Light, Color and Perception

 “If one says ‘red’ – the name of color – and there are fifty people listening, it can be expected that there will be fifty reds in their minds. And one can be sure that all these reds will be very different.” “We never really perceive what color is physically.”

(Josef Albers, American-German artist)

Jonathan, a 65-year-old artist, is involved in a car accident. While driving, he is hit by a truck on the passenger side. He is taken to the emergency room, told he had suffered a concussion, but that it did not appear serious. But then something peculiar happens. While taking an eye exam, he discovers that he is unable to distinguish letters or colors. Letters seem to be Greek and images look like a black and white television screen. “My brown dog is dark grey. Tomato juice is black.” Eventually he is able to see letters as before, but he can never see colors again.

His condition is called cerebral achromatopsia, a type of color-blindness caused by damage to the cerebral cortex. Jonathan’s eyes are fine, but his brain is not. The accident caused irreparable damage to a part of his brain responsible for processing color, a small area of the visual cortex called V4, located in the rear of the skull.

Jonathan’s story illustrates the concept that color vision is humans’ perception of a physical phenomenon that must be interpreted by their brains. Josef Albers’ quote highlights how different this experience is among individuals. To fully understand this complexity, we first need to understand the nature of light and color.

Physics defines the word light in a slightly different manner than our everyday use of it. Physicists refer to light as the entire range of the electromagnetic spectrum. Radio waves are at the low-energy end, while x-rays and gamma rays are at the high-energy side of the spectrum. Between these two extremes there is a narrow range which is referred to as visible light. Visible because it can be detected by our eyes. This is what most of us call light.

All light, including the visible one, is made of subatomic particles called bosons, specifically some called photons. You may remember recently hearing in the news about the discovery of the Higgs boson, more commonly referred to as the “God particle”. Photons are a more common type of boson, a bit less glamorous. We can think of photons as massless small packets of discrete energy or what physicist called quanta of light. Quanta from Latin meaning amount, in this case meaning the smallest possible discrete unit of energy. Photons are the force carriers between electrically charged particles, like protons and electrons.

The difference between a radio wave and what we perceive as Red is just the wavelength or energy level of the photons. The longer the wavelength, the less energy the packet contains; the shorter the wavelength, the higher the energy. The amount or quanta of energy is determined by a simple formula consisting of the wavelength times a constant. We could say that what we perceive as Red is just a higher energy photon than a radio wave. What our eyes perceive as light are all the photons within a range of wavelength or energy levels that correspond to the visible part of the spectrum. What we perceive as different colors really are just photons at different wavelength or energy levels in that spectrum. Red light is the longest wavelength and lowest energy level, while violet corresponds to the shortest wavelength and highest energy level.

The following charts show the entire light spectrum and where visible light falls in, around the middle section, between infrared and ultraviolet. Also take a look at the factors of scale, frequency and temperature.

By Inductiveload, NASA – self-made, information by NASABased off of File:EM Spectrum3-new.jpg by NASAThe butterfly icon is from the P icon set, File:P biology.svgThe humans are from the Pioneer plaque, File:Human.svgThe buildings are the Petronas towers and the Empire State Buildings, both from File:Skyscrapercompare.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2974242

The Sun is a mass generator of photons. It generates them across the entire electromagnetic spectrum from radio waves to gamma rays. It spews them out in all directions into space. Let’s  follow a single photon’s journey, it has a wavelength that most of us perceive as the color green in the range of 560–520 nm. Let’s say that our photon is traveling along with a group of other photons. This group consists of photons in a continuous range of wavelengths in the visible spectrum. This means that some would correspond to what we perceive as the color red others yellow, orange, blue and violet. Our eyes would perceive this mixture as just white light. The group leaves the surface of the Sun and about eight seconds later they start striking the suspended particles of gasses in our atmosphere, then the surface of the earth, a mountain, a rock, a tree, a leaf and so on. This is when things start getting interesting.

There is basically three things that can happen to these photons: their energy can be absorbed, reflected or transmitted. What happens depends on the atomic composition of the objects they strike. Every element in the periodic table responds differently to each wavelength or energy levels of photons. The reaction depends on the natural frequency of the electrons within the atoms. Because of this property of matter, every element in the periodic table has a distinct color signature. This property is used in spectroscopy, using light to determine atomic composition, and it is widely used in many areas of science.

In the case of the leaf, its atomic composition absorbs the energy of most of the group of photons, except for our photon.  That photon has a wavelength which corresponds to what we perceive as green. The leaf reflects that single photon which then strikes a cone cell in our retina in the back of our eye and we perceive the leaf to be green. In reality, it will take much more than a single photon for us to be able to perceive light or color, but it is a simplified example of how the process works.

If you have not noticed by now, I have been using the phrase “we perceive” several times when referring to color. This was a conscious effort because a lot of what we call color is based on perception. Our vision can be thought of as the psychological perception to a physiological response of visible light waves. That is a real mouth full, I know.  Simply, psychological perception refers to what happens in our brains, while physiological response is what happens in our eyes. Speaking about the brain’s process, or the psychological perception of light, Dr. Dennis Eckmeier, explains it best:

“Perception itself is an inner process, that has only little to do with the physical phenomenon. We don’t feel temperature as the movement of molecules, we don’t see light as electromagnetic waves but as colors.”

In other words, our response is not in line with the actual physical phenomena but an abstraction or construct of our minds.

But what about the physiological response, or more specifically, what happens in our eyes?  Eyes are the transducers or the brain’s translator of light, what eventually results in our perception of color and vision. It is estimated that most humans can see around one to two million colors. There are some people, particularly women, who may be able to see up to ten million colors. Let’s not mention other species that can see far more colors, like the zebra finch. Or even others, such as the artic reindeer, who are able to see other parts of the spectrum, like ultraviolet light. How is that possible? How is their eye structure different from ours? How do our eyes translate light, an electromagnetic wave to something that our brains abstract to color and vision? What are the limitations our eyes impose on vision and color perception? We will continue to  explore this part of the puzzle in an upcoming segment.

Written By: Pedro Suarez June 2020

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