Physics
Human Eyes
The human eye is a complex organ that enables vision by capturing and processing light. It consists of various parts, including the cornea, lens, and retina, which work together to focus light onto the retina and convert it into electrical signals that are sent to the brain for interpretation. The eye's ability to perceive color, depth, and detail makes it a remarkable optical instrument.
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10 Key excerpts on "Human Eyes"
eBook - ePub- Michael Peres, Michael R. Peres, Michael Peres, Michael R. Peres(Authors)
- 2013(Publication Date)
- Routledge(Publisher)
Input to the visual system occurs through the eyes. The eyes are sensory organs, and their primary role is to detect light energy, code it into fundamental “bits” of visual information, and transmit it to the rest of the brain for analysis. In the higher levels of the brain, the information bits are eventually recombined to form mental images and our conscious visual perception of the world. The visual system is perhaps the most complex of all sensory systems in humans. This is evidenced by the multiplicity of brain areas devoted to vision, as well as the complexity of cellular organization and function in each of these areas.The human eye is a globe cushioned into place within a bony orbit by extraocular muscles, glands, and fat. The extra-ocular muscles move the eyes in synchrony to allow optimal capture of interesting visual information. Eye movements can be either involuntary, reflex reactions (e.g., nystagmus, convergence during accommodation), or voluntary actions. Light rays enter the eye through a circular, clear, curved cornea, which converges them into the anterior chamber of the eye (Figure 1 ). The number of light rays allowed to pass through the rest of the eye is controlled by the iris — a contractile, pigmented structure that changes in size as a function of light intensity, distance to the object of interest, and even pain or emotional status. The iris controls not only the amount of light entering the posterior chamber of the eye, but also the focal length of this light beam, and thus the overall quality of the image achievable. After the iris, light passes through the lens, a small, onion-like structure whose different layers vary in refractive index which provides final focusing power to precisely position the visual information on the retina. The retina is the most photosensitive component of the human central nervous system and covers most of the inner, back surface of the eye. Its highly regular neuronal structure is normally organized into seven cellular and fiber layers (Figure 2
eBook - ePub- Elizabeth Allen, Sophie Triantaphillidou(Authors)
- 2012(Publication Date)
- Routledge(Publisher)
visual cortex, leads us to perceive images approximately one- to two-tenths of a second after they occur. Understanding the basic functioning of the eye leads to better design and operation of imaging systems, whether it be matching the exit pupil of a pair of binoculars to that of the eye or designing a compression system to be perceptually lossless. The visual systems of animals display incredible variation in complexity, operation and performance. Appreciation of this wide biological diversity, from the compound eye of the bee to the polarized vision of cephalopods, inspires many further advances in numerous areas.THE PHYSICAL STRUCTURE OF THE HUMAN EYE
In brief, the eye is a light-tight sphere, approximately 24 mm in diameter, whose shape is predominantly maintained by the sclera, the white part of the eye, and the vitreous humour (Figure 4.1 ). It has a lens system positioned at the front to focus light on to a photosensitive layer, the retina, which lines the rear of the eye, to form an inverted image. The lens system consists of the cornea and a crystalline lens. It is the function of the retina to convert the incoming light to electrical signals which then travel to the visual cortex and other structures via the optic nerve at the rear of the eyeball. Processing of images, their meaning and the context within which they appear is distributed throughout various parts of the brain and not presently fully understood. The visual cortex, however, is primarily responsible for perception of patterns and shapes encoded by the retina. The coloured portion of the eye, the iris, controls the amount of light entering the visual system by changing the size of the pupil
eBook - PDF- Paul Davidovits(Author)
- 2007(Publication Date)
- Academic Press(Publisher)
Section 15.3 Structure of the Eye 215 is light; the optical components of the eye, which image the light; and the nervous system, which processes and interprets the visual images. 15.2 Nature of Light Experiments performed during the nineteenth century showed conclusively that light exhibits all the properties of wave motion, which were discussed in Chapter 12. At the beginning of this century, however, it was shown that wave concepts alone do not explain completely the properties of light. In some cases, light and other electromagnetic radiation behave as if composed of small packets (quanta) of energy. These packets of energy are called pho-tons . For a given frequency f of the radiation, each photon has a fixed amount of energy E which is E h f (15.1) where h is Planck’s constant, equal to 6 . 63 × 10 − 27 erg-sec. In our discussion of vision, we must be aware of both of these properties of light. The wave properties explain all phenomena associated with the prop-agation of light through bulk matter, and the quantum nature of light must be invoked to understand the e ff ect of light on the photoreceptors in the retina. 15.3 Structure of the Eye A diagram of the human eye is given in Fig. 15.1. The eye is roughly a sphere, approximately 2.4 cm in diameter. All vertebrate eyes are similar in structure but vary in size. Light enters the eye through the cornea, which is a transparent section in the outer cover of the eyeball. The light is focused by the lens system of the eye into an inverted image at the photosensitive retina, which covers the back surface of the eye. Here the light produces nerve impulses that convey information to the brain. The focusing of the light into an image at the retina is produced by the cur-ved surface of the cornea and by the crystalline lens inside the eye. The focus-ing power of the cornea is fixed. The focus of the crystalline lens, however, is alterable, allowing the eye to view objects over a wide range of distances.
