Look around the room you are in - what do you see? Walls, windows, colorful objects - all this seems so familiar and taken for granted. It's easy to forget that we see the world around us only thanks to photons - light particles reflected from objects and striking the retina.
There are approximately 126 million light-sensitive cells in the retina of each of our eyes. The brain deciphers the information received from these cells about the direction and energy of photons falling on them and turns it into a variety of shapes, colors and intensity of illumination of surrounding objects.
Human vision has its limits. Thus, we are neither able to see radio waves emitted by electronic devices, nor to see the smallest bacteria with the naked eye.
Thanks to advances in physics and biology, the limits of natural vision can be determined. "Every object we see has a certain 'threshold' below which we stop recognizing them," says Michael Landy, a professor of psychology and neurobiology at New York University.
Let's first consider this threshold in terms of our ability to distinguish colors - perhaps the very first ability that comes to mind in relation to vision.
Our ability to distinguish, for example, the color violet from magenta is related to the wavelength of the photons hitting the retina. There are two types of light-sensitive cells in the retina - rods and cones. Cones are responsible for color perception (so-called day vision), and rods allow us to see shades of gray in low light - for example, at night (night vision).
The human eye has three types of cones and a corresponding number of types of opsins, each of which is particularly sensitive to photons with a specific range of light wavelengths.
S-type cones are sensitive to the violet-blue, short-wavelength portion of the visible spectrum; M-type cones are responsible for green-yellow (medium wavelength), and L-type cones are responsible for yellow-red (long wavelength).
All of these waves, as well as their combinations, allow us to see the full range of colors of the rainbow. "All human visible light sources, with the exception of some artificial ones (such as a refractive prism or laser), emit a mixture of wavelengths of different wavelengths," says Landy.
Of all the photons existing in nature, our cones are capable of detecting only those characterized by wavelengths in a very narrow range (usually from 380 to 720 nanometers) - this is called the visible radiation spectrum. Below this range are the infrared and radio spectra - the wavelengths of the latter's low-energy photons vary from millimeters to several kilometers.
On the other side of the visible wavelength range is the ultraviolet spectrum, followed by X-rays, and then the gamma ray spectrum with photons whose wavelengths are less than trillionths of a meter.
Although most of us have limited vision in the visible spectrum, people with aphakia—the absence of the lens in the eye (as a result of cataract surgery or, less commonly, a birth defect)—are able to see ultraviolet wavelengths.
In a healthy eye, the lens blocks ultraviolet waves, but in its absence, a person is able to perceive waves up to about 300 nanometers in length as blue-white color.
A 2014 study notes that, in some sense, we can all see infrared photons. If two such photons hit the same retinal cell almost simultaneously, their energy can add up, turning invisible waves of, say, 1000 nanometers into a visible wavelength of 500 nanometers (most of us perceive waves of this length as a cool green color). .
How many colors do we see?
There are three types of cones in a healthy human eye, each of which is capable of distinguishing about 100 different shades of color. For this reason, most researchers estimate the number of colors we can distinguish at about a million. However, color perception is very subjective and individual.
Jameson knows what he's talking about. She studies the vision of tetrachromats - people with truly superhuman abilities to distinguish colors. Tetrachromacy is rare and occurs in most cases in women. As a result of a genetic mutation, they have an additional, fourth type of cone, which allows them, according to rough estimates, to see up to 100 million colors. (Color-blind people, or dichromats, have only two types of cones - they can distinguish no more than 10,000 colors.)
How many photons do we need to see a light source?
In general, cones require much more light to function optimally than rods. For this reason, in low light, our ability to distinguish colors decreases, and rods are taken to work, providing black and white vision.
Under ideal laboratory conditions, in areas of the retina where rods are largely absent, cones can be activated by just a few photons. However, the wands do an even better job of registering even the dimmest light.
As experiments first conducted in the 1940s show, one quantum of light is enough for our eyes to see it. "A person can see a single photon," says Brian Wandell, a professor of psychology and electrical engineering at Stanford University. "It just doesn't make sense for the retina to be more sensitive."
