Physics
Physics of the Eye
The physics of the eye involves the study of how light is refracted and focused by the various structures of the eye to form an image on the retina. The cornea and lens are the primary refractive elements, while the iris controls the amount of light entering the eye. The retina contains photoreceptor cells that convert light into electrical signals that are sent to the brain for processing.
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3 Key excerpts on "Physics of the Eye"
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 - PDF- H. E. Hobbs(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
Chapter 1 THE OPTICAL MECHANISM OF THE EYE Vision is the most accurate of the senses and upon it we depend most for knowledge of our environment and for the co-ordination of our physical skills. Its importance to the higher animals and man is readily apparent when we contrast the primitive photo-sensitive cells of lowly forms of life to the perfection of the complex visual organs of primates. That the increasing independence and control over their environment which the latter enjoy owes much to the evolution of more refined visual mechanisms is clearly indicated by animal experiments which demonstrate the increasing dominance of visual over other sensations in behaviour as the evolutionary scale is ascended. This accuracy of sensation implies a similar precision in the mechanism by which it is mediated and this we find in the anatomical detail of the eye. It is apparent, too, in the physiological processes which subserve visual sensation and those which are responsible for maintaining normal ocular functions. Both of these present adaptations of general physiological processes to suit the needs of visual and ocular function: the former, notably in the rod and cone mechanism of the retina and the latter in the special provisions for maintenance of the ocular tension and the optical system of the eye. Consideration of some of the details of retinal sensation will be reserved for a later chapter; but a brief account of the optical mechanism of the eye is needed as a prelude to discussion of ocular disease. Its anomalies enter into the symptoma-tology and diagnosis of most ocular disorders and some knowledge of it is essential to their interpretation. The analogy with a camera is apt, for the eye possesses in the lens and cornea a refracting apparatus comparable with that of the camera lens.
eBook - PDF- Tom Cornsweet(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
Ill THE PHYSICS OF LIGHT A DEFINITION OF THE information that we have about the visual world, and our percep-SEEING tions of objects and visual events in the world, depend only indirectly upon the state of that world. They depend directly upon the nature of the images formed on the backs of our eyeballs, and these images are different in many important ways from the world itself. It is certainly true that the state of the observer plays a fundamental role in determin-ing the things he sees, and that role will be discussed at length in this book, but the relationship between our perceptions and the world it-self depends first of all upon the nature of our retinal images. If infor-mation about some aspect of an object is not contained in the retinal image of it, the information cannot be recovered or regenerated by later parts of the system. Put more precisely, if two physically different objects produce iden-tical retinal images, it is impossible for an observer visually to tell them 27 28 The Physics of Light apart. He might discover that they are different by feeling them, but then his discrimination is based on something not relevant to a discus-sion of visual perception. He might say, after feeling the difference between the objects, Now I see that they are different/' Even if he in-sists that he is using the word see in its usual sense, it is clear that he is not referring to the properties of his visual perception. After having felt the two objects, he may insist that they really look different to him, and, of course, such a claim cannot be refuted. That claim does not mean that the two objects transmit different visual information, and what it does mean is unclear. It is possible and very useful to establish a definition of seeing that is not subject to such ambiguity.
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