eBook - PDFFascinating Problems for Young Physicists
Discovering Everyday Physics Phenomena and Solving Them
- Nenad Vukmirović, Vladimir Veljić(Authors)
- 2022(Publication Date)
- Cambridge University Press(Publisher)
1 Human Problem 1 Human Eye How do we see? What kind of glasses might we need? When can we distinguish between the two eyes of a cat during the night? A schematic view of the structure of the human eye is presented in Figure 1.1. Light rays that refract at the cornea and eye lens end up at the retina, which produces nerve impulses sent to the brain down the optic nerve. In a simplified model of an eye, the cornea and eye lens can be replaced with one converging lens (called simply the lens in the remainder of the text) while the retina can be modeled as a disk of radius R = 1.00 cm, the axis of which coincides with the optical axis of Figure 1.1 Scheme of the structure of the human eye: (1) cornea, (2) eye lens, (3) retina, (4) optic nerve, (5) ciliary muscles, (6) suspensory ligament 1 2 Human the lens, as shown in Figure 1.2. The distance between the retina and the lens is d = 2.40 cm. A human can adjust the focal length of the lens and therefore has the capability of clearly seeing objects at different distances. This process is called eye accommodation and is enabled by ciliary muscles connected to the eye lens by a suspensory ligament. These muscles act to tighten or relax the ligaments and therefore thin down or thicken the lens. Consequently the focal length of the lens changes. Figure 1.2 A simplified model of the human eye (a) A human has regular eyesight if images of all objects from a distance larger than d 0 = 25.0 cm can be formed at the retina. What is the range of the lens’ focal lengths for a human with regular eyesight? (b) The maximal focal length f max of the lens for a nearsighted man is smaller than the upper limit of the range determined in part (a). This man uses glasses with a diopter value of D 1 = −1.00 m −1 to clearly see very distant objects. Deter- mine f max and find the maximal distance of an object that this man can clearly see without using the glasses. For simplicity neglect the distance between the glasses and the lenses.
eBook - PDFThe Human Eye and the Sun
Hot and Cold Light
- S. I. Vavilov(Author)
- 2016(Publication Date)
- Pergamon(Publisher)
THE HUMAN EYE* The eye owes its existence to light. From among the indifferent auxiliary organs of animals, light calls forth an organ which would become akin to it; and thus is the eye born in light, for light, in order that the inner light meet the outer. Goethe Thy rays create the eyes of all thy creatures. Lesser Hymn to Aten No living creature has a more faithful or potent protector than the eye. To see is to tell friend from foe and know the lie of the land. The other senses do the same, but comparatively poorly. The tactile senses and feeling of warmth provide information about the external world only by direct contact; hearing and smell, which are not so restricted, provide insufficient information about distance, direction and shape. Such expressions as obviously and we live, we see imply that if something can be seen it is authentic. A modern physi-cist can convince others of the reality of atoms because he can point to their paths; persons who formerly denied the existence of atoms constantly argued that no one could see them. This is the meaning of the saying by Anaxagoras Vision is the appearance of the invisible; the invisible world becomes a reality through vision. The functions of an ideal eye as a physical organ are quite clear. * Only a few slight changes have been made in this chapter in accordance with the terminology of physiological optics. Sometimes the author departs from strict scientific terminology. This may have been done intentionally owing to the complexity of this terminology which is unsuited to a work of popular science. We have therefore mainly confined ourselves to the correction of slips of the pen and misprints.— [Russian editor]. 66 THE HUMAN EYE 67 CHARLES DARWIN (1809-1882). 68 THE HUMAN EYE AND THE SUN Light is received from surrounding objects and the eye indicates the direction of the rays, their energy, spectral composition and polariza-tion. Each point of an object should be perceived individually.