In 1941, researchers from Columbia University conducted an experiment - they took subjects into a dark room and gave their eyes a certain time to adapt. The rods require several minutes to achieve full sensitivity; This is why when we turn off the lights in a room, we lose the ability to see anything for a while.
A flashing blue-green light was then directed at the subjects' faces. With a probability higher than ordinary chance, the experiment participants recorded a flash of light when only 54 photons hit the retina.
Not all photons reaching the retina are detected by light-sensitive cells. Taking this into account, scientists have come to the conclusion that just five photons activating five different rods in the retina are enough for a person to see a flash.
Smallest and most distant visible objects
The following fact may surprise you: our ability to see an object does not depend at all on its physical size or distance, but on whether at least a few photons emitted by it will hit our retina.
“The only thing the eye needs to see something is a certain amount of light emitted or reflected by the object,” says Landy. “It all comes down to the number of photons that reach the retina. No matter how small the light source, even if it exists for a fraction of a second, we can still see it if it emits enough photons."
Psychology textbooks often contain the statement that on a cloudless, dark night, a candle flame can be seen from a distance of up to 48 km. In reality, our retina is constantly bombarded by photons, so that a single quantum of light emitted from a great distance is simply lost against their background.
To get an idea of how far we can see, let's look at the night sky, dotted with stars. The size of the stars is enormous; many of those we see with the naked eye reach millions of kilometers in diameter.
However, even the stars closest to us are located at a distance of over 38 trillion kilometers from Earth, so their apparent sizes are so small that our eyes are not able to distinguish them.
On the other hand, we still observe stars in the form of bright point sources of light, since the photons emitted by them overcome the gigantic distances separating us and land on our retina.
All individual visible stars in the night sky are located in our galaxy, the Milky Way. The most distant object from us that a person can see with the naked eye is located outside the Milky Way and is itself a star cluster - this is the Andromeda Nebula, located at a distance of 2.5 million light years, or 37 quintillion km, from the Sun. (Some people claim that on particularly dark nights, their keen vision allows them to see the Triangulum Galaxy, located about 3 million light years away, but leave this claim to their conscience.)
The Andromeda nebula contains one trillion stars. Due to the great distance, all these luminaries merge for us into a barely visible speck of light. Moreover, the size of the Andromeda Nebula is colossal. Even at such a gigantic distance, its angular size is six times the diameter of the full Moon. However, so few photons from this galaxy reach us that it is barely visible in the night sky.
Visual acuity limit
Why are we unable to see individual stars in the Andromeda Nebula? The fact is that resolution, or visual acuity, has its limitations. (Visual acuity refers to the ability to distinguish elements such as a point or line as separate objects that do not blend into adjacent objects or the background.)
In fact, visual acuity can be described in the same way as the resolution of a computer monitor - in the minimum size of pixels that we are still able to distinguish as individual points.
Limitations in visual acuity depend on several factors, such as the distance between the individual cones and rods of the retina. An equally important role is played by the optical characteristics of the eyeball itself, due to which not every photon hits the light-sensitive cell.
In theory, research shows that our visual acuity is limited to the ability to distinguish about 120 pixels per angular degree (a unit of angular measurement).
A practical illustration of the limits of human visual acuity can be an object located at arm's length, the size of a fingernail, with 60 horizontal and 60 vertical lines of alternate white and black colors applied to it, forming a semblance of a chessboard. “Apparently, this is the smallest pattern that the human eye can still discern,” says Landy.
The tables used by ophthalmologists to test visual acuity are based on this principle. The most famous table in Russia, Sivtsev, consists of rows of black capital letters on a white background, the font size of which becomes smaller with each row.
A person’s visual acuity is determined by the size of the font at which he ceases to clearly see the outlines of letters and begins to confuse them.
It is the limit of visual acuity that explains the fact that we are not able to see with the naked eye a biological cell, the dimensions of which are only a few micrometers.
