Ultraviolet Radiation, X-Ray and even the one we can hardly live without nowadays, Wi-Fi. In the end, however, they are all the same kind of energy, sorted by us in these groups for communicatitative convenience.
Long before science was structured as an academic field, humans did the same with the visible part of the electromagnetic spectrum. We gave names to the different wavelengths of light and called them colors.
At the very beginning, it was only Black, White and for some reason Red. These are just words we made up for the extremes in our environment.
No light is Black. Too much light is White. Most likely the burning of wood and all the blood spilled from battles between tribes left a strong impression on our minds and so the word Red came to be.
As speech developed, we eventually went and detailed colors by making even more words like Blue, Green, Yellow and etc.
Shorter wavelengths from the visible spectrum appear to us bluish, while longer wavelengths appear reddish. It is important to note that the word “appear” is crucial because electromagnetic radiation carries no color by itself.
The photons that make up light do not know what they look like to you nor do they care what you think they look like or even what you name them. Our existence is simply meaningless to them.
Every single thing regarding colors is subjective. All of it happens only in our minds and can even differ slightly per person, e.g. various degrees of colorblindness. The reason is simple and it is because color is how we interpret our reality, the same way we do with pain, taste, smell or any other cause-effect relationship.
An object does not carry emotions. It just lays there minding its own business. It is we who can use objects as means to stimulate emotions. Sitting on a comfortable chair can make you feel good, but the chair does not really care. In the same way, light does not carry color. We “feel” colors when reflected light meets our eyes.
When light enters the eye it hits a thin layer of specialised nerve tissue called retina. This is the key part for seeing in general. There, the light rays are sorted out and interpreted in order to build a picture and only after this complex initial processing of the information is passed down to the brain via an optic nerve do we achieve color vision.
The combination between just three optic signals is enough for the brain to allow us to see millions of colors. How can just these three types of cells give the impression of all the possible colors we see?
In the human eye there are 6 to 7 millions retinal cells known as “cones” that are responsible for color vision. These receptors are known as “cones” because, well, they look like cones.
S cones are most sensitive to short wavelengths at around 445 nm which we perceive as “blue”.
M cones respond the most to medium wavelengths at around 540 nm which we perceive as “green”.
L cones respond the most to long wavelengths at around 565 nm which we perceive as “yellow”.
The three ranges do not actually correspond to the final variety of colors we perceive. In fact several combinations of wavelengths can create the same perception of color while some colors, such as pink or brown, occur only through the combination of different wavelengths.
For example, the color pink is visible when a lot of red wavelengths (L-cones) and a small amount of all other visible wavelengths (M + S cones) enter our eye. Brown on the other hand requires a lot of red stimulus (L-cones), some orange (M-cones), a bit of yellow (M-cones) and none of the remaining visible colors (S-cones). When cones react to extremes such as all visible colors at the same time (L+M+S cones), we see white. When there is absence of visible wavelengths we see black.
To reach the same reaction by using different colors is also possible. For example, as was just described, when cones react to all visible colors at the same time (L+M+S cones) we see white. We can reach the same result by just using three colors that stimulate all three types of cones to their maximum with Red (L-cones), Green (M-cones) and Blue (S-cones).
Another good example of this is Yellow. We can see Yellow by catching the wavelength that directly corresponds to the color or by catching both Red and Green at the same time. In short, the brain can easily be tricked to see any of the visible colors by using a combination of just Red, Green and Blue.
The retina makes color vision possible by essentially acting as a light sensor. Roughly six to seven million retinal cells known as “cone cells'' are constantly looking for changes in the reflected light back from the objects we look to create our trichromatic color vision. This principle is also used as the basis for the RGB color model and the foundation of our digital age.