Why can one only see light from red to purple?
Broadly speaking, we can see light because objects emit electromagnetic radiation, and after this radiation sends a signal to our brain, we see different colors of light.
But why can we only see red to purple, that is, 400 to 700 nanometers of light? And others with such a large wavelength range that we all turn a blind eye?
<h1 class="pgc-h-arrow-right" data-track="5" > see that light is a chemical effect of electromagnetic radiation</h1>
Electromagnetic radiation has physical and chemical effects, and we first need to figure out which effect we see light at play.
The physical effects of electromagnetic radiation include heating objects and generating pressure, both of which are very weak. For example, the pressure of light can make a light sail spacecraft, although the force is very weak, but the continuous acceleration can make it reach one-tenth of the speed of light, so as to reach the nearest star system within 40 years, and even if the fastest spacecraft is used now, it will take more than 100,000 years.
Obviously, electromagnetic radiation does not allow us to see objects through physical effects, but should cause chemical changes in molecules, and transmit the signals of these changes to the nervous system to let us perceive light, which is the basic working principle of vision.
<h1 class="pgc-h-arrow-right" data-track="47" > where does the photochemical action of vision occur? </h1>
This action occurs in the retina of the human eye. The retina contains photoreceptor cells, including rod cells, cone cells, and retinal ganglion cells, and this is where magic happens.
There are about 6 million cone cells and 125 million rod cells in the human retina.
Cone cells work in a brighter environment and can distinguish colors.
Rod cells work in a relatively dark environment, have lower resolution, and are unable to distinguish colors.
In simple terms, cone-like cone cells distinguish the color of light, and rod cells like rods perceive the intensity of light.
Humans and animals such as higher primates have three different kinds of cone cells, while other mammals lack red cone cells, so their ability to distinguish colors is poor. For example, cats can only distinguish blue-green tones, and dogs can only distinguish yellow-blue tones.
Some people lack red, blue, or green cone cells, so they develop different color blindness, and they see the world more or less like what cats and dogs see.
You may have developed a sense of superiority, because the higher the animal, the more cones there are, and the more abundant the world can be seen.
But the birds will come and punch you in the face right away: we have four types of cone cells that can see ultraviolet rays;
Pigeons and butterflies: cut! We have 5 kinds;
Pippi shrimp: I have 12 species, can I still see polarized light, am I proud?
It's a bit far.
<h1 class="pgc-h-arrow-right" How is the vision > data-track="49"? </h1>
Rod cells and cone cells convert the perceived light into neural signals, which are processed by other nerve cells on the retina and transformed into the action potential of the retinal ganglion cells, which are transmitted to the brain through the optic nerve tube, causing visual impulses to be transmitted to the visual center of the cerebral cortex to produce vision.
The key here is the generation of the action potential, how does light cause the action potential to be generated?
The part of each cone cell exposed to light is filled with opsin containing photosensitive molecules, and 11-cis-retinal linked together, which together make up the optic pigment.
When light with the right energy falls on the molecule, 11-cis-retinal absorbs photons and isomerizes into total trans retinoal, activates rhodopsin, initiates nerve impulses to the brain, and produces a series of downstream reactions to form vision.
At this point you may have seen the key to the question "Why can we only see red to purple light", only light of the right energy can achieve cis-trans isomerization of 11-cis-retinal on the retina, so what is the wavelength of these lights?
Cis-transgenic isomerism is essentially an electron transition that has higher energies than rotational and vibrational transitions, whereas the transition energy of a typical organic molecule is only a few ev magnitudes. So as long as we know that there are electron transitions in the visual chemistry, we can immediately say that the wavelength of the visible spectrum should be around a few hundred nanometers.
<h1 class="pgc-h-arrow-right" data-track="50" > so why is it exactly 400 to 700 nanometers? </h1>
This is the specific absorption spectrum of 11-cis-retinoal, which is influenced by opsin, so the question becomes, how does our retina get opsin that absorbs only this wavelength range? Or rather, how do we get the photoreceptor molecules in this wavelength range?
This is because, to detect light of a given wavelength, these wavelengths of light need to be enough, most of the ultraviolet light in the sunlight is absorbed by the atmosphere, and most of the energy that falls into our eyes is in the visible to infrared region, so we have reason to believe that the eye will evolve to see the wavelength band from visible light to near infrared.
But this is not all, the basic evolution of the animal eye takes place in the water under the sun, and 99% of the vitreous body inside our eyeball is water, which means that light must pass through the water to see for us, and water has only a narrow transparent window in the visible wavelength range, so the eye naturally evolved into a photosensitive system with a suitable visible range, and evolution is to find and use this bug of water.
So the ultimate answer to "why we can only see visible light from red to purple" is that it is determined by the electron transition of the cis-inverse isomer of 11-cis retinal, and whether various lights can pass through the atmosphere and through water. Only this band of light is suitable for the absorption of 11-cis retinal in these three types of cone cells, so our eyes evolved to see visible light from red to purple. Taken together, this is determined by evolution, sunlight, atmosphere, and water. Only this wavelength of light can penetrate the atmosphere and water, triggering the reaction of the opsin in our eyes.
The cone cells of other animals are different from us, and they are differently sensitive to different wavelengths of light, so they are also different from our "visible light", especially the four-color and five-color vision, especially the 12 colors of pippi shrimp, I am really curious, what does the visible light in their eyes look like?
The main content of this article comes from the views of Raziman T.V, a postdoctoral researcher in nanophotonics at Eindhoven University of Science and Technology, referring to the explanation of relevant terms in Wikipedia, Raziman also said that he only studies nanophotonics, not biology and chemistry, and asked subject experts to correct it. I am not this professional, just feel curious, according to their own understanding to write this article, there may be inaccurate or even wrong expression, just for reference.