Chapter 26: Vision PDF

Chapter 26 : Vision Mrs. Pooja Brahmaiahchari Introduction Physics is called an enabling science—a science that enables development and advancement in other areas. It is through optics and imaging that physics enables advancement in major areas of biosciences. This chap...

Chapter 26 : Vision Mrs. Pooja Brahmaiahchari Introduction Physics is called an enabling science—a science that enables development and advancement in other areas. It is through optics and imaging that physics enables advancement in major areas of biosciences. This chapter illustrates the enabling nature of physics through an understanding of how a human eye is able to see and how we are able to use optical instruments to see beyond what is possible with the naked eye. Physics of the Eye The eye is remarkable in how it forms images and in the richness of detail and color it can detect. However, our eyes commonly need some correction, to reach what is called “normal” vision, but should be called ideal rather than normal. Figure shows the basic anatomy of the eye. The cornea and lens form a system that acts as a single thin lens.  For clear vision, a real image must be projected onto the light-sensitive retina, which lies at a fixed distance from the lens. The lens of the eye adjusts its power to produce an image on the retina for objects at different distances. The center of the image falls on the fovea, which has the greatest density of light receptors and the greatest acuity (sharpness) in the visual field. The variable opening (or pupil) of the eye along with chemical adaptation allows the eye to detect light intensities from the lowest observable to times greater (without damage). Processing of visual nerve impulses begins with interconnections in the retina and continues in the brain. The optic nerve conveys signals received by the eye to the brain.  Refractive indices are crucial to image formation using lenses. The biggest change in the refractive index, and bending of rays, occurs at the cornea rather than the lens. The cornea provides about two-thirds of the power of the eye, owing to the fact that speed of light changes considerably while traveling from air into cornea. Cornea provides most of the power The rays bend according to the refractive indices. Figure shows image formation by cornea and lens.  The lens provides the remaining power needed to produce an image on the retina. The image formed is much like the one produced by a single convex lens. Images formed in the eye are inverted but the brain inverts them once more to make them seem upright. The image must fall precisely on the retina to produce clear vision — that is, the image distance must equal the lens-to- retina distance. Because the lens-to-retina distance does not change, the image distance must be the same for objects at all distances.  The eye manages this by varying the power (and focal length) of the lens to accommodate for objects at various distances. The process of adjusting the eye’s focal length is called accommodation. Done by cilliary muscles changing the lens the cornea is constant A person with normal (ideal) vision can see objects clearly at distances ranging from 25 cm to essentially infinity. Since light rays from a nearby object can diverge and still enter the eye, the lens must be more converging (more powerful) for close vision than for distant vision.  To be more converging, the lens is made thicker by the action of the ciliary muscle surrounding it. The eye is most relaxed when viewing distant objects, one reason that microscopes and telescopes are designed to produce distant images. Vision of very distant objects is called totally relaxed, while close vision is termed accommodated, with the closest vision being fully accommodated. Relaxed and accommodated vision for distant and close objects.  Rewriting the thin lens equation by replacing 1 𝑃= 𝑓 Thus, We understand that di must equal the lens-to-retina distance to obtain clear vision, and that normal vision is possible for objects at distances do = 25cm to infinity. Vision Correction Common vision defects are easy to understand, and some are simple to correct. Nearsightedness, or myopia, is the inability to see distant objects clearly while close objects are clear. The eye over converges the nearly parallel rays from a distant object, and the rays cross in front of the retina. More divergent rays from a close object are converged on the retina for a clear image. The distance to the farthest object that can be seen clearly is called the far point of the eye (normally infinity).  Farsightedness, or hyperopia, is the inability to see close objects clearly while distant objects may be clear. A farsighted eye does not converge sufficient rays from a close object to make the rays meet on the retina. Less diverging rays from a distant object can be converged for a clear image. The distance to the closest object that can be seen clearly is called the near point of the eye.