But there is no need to grieve over this. The ability to distinguish a million colors, capture single photons and see galaxies several quintillion kilometers away is quite a good result, considering that our vision is provided by a pair of jelly-like balls in the eye sockets, connected to a 1.5 kg porous mass in the skull.
Anatomical questions have always been of some interest. After all, they affect each of us directly. Almost everyone has at least once been interested in what the eye is made of. After all, it is the most sensitive sense organ. It is through the eyes, visually, that we receive about 90% of information! Only 9% - using hearing. And 1% - through other organs. Well, the structure of the eye is a really interesting topic, so it’s worth considering it in as much detail as possible.
Shells
It's worth starting with terminology. The human eye is a paired sensory organ that perceives electromagnetic radiation in the light wavelength range.
It consists of membranes surrounding the inner core of the organ. Which, in turn, includes aqueous humor, the lens and But more on that a little later.
When talking about what the eye consists of, special attention should be paid to its membranes. There are three of them. The first one is external. Dense, fibrous, the external muscles of the eyeball are attached to it. This shell performs a protective function. It is also what determines the shape of the eye. Consists of the cornea and sclera.
The middle layer is also called the choroid. It is responsible for metabolic processes and provides nutrition to the eyes. Consists of the iris and choroid. In the very center is the pupil.
And the inner shell is often called reticular. The receptor part of the eye, in which light is perceived and information is transmitted to the central nervous system. In general, this can be said briefly. But, since each component of this organ is extremely important, it is necessary to pay attention to each of them separately. This will help you better understand what the eye is made of.
Cornea
So, this is the most convex part of the eyeball, making up its outer shell, as well as a light-refracting transparent medium. The cornea looks like a convex-concave lens.
Its main component is the connective tissue stroma. In front, the cornea is covered with stratified epithelium. However, scientific words are not very easy to understand, so it is better to explain the topic popularly. The main properties of the cornea are sphericity, specularity, transparency, increased sensitivity and the absence of blood vessels.
All of the above determines the “purpose” of this part of the organ. Essentially, the cornea of the eye is the same as the lens of a digital camera. Even in structure they are similar, because both one and the other are lenses that collect and focus light rays in the required direction. This is the function of the refractive medium.
Talking about what the eye is made of, one cannot help but touch upon the negative influences that it has to cope with. The cornea, for example, is most susceptible to external irritants. To be more precise - exposure to dust, changes in lighting, wind, dirt. As soon as something in the external environment changes, the eyelids close (blinking), photophobia occurs, and tears begin to flow. So, one might say, damage protection is activated.
Protection
A few words should be said about tears. This is a natural biological fluid. It is produced by the lacrimal gland. A characteristic feature is slight opalescence. This is an optical phenomenon due to which light begins to scatter more intensely, which affects the quality of vision and the perception of the surrounding image. 99% consists of water. One percent is inorganic substances, which are magnesium carbonate, sodium chloride, and also calcium phosphate.
Tears have bactericidal properties. They are the ones who wash the eyeball. And its surface thus remains protected from the effects of dust particles, foreign bodies and wind.
Another component of the eye is the eyelashes. On the upper eyelid their number is approximately 150-250. On the bottom - 50-150. And the main function of eyelashes is the same as that of tears - protective. They prevent dirt, sand, dust, and in the case of animals, even small insects from getting on the surface of the eye.
Iris
So, above we talked about what the outer one consists of. Now we can talk about the middle one. Naturally, we will talk about the iris. It is a thin and movable diaphragm. It is located behind the cornea and between the chambers of the eye - right in front of the lens. Interestingly, it practically does not transmit light.
The iris consists of pigments that determine its color and circular muscles (due to them the pupil narrows). By the way, this part of the eye also includes layers. There are only two of them - mesodermal and ectodermal. The first is responsible for the color of the eye, as it contains melanin. The second layer contains pigment cells with fuscin.
If a person has blue eyes, it means that his ectodermal layer is loose and contains little melanin. This hue is the result of light scattering in the stroma. By the way, the lower its density, the more saturated the color.
People with a mutation in the HERC2 gene have blue eyes. They produce minimal melanin. The density of the stroma in this case is higher than in the previous case.
