What the human eye can perceive are only a small part of the electromagnetic radiation that surrounds it: this small part comescalled ” visible spectrum “.
Visible spectrum
The visible spectrum is that part of the electromagnetic spectrum that falls between red and violet, including all the colors perceptible by the human eye which therefore give life to the phenomenon of light. The wavelength of light visible in the air ranges from 390 to 700 nm while the frequencies of the visible spectrum vary between 770 and 430 THz. Wavelength and frequency are inversely proportional. As you can see from the image above:
- radiation with shorter wavelength (and therefore higher frequency) are ultraviolet, X-rays, gamma rays and cosmic rays;
- radiation with longer length (and lower frequency) are infrared, microwave and radio waves;
- the visible spectrum is between ultraviolet and infrared.
All these radiations have the same nature, in fact they are all composed of photons. The study of objects based on the spectrum of visible light that they emit is called spectroscopy and one of the most important applications of spectroscopy is in the astronomical field: it is fundamental for the analysis of the physical properties of celestial bodies even very far from the Earth.
The seven colors of Newton’s prism
Newton observed that a ray of white light, if passed through a crystal prism, was split into a rainbow of colors. This is due to the fact that energies with different wavelengths are refracted differently when they pass through “means” of different densities and therefore with different wave propagations. Newton described the rainbow with 7 colors, of which we now know the wavelength and frequency:
| Color | Frequency | Wavelength |
| Violet | 668-789 THz (Terahertz) | 380–450 nm (nanometers) |
| Indigo | 631-668 THz | 450–475 nm |
| Blue | 606-631 THz | 476-495 nm |
| Green | 526-606 THz | 495–570 nm |
| Yellow | 508-526 THz | 570–590 nm |
| Orange | 484-508 THz | 590–620 nm |
| Red | 400-484 THz | 620–750 nm |
White light is the sum of all these colors: it appears white until it is broken down into the 7 colors.
How do we distinguish colors?
The light penetrates our eye and reaches the retina. In it there are the ” photoreceptors “, that is, a particular type of highly specialized neurons that have the function of “translating” the light that reaches the bottom of the eye from the outside and converting it into bioelectrical signals, sent to the visual cortex of the brain through the optic nerve. There are two types of photoreceptors, rods and cones: the latter show us the colors. While the rods are concentrated in the peripheral area of the retina and are more sensitive to the vision of moving objects, in addition to being used above all for vision in the dark (“scotopic” vision) , on the contrary the cones are concentrated in the central area of the retina(called “fovea”). There are at least three different types of cones, respectively for red, green and blue: each one processes the signals belonging to a given wavelength. The brain, collecting the impulses from each cone, processes a whole image, with each single color.
Do the colors exist?
No, the colors you see around you do not exist, at least not in the way you have always believed. In reality the colors are nothing but electromagnetic radiation with different wavelengths that are perceived by the central photoreceptors of our eye (the cones of the retina) , transformed into electrical signals, transported by the optic nerve and sent to the cortex visual cortex (which has located in the occipital lobe of the brain) which is responsible for translating and interpreting them to supply them to our consciousness as we see them.
The objects are not colored
At this point, after asserting that the colors do not exist in an objective condition but are electromagnetic waves of different wavelength translated subjectively, it follows that the objects, and everything that surrounds us, are not colored, or at least are not in the way we believe. To understand this concept, we must understand why we see different objects with a given color and not with another.
As we have seen before, white light appears white to us because it is the sum of all visible wavelengths. But what happens when a white light (the sun) hits an object? In this case:
- a light projected onto an object can be fully reflected, and our eye will perceive the white hue;
- it may not be reflected at all, and then we will perceive black;
- it can be partly reflected and partly absorbed: the reflected light will be the one that will make us perceive a given color rather than another.
Let’s take a practical example: light hits an object and:
- the wavelength belonging to the violet is absorbed by the surface of the object;
- the wavelength belonging to the indigo is absorbed by the surface of the object;
- the wavelength belonging to the blue is absorbed by the surface of the object;
- the wavelength belonging to the green is absorbed by the surface of the object;
- the wavelength belonging to the yellow is absorbed by the surface of the object;
- the wavelength belonging to the orange is absorbed by the surface of the object;
- the wavelength belonging to the RED is REFLECTEDfrom the surface of the object, reaching our eye.
In this case the object will appear to us red because “it is the color that the object will have rejected and rejected ” making it reach us. Look at the objects around you: nothing is really “colored”: you only have around objects capable of absorbing some electromagnetic waves and reflecting others. If the brain of a hypothetical alien, for example, were organized in such a way as to interpret the wavelength that we see red, as “blue”, a strawberry would appear blue, not red. It ‘important to stress that g objects that reflect them all wavelengths appear white , while those that absorb all wavelengths appear blacks: this is the reason why a room with white walls, with the same sunlight, appears brighter than one with gray or black walls, since the surface that we see as white we see it like this because it reflects all wavelengths , while what we see black does not reflect any.
Why do warm clothes tend to be warmer than white ones?
The same explanation just given, is useful for understanding why dark or black clothes tend to be warmer than light and white ones: the waves that make up light are energy and white is the color that repels more radiation , while black is the one that absorbs them most .
Color is subjective: it is not the same for everyone
Color, therefore, is a psychophysical and subjective quantity , which depends largely on the final interpretation of the cerebral cortex of each of us: our translation / vision of a certain frequency is not exactly the same perceived by another person. If there are no problems related to color blindness, it is easy between two people to agree on the ” strong ” colors : a blue, a red or a yellow, for example, will be well distinct and cataloged as “blue, red or yellow” also by two different people, but what if we are faced with intermediate colors? For example, some will consider aquamarine blue while others will say it is green . Same thing can happen with lemon yellow(some will say yellow, others green), or with indigo (some will say blue, others violet) or with dark colors like the grenade (dark red for some, brown for others). However, it should be emphasized that often the lack of recognition is given by our ignorance of the names of the colors: ask the average man to distinguish between pink, fuchsia, amethyst, violet, wisteria, cotton candy, iris, lilac, purple, burgundy violet, mauve, mauve opera, plum, lavender, shocking pink, coral, carmine red, crimson , complexion, terra cotta, burgundy, orchid … he will probably call them all “pink” or “purple”! However, in many cases, the more the color is intermediate between two primary or secondary colors (the easiest to distinguish), the more the differences between one visual cortex and the other will be felt and each will interpret a given color differently not because he does not know the specific name of the color, but precisely because his brain interprets it differently or because he is not particularly trained to distinguish that given color gradation.