(normally 25cm) Near sightedness, you can see far but not close = myopia lens too strong or eye too long Far sightedness you can see close but not far = hyper opia = lens too weak eye too short Correct myopia with diverging lens correct hyper opia with converging lens  Since the nearsighted eye over converges light rays, the correction for nearsightedness is to place a diverging spectacle lens in front of the eye. This reduces the power of an eye that is too powerful. To determine the spectacle power needed for correction, you must know the person’s far point—that is, you must know the greatest distance at which the person can see clearly.  Then the image produced by a spectacle lens must be at this distance or closer for the nearsighted Person to be able to see it clearly. It is worth noting that wearing glasses does not change the eye in any way. The eyeglass lens is simply used to create an image of the object at a distance where the nearsighted person can see it clearly. Whereas someone not wearing glasses can see clearly objects that fall between their near point and their far point, someone wearing glasses can see images that fall between their near point and their far point.  Since the farsighted eye under converges light rays, the correction for farsightedness is to place a converging spectacle lens in front of the eye. This increases the power of an eye that is too weak.  To determine the spectacle power needed for correction, you must know the person’s near point—that is, you must know the smallest distance at which the person can see clearly. Then the image produced by a spectacle lens must be at this distance or farther for the farsighted person to be able to see it clearly. Another common vision defect is astigmatism, an unevenness or asymmetry in the focus of the eye. For example, rays passing through a vertical region of the eye may focus closer than rays passing through a horizontal region, resulting in the image appearing elongated.  This is mostly due to irregularities in the shape of the cornea but can also be due to lens irregularities or unevenness in the retina. Because of these irregularities, different parts of the lens system produce images at different locations.  Astigmatism can be at least partially corrected with a spectacle having the opposite irregularity of the eye. If an eyeglass prescription has a cylindrical correction, it is there to correct astigmatism. you can correct astigmatism with a cylindrical correction in the eyeware. myopia/hyperopia is with spheical corrections The normal corrections for short- or farsightedness are spherical corrections, uniform along all axes. Contact lenses have advantages over glasses beyond their cosmetic aspects. One problem with glasses is that as the eye moves, it is not at a fixed distance from the spectacle lens. Contacts rest on and move with the eye, eliminating this problem. Because contacts cover a significant portion of the cornea, they provide superior peripheral vision compared with eyeglasses. Contacts also correct some corneal astigmatism caused by surface irregularities.  Laser vision correction has progressed rapidly in the last few years. It is the latest and by far the most successful in a series of procedures that correct vision by reshaping the cornea. As noted at the beginning of this section, the cornea accounts for about and is mostly invariable. the lens has a variable power which is changed via capillary muscle two-thirds of the power of the eye. action called accomidation Thus, small adjustments of its curvature have the same effect as putting a lens in front of the eye. To a reasonable approximation, the power of multiple lenses placed close together equals the sum of their powers.  Today, the most commonly used laser vision correction procedure is Laser in situ Keratomileusis (LASIK). Lasers are used because they not only produce well-focused intense light, but they also emit very pure wavelength electromagnetic radiation that can be controlled more accurately than mixed wavelength light. The 193 nm wavelength UV commonly used is extremely and strongly absorbed by corneal tissue, allowing precise evaporation of very thin layers. Nearsightedness, farsightedness, and astigmatism can be corrected with an accuracy that produces normal distant vision in more than 90% of the patients, in many cases right away. can treat astigmatism with lasik, contact lenses, or cylindrical eye ware The corneal flap is replaced; healing takes place rapidly and is nearly painless. Color and Color Vision Objects and lights abound with thousands of hues that stimulate our eyes, brains, and emotions. Two basic questions are addressed in this brief treatment—what does color mean in scientific terms, and how do we, as humans, perceive it? The gift of vision is made richer by the existence of color. Simple Theory of Color Vision When our eyes receive pure-wavelength light, we tend to see only a few colors. These are the rainbow of colors produced when white light is dispersed according to different wavelengths. There are thousands of other hues that we can perceive. One simple theory of color vision implies that all these hues are our eye’s response to different combinations of wavelengths. This is true to an extent, but we find that color perception is even subtler than our eye’s response for various wavelengths of light.  