Green eyes have the most melanin. By the way, the red hair gene plays an important role in the formation of this shade. Pure green color is very rare. But if there is even a “hint” of this shade, then they are called as such.
But still, the most melanin is found in brown eyes. They absorb all the light. Both high and low frequencies. And reflected light gives a brown tint. By the way, initially, many thousands of years ago, all people were brown-eyed.
There is also a black color. Eyes of this shade contain so much melanin that all light entering them is completely absorbed. And, by the way, often this “composition” causes a grayish tint to the eyeball.
Choroid
It also needs to be noted with attention, telling what the human eye consists of. It is located directly under the sclera (tunica albuginea). Its main property is accommodation. That is, the ability to adapt to dynamically changing external conditions. In this case, this concerns changes in refractive power. A simple visual example of accommodation: if we need to read what is written on the package in small print, we can look closely and distinguish the words. Need to see something in the distance? We can do this too. This ability lies in our ability to clearly perceive objects located at a particular distance.
Naturally, when talking about what the human eye is made of, we cannot forget about the pupil. This is also a rather “dynamic” part of it. The diameter of the pupil is not fixed, but constantly narrowing and expanding. This happens because the flow of light that enters the eye is regulated. The pupil, changing in size, “cuts off” too bright sun rays on a particularly clear day, and lets in the maximum amount of them in foggy weather or at night.
Should know
It is worth paying attention to such an amazing component of the eye as the pupil. This is perhaps the most unusual thing about the topic under discussion. Why? If only because the answer to the question of what the pupil of the eye consists of is nothing. In fact, it is! After all, the pupil is a hole in the tissues of the eyeball. But next to it there are muscles that allow it to perform the above-mentioned function. That is, to regulate the flow of light.
A unique muscle is the sphincter. It surrounds the outermost part of the iris. The sphincter consists of fibers woven together. There is also a dilator - the muscle that is responsible for dilating the pupil. It consists of epithelial cells.
It is worth noting another interesting fact. The middle one consists of several elements, but the pupil is the most fragile. If you believe medical statistics, then 20% of the population has a pathology called anisocoria. It is a condition in which the size of the pupils is different. They may also be deformed. But not all of these 20% have a pronounced symptom. Most people don't even know they have anisocoria. Many people become aware of it only after visiting a doctor, which people decide to do, feeling foggy, pain, etc. But some experience diplopia - “double pupil”.
Retina
This is the part that needs special attention when talking about what the human eye is made of. The retina is a thin membrane that is closely adjacent to the vitreous body. Which, in turn, is what fills 2/3 of the eyeball. The vitreous body gives the eye its correct and constant shape. It also refracts light entering the retina.
As already mentioned, the eye consists of three membranes. But this is just the basis. After all, the retina of the eye consists of another 10 layers! And to be more precise, its visual part. There is also a “blind” one, in which there are no photoreceptors. This part is divided into ciliary and iridescent. But it's worth going back to ten layers. The first five are: pigmentary, photosensory and three external (membranous, granular and plexus). The remaining layers have similar names. These are three internal ones (also granular, plexus and membranous), as well as two more, one of which consists of nerve fibers, and the other of ganglion cells.
But what exactly is responsible for visual acuity? The parts that make up the eye are interesting, but I want to know the most important thing. So, the central fovea of the retina is responsible for visual acuity. It is also called the “yellow spot”. It has an oval shape and is located opposite the pupil.
Photoreceptors
An interesting sensory organ is our eye. What it consists of - photo provided above. But nothing has been said yet about photoreceptors. And, to be more precise, about those on the retina. But this is also an important component.
It is they who contribute to the transformation of light stimulation into information that enters the central nervous system along the fibers of the optic nerve.
Cones are highly sensitive to light. And all because of the iodopsin content in them. This is a pigment that provides color vision. There is also rhodopsin, but this is the complete opposite of iodopsin. Since this pigment is responsible for twilight vision.