eBook - PDF- Graham Saxby(Author)
- 2016(Publication Date)
- CRC Press(Publisher)
33 3 Chapter Visual Perception The Eye and Evolution The eye is such a complicated organ that creationists have used it for many years as evidence against the theory of evolution, saying, “What use to an animal would an only half-developed eye be?” In his book The Blind Watchmaker , Richard Dawkins demolishes such arguments, showing that at every stage of the building of the mam-malian eye, from a simple light-sensitive patch to its present complexity, the organ became steadily more useful to its owner, and moreover that every stage in that development is still extant, in creatures ranging from the earthworm to the eagle. Optics of the Eye Older textbooks on light and optics always contained a section on the eye, and it was often the weakest section in the book. The authors usually treated the eye as a kind of camera, and got the optics wrong too, showing all the refraction taking place at the lens—in fact, most of the refraction occurs at the cornea. Discussion of the visual process has now largely moved to textbooks on the psychology of per-ception, where the eye is treated as having a resemblance to a computer input. Both these views nevertheless contain a measure of truth, and both may be helpful in an appreciation of what really goes on in the process of visual perception. Certainly, the eye is an optical device that focuses an image on a light-sensitive surface, so that is probably the best place to begin. You can demonstrate the optical workings of the eye quite easily, though unless you have some rather old-fashioned equipment such as a large goldfish bowl and the condenser lens from an old photographic enlarger (or a large and powerful mag-nifying glass) it will have to be a thought experiment. If you fill the goldfish bowl with water you will have made a lens—a very thick one, but a lens nonetheless.
eBook - PDF- Peter Robert Boyce(Author)
- 2014(Publication Date)
- CRC Press(Publisher)
43 2 Visual System 2.1 INTRODUCTION Light is necessary for the human visual system to operate� With light, we can see; without light, we cannot� This chapter describes the structural, operational and per-ceptual characteristics of the human visual system� 2.2 STRUCTURE OF THE VISUAL SYSTEM The first thing to appreciate about the visual system is that it is not the eye alone� Rather, it is the eye and brain working together� The visual system is often likened to a camera but this analogy is misleading� The only parts of the visual system that resemble a camera are the optical components of the eye� Once the optical compo-nents have formed an image of the world on the retina, the camera analogy fails� All the rest of the visual system is an image-processing system that extracts specific aspects of the retinal image for interpretation by the brain� Despite this fact, the obvi-ous starting point for a consideration of the visual system is the eye� 2.2.1 V ISUAL F IELD Humans have two eyes, mounted frontally� This is the classic position of the eyes for a predator, the two frontally mounted eyes providing considerable overlap between the two visual fields and hence the good depth perception necessary to stalk and cap-ture prey� Animals that are prey typically have their eyes mounted laterally so that their visual fields cover a larger portion of the world around them� Figure 2�1 shows the approximate extent of the visual field of the two eyes in humans and the overlap between them� Given the limited field of view imposed by the frontal mounting of the two eyes, it is necessary for the two eyes to be able to move� There are two ways this can be done: by moving the head and by moving the eyes in the head� Most animals do both, although some creatures show a bias to one extreme or the other� Owls, for example, have very limited ability to move their eyes but can move their
eBook - PDF- José Alonso, José A. Gómez-Pedrero, Juan A. Quiroga(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
On the contrary, if the head is still, spectacle lenses are also still when the eye scans the available field of view behind them, rotating to aim and fixate the point of interest. We are quite used to this arrangement, as we have been using spectacle lenses for centuries, but think about it: There are very few optical systems that work like that. When we talk about field of view, magnification, and aberrations, we have to modify to a greater or lesser extent the typical definitions and methods used for optical systems whose parts “move together.” The spectacle lens-eye system becomes even more different from standard optical systems when we study both eyes at the same time, as the spectacle lenses may cause different pointing errors for each eye, altering the balance of the binocular vision. In this chapter we study the spectacle lens-eye system, first with the static eye, and then with the moving eye, accurately defining and quantifying the concepts we have just outlined here. 5.2 Basic Optical Physiology 5.2.1 The Eye The eye is an organ whose function is to provide neural information encoding planar images of the world around us. The combination of the information from the two eyes provides further information that the brain uses to construct a three-dimensional image of our environment. The image-forming system of the eye is composed of two lenses and a variable shutter, the cornea, the crystalline lens, and the iris, and the detection system is mainly formed by the retina, a tissue with photoreceptor cells and neural connections that codify the information imaged onto it. The physiology of the eye and its optical function are fascinating subjects, but we do not need a very detailed description of the eye to study most of the properties of the lens-eye system. As we will see, some fundamental principles of the theory of spectacle lenses can be established with a very simple and conceptual model of the eye, without even considering any of its actual structures.
