Chapter 20 Human Vision

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1 Chapter 20 GOALS When you have mastered the contents of this chapter, you will be able to achieve the following goals: Characterize the physical parameters that are significant in human vision. Visual Defects Explain the causes and corrections for the visual defects of myopia, hypermetropia, presbyopia and astigmatism. Characteristics of Vision Define the following terms: visual acuity scotopic vision photopic vision accommodation dichromat Corrective Lenses Solve problems involving visual defects and their corrections using lenses. PREREQUISITES Before beginning this chapter you should have achieved the goals of Chapter 18, Optical Elements, and Chapter 19, Wave Properties of Light. 164

2 Chapter 20 OVERVIEW - In the simplest sense, the human eye is a single lens, refractive system with a retina as a screen. But in practice, the operation of the eye is amazing. In this chapter, you will learn about many of the intricate and delicate operations performed by eye parts. In addition, optically defective eyes will be classified and corrected using additional converging and/or diverging eye glasses. SUGGESTED STUDY PROCEDURE - As you begin to study this chapter, become familiar with the following Chapter Goals:, Visual Defects, Characteristics of Vision and Corrective Lenses. Please note that the terms listed under the goal of Characteristics of Vision are discussed under the first section of this Study Guide, Definitions. Next, read text sections Answers to the questions you encounter in your reading can be found in the second section of this Study Guide chapter. Next, turn to the end of the chapter and read the Chapter Summary and complete Summary Exercises Check your answers carefully with those provided. Now do Algorithmic Problems 1 and 2, and completeexercises and Problems 1, 2, 5, and 12. For additional work with the concepts presented in this chapter, see the Examples section of this Study Guide chapter. Now you should be prepared to test your understanding of the important parts of this chapter. Attempt the Practice Test provided at the end of this chapter. If you have difficulties with any of the individual parts of the test, see that part of the text or refer to this chapter. This study procedure is outlined below Chapter Goals Suggested Summary Algorithmic Exercises Text Readings Exercises Problems & Problems ,20.2, Visual Defects 20.3,20.4 2, 3 Characteristics 20.1, 20.2,20.5, 4-9 of Vision 20.6, 20.7 Corrective , 5, 12 Lenses 165

3 DEFINITIONS VISUAL ACUITY OF THE EYE - The minimum separation between two equidistant point objects that can be resolved as separate objects. The resolving power of the eye allows a driver to detect that an oncoming light source is actually two headlights of an approaching automobile. SCOPTIC VISION - Vision of the dark-adapted eye. The rod cells of the retina are more concentrated on the periphery around the fovea and are more sensitive to low light intensity. PHOTOPIC VISION - Vision of bright-light adapted eye. The cone cells of the retina are more concentrated in the center fovea and are most sensitive to high light intensity. ACCOMMODATION - The ability of the eye to focus on objects between the near point and far point of the eye. The maximum change in power of the eye as it focuses on distant objects and then on near objects. The accommodation is measured in diopters and is produced by the change in power of the eye lens. DICHROMAT - Color vision resulting from only two of the three pigments required for normal color vision. ANSWERS TO QUESTIONS FOUND IN THE TEXT SECTION 20.5 The Receptive Eye The relative sensitivity of the human eye is shown as normalized to the most sensitive wavelength in figure You will notice that the eye is sensitive to light that is within about 40% of the most sensitive wavelength. Let us compare that to the frequency response of the human ear. Ear Eye Lowest frequency 2x10 1 Hz 4.3x10 14 Hz Most sensitive 2x10 3 Hz 5.5x10 14 Hz Highest frequency 2x10 4 Hz 7.5x10 14 Hz Compared to Figure 20.4 the relative sensitivity of the human eye is a broad peak with a maximum at 2000 Hz. The ear stretches over three orders of magnitude in its frequency response! Notice also that monochromates are able to function in our society with little loss in information input. A person who could hear only one frequency would appear to have great difficulty with human communication. 166

4 SECTION 20.6 The Perceptive Eye If you use the Raleigh criterion for acuity of the human eye, assuming an iris size of 3 mm and light of 550 nm wavelength, the angular separation between just resolvable sources of light is about 2x10-4 radians. This is a larger angle than the angle between the cones. For some conditions human vision seems to be Rayleigh limited which changes only a small amount for visible light. EXAMPLES CORRECTIVE LENSES 1. An elderly person can see distinctly with unaided vision only objects between 64 cm and 540 cm from her eyes. What kind of eyeglasses does she require? For what distances does she not see objects clearly when she is wearing her spectacles? What data are given? Persons near point = 64 cm. Persons far point = 540 cm. What data are implied? The use of simple lens equations are assumed to be adequate. There is an implication that the person needs bifocal eye glasses since both her near point and far points are not the normal values of 25 cm. and infinity. What physics principles are involved? This problem is just an additional application of the physics principles discussed in Section What equations are to be used? 1/p + 1/q = 1/f (18.13) Solutions In order to compensate for the person's inability to see near objects she needs to have a lens that will put the virtual, upright image of an object at 25cm at her near point of 64cm. 1/25 + 1/-64 = 1/f; so f = +41 cm. To compensate for her farsightedness she needs a lens of +2.4 diopters. To compensate for her nearsightedness, she needs a lens that will make objects at infinity appear to be at 540 cm. This clearly demands a diverging lens because an object a long distance from a converging lens has an image at its focal point. So the image will be a virtual image 1/ - 1/q = -(1/f) 1/ - 1/5.4m = 1/f; f = -5.4m She needs a lens of diopters. In general such eyeglasses are composed of a lower portion of positive power and an upper portion of negative power. For objects farther away from her than 38 cm, the positive power portion of her lens will give her a virtual image outside of her far point of 540 cm. So for all objects between 38 cm and 64 cm she will not see them clearly when she is wearing her spectacles. 167

5 PRACTICE TEST 2. A person, looking at an eye chart that contains an arrangement of lines similar to those at right, finds that some of the lines appear blurred to him or of unequal intensity. A. What visual defect does this person probably have? B. What is the usual cause of this defect? C. How can this defect be corrected? 3. Typical optical data for an elderly man is shown below. Near Point Far Point Without Glasses 75 cm 250 cm With glasses 25 cm A. What is the power of accommodation of this man's eye without glasses? B. What is the power of accommodation of his optical system, with glasses? C. What type of eyeglasses does the man wear? D. Calculate the power of these eyeglasses. ANSWERS: 1. A) Cornea, lens, muscles (refracts and focus of light on retina, muscles contract changing curvature and power of eye lens) B) Iris (adjusts the opening of the eye thus controlling the intensity of light admitted) C) Rods & Cones - Retina (transducers which convert light energy to electrical nerve impulses which are imparted to the brain via the optic nerve) 2. Astigmatism, nonuniform curvature of the cornea, using a cylindrical lens which is produced to compensate for the defect in curvature of the cornea diopters, 4 diopters, bifocals, -.4 diopters and +2.7 diopters 168

Vision. The eye. Image formation. Eye defects & corrective lenses. Visual acuity. Colour vision. Lecture 3.5

Vision. The eye. Image formation. Eye defects & corrective lenses. Visual acuity. Colour vision. Lecture 3.5 Lecture 3.5 Vision The eye Image formation Eye defects & corrective lenses Visual acuity Colour vision Vision http://www.wired.com/wiredscience/2009/04/schizoillusion/ Perception of light--- eye-brain