A person with 100% good vision has approximately 6-7 million cones. Interestingly, they are less sensitive to light (it is about 100 times worse) than sticks. However, they perceive fast movements better. By the way, there are more sticks - about 120 million. They contain the notorious rhodopsin.
It is the rods that provide a person’s visual ability in the dark. Cones are not active at all at night - since they need at least a minimal flow of photons (radiation) to work.
Muscles
They also need to be talked about when discussing the parts that make up the eye. The muscles are what keep the apples straight in the eye socket. They all originate from the notorious dense connective tissue ring. The core muscles are called obliques because they attach to the eyeball at an angle.
It is better to explain the topic in simple language. Every movement of the eyeball depends on how exactly the muscles are fixed. We can look to the left without turning our heads. This is due to the fact that the rectus motor muscles coincide in their location with the horizontal plane of our eyeball. By the way, they, in combination with oblique ones, provide circular turns. Which includes every gymnastics for the eyes. Why? Because when performing this exercise, all eye muscles are involved. And everyone knows: for this or that training (no matter what it is connected with) to give a good effect, every component of the body needs to work.
But that, of course, is not all. There are also longitudinal muscles that begin to work the moment we look into the distance. Often people whose activities involve painstaking or computer work experience pain in their eyes. And it becomes easier if you massage them, close your eyes, and rotate them. What causes pain? Due to muscle strain. Some of them work constantly, while others rest. That is, for the same reason why hands may hurt if a person was carrying some kind of heavy thing.
Lens
When talking about what parts the eye consists of, one cannot help but touch upon this “element.” The lens, which was already mentioned above, is a transparent body. This is a biological lens, in simple terms. And, accordingly, the most important component of the light-refracting ocular apparatus. By the way, the lens even looks like a lens - it is biconvex, round and elastic.
It has a very fragile structure. On the outside, the lens is covered with a thin capsule that protects it from external factors. Its thickness is only 0.008 mm.
The lens is susceptible to various diseases. The hardest thing is cataracts. With this disease (usually associated with age), a person sees the world in a cloudy, blurry way. And in such cases, it is necessary to replace the lens with a new, artificial one. Fortunately, it is located in such a place in our eye that it can be changed without affecting the other parts.
In general, as you can see, the structure of our main sense organ is very complex. The eye is small, but it includes a simply huge number of elements (remember, at least 120 million rods). And we could talk for a long time about its components, but we managed to list the most basic ones.
Eyes- an organ that allows a person to live a full life, admire the beauty of the surrounding nature and exist comfortably in society. People understand how important the eyes are, but they rarely think about why they blink, why they cannot sneeze with their eyes closed, and other interesting facts related to this unique organ.
10 interesting facts about the human eye
The eyes are the conductor of information about the world around us.
In addition to vision, a person has organs of touch and smell, but it is the eyes that conduct 80% of the information that tells about what is happening around. The ability of the eyes to capture images is very important, since it is visual images that retain memory longer. When meeting a specific person or object again, the organ of vision activates memories and gives rise to thought.
Scientists compare the eyes to a camera, the quality of which is many times higher than that of ultra-modern technology. Bright and content-rich pictures allow a person to easily navigate the world around them.
The cornea of the eye is the only tissue in the body that does not receive blood.
The cornea of the eye receives oxygen directly from the air
The uniqueness of such an organ as the eyes lies in the fact that no blood flows into its cornea. The presence of capillaries would negatively affect the quality of the image captured by the eye, so oxygen, without which not a single organ of the human body can function effectively, receives oxygen directly from the air.
Highly sensitive sensors transmitting signals to the brain
The eye is a miniature computer
Ophthalmologists (vision specialists) compare the eyes to a miniature computer that captures information and instantly transmits it to the brain. Scientists have calculated that the “RAM” of the organ of vision can process about 36 thousand bits of information within an hour; programmers know how large this volume is. Meanwhile, the weight of miniature laptop computers is only 27 grams.
What does having close eyes give a person?
A person sees only what is happening directly in front of him
The location of the eyes in animals, insects and humans is different, this is explained not only by physiological processes, but also by the nature of life and the gray habitat of a living creature. The close placement of the eyes provides depth of image and three-dimensionality of objects.
