Everything in the world seems to us to be colorful and colorful, but this is not necessarily the real color of the object, this is just the light shining on the object, reflected into the human eye, the brain then some processing of these reflected lights, which allows you to see the colorful light.
But there may be one color that doesn't exist, and that's purple.
To know when humans learned about the color of light, you have to start 300 years ago.
At that time, the British physicist Newton saw a chance that when light passed through special glass, it decomposed into different colored light bands. Through subsequent experiments, Newton finally found that the use of triangular prism can be used to decompose light into colors visible to the naked eye, confirming that light is not a single transparent, but is composed of light waves of different colors, and the light waves visible to the naked eye have seven colors of red, orange, yellow, green, blue, and purple.
Now, through the efforts of scientists, we have found that in addition to the light waves visible in seven colors, there are also two light waves that are invisible to the naked eye, infrared and ultraviolet rays.
But now there is a saying that purple does not exist, and to understand the truth of this sentence, we must first understand how humans can distinguish colors.
How exactly do humans distinguish colors?
When we studied junior high school physics, we once learned the physical concept of electromagnetic waves, which is a wave with energy with specific amplitude, frequency and phase, and light is also a kind of electromagnetic wave, and electromagnetic waves also have an attribute called color, which is because color is not an objective property of electromagnetic waves, but a subjective judgment of the brain for the light reflected into the eye.
There are two kinds of cells responsible for light sensitivity in the human retina, which are cone cells and rod cells.
Rod cells are distributed in a large number of more than 100 million in the retina, and their main function is to be responsible for responding to what is seen in front of them in a low-light environment. The number of cone cells is also nearly 6 million, they are mainly responsible for the perception of the surrounding environment in a strong light environment, which has the perception of color, and these more than 6 million cone cells can divide their perception of different colors into 3 categories, these three longer wavelength colors are red, green and blue three colors.
When different wavelengths of light enter the eye, it will cause different cone cells to be excited, and the cells in the excitement can combine with each other, such as yellow light into the human eye, it will stimulate both red and green cone cells, these stimuli are transmitted to the brain, and the brain remembers the corresponding light seen by the corresponding stimulus.
To put it simply, the reason why the colors we see today are colorful is because in the process of long-term evolution, we have grown so many cone cells that can distinguish between red, green and blue, but such cone cells only exist in the eyes of humans and some primates.
The world of cats and dogs is only black and white
Mammals like dogs and cats have only black and white cone cells, so in the eyes of these animals, the world is actually a black and white, which feels like we used to watch programs from black and white televisions. Birds have four kinds of color-aware cone cells, so birds perceive many more colors than humans.
Why do you say that purple does not exist?
First of all, humans cannot distinguish whether the light they see is monochromatic or mixed by looking at it.
The cone cells on the retina are only the most sensitive to the three colors of red, green and blue, but they can also feel the light of other colors. Taking yellow light as an example, if you see yellow monochromatic light, then the cone cells will respond to the brain to see yellow light, but if you see a mixture of red and green light, then your cone cells will also respond to the brain to see yellow light.
So the same kind of color that people react to in the brain may be different light waves, which is the "isochromatic spectrum" in physics.
The yellow you see is not necessarily a single yellow, it may be a mixture of red and green. But the violet monochromatic light is a different matter, because the violet light is at the edge of the spectrum. The wavelength of purple monochromatic light is very narrow and long, and it cannot be accepted by the cone cells to the brain, that is, it is almost invisible to the naked eye.
To put it simply, when you see a purple object in your life, what you see is not really "purple", but the blue and red in the sunlight are reflected into your eyes through the surface of the "purple" at the same time, thus forming a mixed color.
So, from this point of view, purple doesn't exist.
But from another point of view, the colors we see today are all because we have been taught from an early age to say what color is what color, and human cognition of various colors is only the response of the human brain to different light waves. If the "number of color blind people" in the world accounts for the majority, our definition of color will be different.
But it is also possible that a long time ago, when human beings have not evolved cone cells that can distinguish red, green and blue colors, human cognition of color may only stay at the level of black and white, with the change of human living environment, human beings need to distinguish colors, so they began to slowly evolve color-resolving cone cells.