eBook - PDFFrom Humans To Computers: Cognition Through Visual Perception
Cognition Through Visual Perception
- Victor V Alexandrov, N D Gorsky(Authors)
- 1991(Publication Date)
- World Scientific(Publisher)
Despite the fact that peripheral vision does not give us sharp images, it is important for the orientation or detection of moving objects. If you shield with your hands the lateral areas of your visual field or put on special glasses to limit it (for example, a diver's mask), you will realize at once how impoverished your visual world becomes. All this substantiates the fact that the eye's structure is set up to transfer to the brain the necessary information at the moment it is received in the necessary volume from a specific place. 50 FROM HUMANS TO COMPUTERS Human and animal eyes clearly differ in this respect from machine eyes, TV-cameras, scanners, etc., which convey information with the same resolution both at the center and at the periphery into the computer memory in succession. Devices of image data input in a computer seem to resemble, in this characteristic, an aspect of an insect's eye where each photosensitive element is a small tube catching the light that has come from a specific direction. Before the image passes from the retina to the brain via the optic nerve, it undergoes an important change. However, it is more accurate to speak of the decomposition of an input image into several others since the same information that was present in the original picture is also present in the aggregate of images. The first image received by the brain is the roughest copy of the retinal image and looks as if it were taken by an out-of-focus camera. It has only low spatial frequencies. The image that follows adds more precise details from the observed situation; it conveys even finer details and adds higher spatial frequencies, etc. (see Fig. 23). According to physiologists, several images then follow into the brain. This process is repeated every time we shift our glance, that is, when a new retinal image appears which differs from the preceding one [11, 43]. Two basic hypotheses exist which account for this phenomenon.
eBook - PDF- David M. Miller(Author)
- 2000(Publication Date)
- Academic Press(Publisher)
(From Prause JO, Jensen OA: Scanning electron micrograph of frozen-cracked, dry cracked, and enzyme digested retinal tissue of a monkey and man. Graefe'sArch Ophthalmol. 202:261-270, 1980, with permission.) 36 THE WISDOM OF THE EYE However, there is a complicating factor that must be kept in mind. The fixating eye, as opposed to a camera on a tripod, is in constant motion. These small move- ments (called either tremors, drifts, or microsaccades) range in amplitude from seconds to minutes of angular arc. Such movements tend to smear rather than enhance visual resolution. One can only presume that the visual system takes quick, short samples of the retinal image during those smearing movements. 16-18 Knowing the size of the optimal diffraction pattern of a focused point of light allows us to predict the optimal visual acuity for a human eye. 3 Such an eye should have a visual acuity of 20/20.* In practical terms, a person with 20/20 vision could see a space between two people (i.e., recognizing that there are two people instead of one), which just subtends an angle of 1 minute. To sum up, the unique essence of the vertebrate eye is that the structure of the transparent opti- cal components, the rhodopsin molecule, and the size of the foveal cones are all tuned to interact optimally with wavelengths of visible light. 19,2~ It is earth's unique atmosphere and unique relationship to the sun that has allowed primarily visible light, a tiny band from the enormous electromagnetic spectrum of the sun, to rain down upon us at safe energy levels.* Our eyes are a product of an evolutionary process that has tuned to these unique wavelengths and at these lev- els of intensity. 21,22 2. OPTICAL ABERRATIONS Herman von Helmholtz, the famous nineteenth-century German physiologist, in volume I of his Treatise on Physiologic Optics, had written that the optical aberrations of the human eye are of a kind that is not permissible in well con- structed instruments.
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