Humans are more advanced creatures, therefore they have high-quality vision, especially when compared with marine life and animals. True, such an arrangement has its drawback - a person sees only what is happening directly in front of him, the view is significantly reduced. In many animals, an example is a horse, the eyes are located on the sides of the head, this structure allows you to “capture” more space and react in time to approaching danger.
Do all the inhabitants of the earth have eyes?
Approximately 95 percent of living creatures on our planet have vision
Approximately 95 percent of living creatures on our planet have an organ of vision, but most of them have a different eye structure. In the inhabitants of the deep sea, the organ of vision consists of light-sensitive cells that are not capable of distinguishing color and shape; all that such vision is capable of is perceiving light and its absence.
Some animals determine the volume and texture of objects, but at the same time see them exclusively in black and white. A characteristic feature of insects is the ability to see many pictures at the same time, but they do not recognize colors. Only human eyes have the ability to accurately convey the colors of surrounding objects.
Is it true that the human eye is the most perfect?
There is a myth that a person can only recognize seven colors, but scientists are ready to debunk it. According to experts, the human visual organ is capable of perceiving over 10 million colors; no living creature has such a feature. However, there are other criteria that are not characteristic of the human eye, for example, some insects are able to recognize infrared rays and ultraviolet signals, and the eyes of flies have the ability to detect movement very quickly. The human eye can only be called the most perfect in the field of color recognition.
Who on the planet has the most island eyesight?
Veronica Seider - the girl with the sharpest eyesight on the planet
The name of a student from Germany, Veronica Seider, is listed in the Guinness Book of Records; the girl has the sharpest eyesight on the planet. Veronica recognizes a person's face at a distance of 1 kilometer 600 meters, this figure is approximately 20 times higher than the norm.
Why does a person blink?
If a person did not blink, his eyeball would quickly dry out and quality vision would be out of the question. Blinking causes the eye to become covered with tear fluid. It takes about 12 minutes per day for a person to blink – once every 10 seconds, during which time the eyelids close over 27 thousand times.
A person begins to blink for the first time at six months.
Why do people start sneezing in bright light?
The human eyes and nasal cavity are connected by nerve endings, so often when exposed to bright light we begin to sneeze. By the way, no one can sneeze with their eyes open; this phenomenon is also associated with the reaction of nerve endings to external stimuli of calm.
Restoring vision with the help of sea creatures
Scientists have found similarities in the structure of the human eye and sea creatures, in this case we are talking about sharks. Modern medicine methods make it possible to restore human vision by transplanting a shark cornea. Similar operations are very successfully practiced in China.
Sincerely,
The human eye is often cited as an example of amazing natural engineering - but judging by the fact that this is one of 40 variants of devices that appeared in the process of evolution in different organisms, we should moderate our anthropocentrism and recognize that the structure of the human eye is not something then perfect.
It’s best to start the story about the eye with a photon. A quantum of electromagnetic radiation slowly flies directly into the eye of an unsuspecting passerby, who squints from an unexpected glare from someone’s watch.
The first part of the eye's optical system is the cornea. It changes the direction of light. This is possible due to such a property of light as refraction, which is also responsible for the rainbow. The speed of light is constant in vacuum - 300,000,000 m/s. But when moving from one medium to another (in this case, from air to the eye), light changes its speed and direction of movement. Air has a refractive index of 1.000293, and the cornea has a refractive index of 1.376. This means that the light beam in the cornea slows down by a factor of 1.376 and is deflected closer to the center of the eye.
A favorite way to split partisans is to shine a bright lamp in their face. This hurts for two reasons. Bright light is powerful electromagnetic radiation: trillions of photons attack the retina, and its nerve endings are forced to transmit a crazy number of signals to the brain. From overstrain, nerves, like wires, burn out. This forces the iris muscles to contract as hard as they can, desperately trying to close the pupil and protect the retina.