The two major types of light-sensing cells (photoreceptors) in the retina are rods and cones. Rods are more sensitive than cones by a factor of about 1000 and are solely responsible for peripheral vision as well as vision in very dark environments. Rods see gray and are used in dark environments and periphreal vision. Cones are for color They are also important for motion detection. There are about 120 million rods in the human retina. Rods do not yield color information. You may notice that you lose color vision when it is very dark, but you retain the ability to discern grey scales. Know the difference b/w rods and cons and know where they are most dense  Cones are most concentrated in the fovea, the central region of the retina. There are no rods here. The fovea is at the center of the macula, a 5 mm diameter region responsible for our central vision. The cones work best in bright light and are responsible for high resolution vision. There are about 6 million cones in the human retina. There are three types of cones, and each type is sensitive to different ranges of wavelengths, as illustrated in Figure. 120 million rods 6 million cones  A simplified theory of color vision is that there are three primary colors corresponding to the three types of cones. The combinations will create thousands of different hues. Color television uses RGB. The broad range of hues a viewer sees is produced by various combinations of these three colors. Another set that works is yellow, green, and blue. A further indication of the need for a more complex theory of color vision is that various different combinations can produce the same hue. The true color of an object is defined by its absorptive or reflective Characteristics.  The figure in previous slide shows white light falling on three different objects, one pure blue, one pure red, and one black, as well as pure red light falling on a white object. Other hues are created by more complex absorption characteristics. An object can appear a different color under non-white illumination. For example, a pure blue object illuminated with pure red light will appear black, because it absorbs all the red light falling on it. But, the true color of the object is blue, which is independent of illumination. Ture color is what color it will be under white light Color Constancy and a Modified Theory of Color Vision The eye-brain color-sensing system can, by comparing various objects in its view, perceive the true color of an object under varying lighting conditions—an ability that is called color constancy. We can sense that a white tablecloth, for example, is white whether it is illuminated by sunlight, fluorescent light, or candlelight. The wavelengths entering the eye are quite different in each case but our color vision can detect the true color by comparing the tablecloth with its surroundings.  There are nerve connections among the light receptors on the retina, and there are far fewer nerve connections to the brain than there are rods and cones. This means that there is signal processing in the eye before information is sent to the brain. For example, the eye makes comparisons between adjacent light receptors and is very sensitive to edges as seen in Figure Rather than responding simply to the light entering the eye, which is uniform in the various rectangles in this figure, the eye responds to the edges and senses false darkness variations. essentially this says that the eye actively processes visual information, enhancing edges and contrasts. This preprocessing makes vision more efficient by highlighting important features, like edges, so edges are more extreme.  One theory that takes various factors into account was advanced by Edwin Land (1909 – 1991), the creative founder of the Polaroid Corporation. Land proposed, based partly on his many elegant experiments, that the three types of cones are organized into systems called retinexes. Each retinex forms an image that is compared with the others, and the eye-brain system thus can compare a candle-illuminated white table cloth with its generally reddish surroundings and determine that it is actually white. This retinex theory of color vision is an example of modified theories of color vision that attempt to account for its subtleties. 1. What is the power of the eye when viewing an object 50.0 cm away assuming lens-to-retina distance of 2.00cm(a typical value)? 2. People who do very detailed work close up, such as jewelers, often can see objects clearly at much closer distance than the normal 25 cm. (a) What is the power of the eyes of a woman who can see an object clearly at a distance of only 8.00 cm? (b) What is the size of an image of a 1.00 mm object, such as lettering inside a ring, held at this distance? (c) What would the size of the image be if the object were held at the normal 25.0 cm distance? 3.The power of a physician’s eyes is 53.0 D while examining a patient. How far from her eyes is the feature being examined? 4. What is the far point of a person whose eyes have a relaxed power of 50.5 D? THANK YOU