And flies up to the pupil. Everything is simple with it - it is a hole in the iris. Using the circular and radial muscles, the iris can constrict and dilate the pupil accordingly, regulating the amount of light entering the eye, like the diaphragm in a camera. The diameter of the human pupil can vary from 1 to 8 mm depending on the lighting.
Having flown through the pupil, the photon hits the lens - the second lens responsible for its trajectory. The lens refracts light weaker than the cornea, but it is mobile. The lens hangs on ciliary muscles, which change its curvature, thereby allowing us to focus on objects at different distances from us.
Visual impairment is associated with focus. The most common are myopia and farsightedness. In both cases, the image is not focused on the retina, as it should, but in front of it (myopia) or behind it (farsightedness). This is due to the eye, which changes shape from round to oval, and then the retina moves away from the lens or approaches it.
After the lens, the photon flies through the vitreous body (transparent jelly - 2/3 of the volume of the entire eye, 99% is water) straight to the retina. Here photons are detected and arrival messages are sent along nerves to the brain.
The retina is lined with photoreceptor cells: when there is no light, they produce special substances - neurotransmitters, but as soon as a photon hits them, the photoreceptor cells stop producing them - and this is a signal to the brain. There are two types of these cells: rods, which are more sensitive to light, and cones, which are better at detecting movement. We have about one hundred million rods and another 6-7 million cones, in total more than one hundred million light-sensitive elements - that’s more than 100 megapixels, which no “Hassel” could ever dream of.
The blind spot is a breakthrough point where there are no light-sensitive cells at all. It is quite large - 1-2 mm in diameter. Fortunately, we have binocular vision and a brain that combines two pictures with spots into one normal one.
At the moment of signal transmission, a problem with logic arises in the human eye. The underwater resident octopus, which does not particularly need vision, is much more consistent in this sense. In octopuses, a photon first hits the layer of cones and rods on the retina, immediately behind which a layer of neurons waits and transmits the signal to the brain. In humans, light first breaks through layers of neurons - and only then hits the photoreceptors. Because of this, there is a first spot in the eye - a blind spot.
The second spot is yellow, this is the central area of the retina directly opposite the pupil, just above the optic nerve. The eye sees best in this place: the concentration of light-sensitive cells here is greatly increased, so our vision in the center of the visual field is much sharper than the peripheral one.
The image on the retina is inverted. The brain knows how to correctly interpret the picture, and restores the original image from the inverted one. Children see everything upside down for the first couple of days while their brain installs its Photoshop. If we put on glasses that reverse the image (this was first done back in 1896), then after a couple of days our brain will learn to interpret such an inverted picture correctly.
The eye is sometimes called a living camera, since the optical system of the eye that produces the image is similar to a camera lens, but it is much more complex.
The human eye (and many animals) has an almost spherical shape (Fig. 163), it is protected by a dense membrane called the sclera. The anterior part of the sclera - the cornea 1 - is transparent. Behind the cornea (cornea) is the iris 2, which can be a different color in different people. Between the cornea and the iris there is a watery fluid.
Rice. 163. Human eye
There is a hole in the iris - pupil 3, the diameter of which, depending on the lighting, can vary from approximately 2 to 8 mm. It changes because the iris is able to move apart. Behind the pupil there is a transparent body, similar in shape to a converging lens - this is the lens 4, it is surrounded by muscles 5 that attach it to the sclera.
Behind the lens is the vitreous body 6. It is transparent and fills the rest of the eye. The back of the sclera - the fundus of the eye - is covered with a retina 7 (retina). The retina consists of the finest fibers that, like villi, cover the fundus of the eye. They are branched ends of the optic nerve that are sensitive to light.
How is an image produced and perceived by the eye?
The light falling into the eye is refracted on the front surface of the eye, in the cornea, lens and vitreous body (i.e. in the optical system of the eye), due to which a real, reduced, inverted image of the objects in question is formed on the retina (Fig. 164).
Rice. 164. Formation of an image on the retina
Light falling on the endings of the optic nerve, which make up the retina, irritates these endings. Irritations are transmitted along nerve fibers to the brain, and a person receives a visual impression and sees objects. The process of vision is corrected by the brain, so we perceive the object as straight.
How is a clear image created on the retina when we move our gaze from a distant object to a close one or vice versa?
As a result of its evolution, the optical system of the eye has developed a remarkable property that provides images on the retina at different positions of the object. What kind of property is this?
The curvature of the lens, and therefore its optical power, can change. When we look at distant objects, the curvature of the lens is relatively small because the muscles surrounding it are relaxed. When looking at nearby objects, the muscles compress the lens, its curvature, and therefore the optical power, increases.
The ability of the eye to adapt to vision both at close and far distances is called accommodation of the eye (translated from Latin as “adaptation”). The limit of accommodation occurs when the object is at a distance of 12 cm from the eye. The best vision distance (this is the distance at which the details of an object can be viewed without strain) for a normal eye is 25 cm. This should be taken into account when writing, reading, sewing, etc.
Firstly, we see more space, i.e. the field of view increases. Secondly, vision with two eyes allows us to distinguish which object is closer and which is further from us. The fact is that the retinas of the right and left eyes produce images that are different from each other; we seem to see objects on the left and on the right. The closer the object, the more noticeable this difference is; it creates the impression of a difference in distances, although, of course, the images merge in our minds into one. Thanks to vision with two eyes, we see an object in volume, not flat.
Questions
- How is an image produced and perceived by the eye?
- How is a clear image created on the retina when looking from a distant object to a close one?
- What advantage does seeing with both eyes give?
Exercise
- Using additional literature and the Internet, draw a diagram of the image structure in the camera.
- Prepare a presentation about modern cameras and their use in everyday life and technology.
This is interesting...
Myopia and farsightedness. Glasses
Thanks to accommodation, the image of the objects in question is obtained precisely on the retina of the eye. This is done if the eye is normal.
An eye is called normal if, in a relaxed state, it collects parallel rays at a point lying on the retina (Fig. 165, a). The two most common eye defects are myopia and farsightedness.
Myopic is an eye whose focus, when the eye muscle is at rest, lies inside the eye (Fig. 165, b). Myopia can be caused by a greater distance between the retina and the lens compared to a normal eye. If an object is located at a distance of 25 cm from a myopic eye, then the image of the object will not appear on the retina (as in a normal eye), but closer to the lens, in front of the retina. In order for the image to appear on the retina, you need to bring the object closer to the eye. Therefore, in a myopic eye, the distance of best vision is less than 25 cm.
Rice. 165. Visual impairment
Farsighted is an eye whose focus, when the eye muscle is at rest, lies behind the retina (Fig. 165, f).
Farsightedness may be caused by the retina being closer to the lens than in a normal eye. The image of an object is obtained behind the retina of such an eye. If an object is removed from the eye, the image falls on the retina, hence the name of this deficiency - farsightedness.
A difference in the location of the retina, even within one millimeter, can already lead to noticeable myopia or farsightedness.
People who had normal vision in youth become farsighted in old age. This is explained by the fact that the muscles that compress the lens weaken and the ability of accommodation decreases. This also happens due to the compaction of the lens, which loses its ability to compress. Therefore, the image is obtained behind the retina.
Myopia and farsightedness are corrected by using lenses. The invention of glasses was a great boon for people with visual impairments.
What lenses should be used to correct these vision defects?
In a nearsighted eye, the image is obtained inside the eye in front of the retina. In order for it to move to the retina, the optical power of the refractive system of the eye must be reduced. For this, a diverging lens is used (Fig. 166, a).
Rice. 166. Correction of visual impairments using lenses
The optical power of the farsighted eye system, on the contrary, must be strengthened in order for the image to fall on the retina. For this, a converging lens is used (Fig. 166.6).
So, to correct myopia, glasses with concave, diverging lenses are used. If, for example, a person wears glasses whose optical power is -0.5 diopters (or -2 diopters, -3.5 diopters), then he is myopic.
Glasses for farsighted eyes use convex, converging lenses. Such glasses can have, for example, an optical power of +0.5 diopters, +3 diopters, +4.25 diopters.
