1. Physiology of Optics Dr. Abdul Ahad Shaikh 1

2. Objectives Describe the refraction of light as it passes through the eye to the retina, identifying the eye components that account for refraction of light at the center of the eye and away from the center. Describe the refractive deficits that account for myopia, hyperopia, presbyopia and astigmatism, and their correction by eyeglasses or contact lenses. Describe the mechanism of measuring visual acuity using Snellen chart. 2

3. Horizontal section of the right eye. AP, anterior pole; PP, posterior pole; VA, visual axis. Eyeball Our brain derives knowledge of the world around us from visual stimuli The three tunics of eyeball are; – Outer fibrous tunic made up of sclera and cornea – Middle vascular tunic made up of choroid, ciliary body and iris – Inner nervous tunic called retina covers only posterior 5/6 th portion of the eye ball 3

4. Refraction Bending of light at an angulated interface is known as refraction If the interface between two media is perpendicular to the beam of light there is no change in the direction of light When the interface between two media is angulated the light rays change their direction at the interface Refraction depends on two factors – Degree of angulation interface between two media – Difference in the refractive indices of two media 4

5. Optics - Basic Principles AIR Transparent substance Refractive index of a transparent substance - ratio of velocity of light in air to the velocity in the substance. Refractive index Perpendicular interface - decreased velocity and shorter wavelength. 5

6. Types of Lenses Two types of lenses Spherical (convex or concave) refracts the light in all planes Cylindrical (convex or concave) refracts the light only in one plane The signs of the lens radii indicate whether the corresponding surfaces are convex (R > 0, bulging outwards from the lens) or concave (R < 0, depressed into the lens). If R is infinite, the surface is flat, or has zero curvature, and is said to be planar. 6

7. Optics - Basic Principles Refraction A convex lens focuses light rays while concave lenses diverge parallel rays. Focal length distance at which parallel rays converge to a focal point 7

8. Optics - Basic Principles Parallel rays Point source Parallel rays - considered to be coming from infinity. Light sources closer than infinity (>6 meters) are divergent. Parallel rays and divergent rays (point source) may converge at the same focal point however, provided the lens changes its convexity. 8

9. Optics - Basic Principles Refractive power of a lens - measured in diopters In case of a convex lens the refractive power is equal to 1 divided by its focal length of the lens in meter If focal length is 1 m refractive power of the lens is +1D The power of concave lens is expressed by comparing it with convex lens Refractive power of a convex lens = 1/focal length in meters 9

10. Refractive Interfaces of the Eye The eye has 4 refractive interfaces - Air/anterior cornea - posterior cornea/aqueous humour - Ant. Lens/aqueous humour - Post. Lens/vitreous humour 10

11. The cornea provides most of the refractive power of the eye because its refractive index is markedly different from that of air (1.38 - 1.0 = 0.38) Refractive power of the optical system of the eye 11

12. Refractive Surface Anterior surface of cornea + 48D Posterior surface of cornea - 4D Lens of the eye +15D Maximum refraction occurs at the anterior surface of cornea because; – Greater curvature – Greater difference between refractive indices Refractive power of a lens inside the eye is +15D and outside the eye is +150D Reduced eye is schematic representation of the eye considering the eye has a single lens of +59 D placed at a distance of 17 mm from retina (refractive power= 1000/17 ) The axial length of the eye at birth is approximately 17 mm and reaches approximately 24 mm in adulthood. It is typically longer than 24 mm in myopes and shorter than 24 mm in hyperopes. The centre of the lens is called nodal point lies 7.08 mm behind the cornea 12

13. The Eye as a camera The human eye is a camera! Iris - colored annulus with radial muscles Pupil - the hole (aperture) whose size is controlled by the iris What’s the “film”? –photoreceptor cells (rods and cones) in the retina 13

14. The degree to which the details and contours of objects are perceived. Defined in terms of the minimum separable i.e. the minimum distance by which two lines can be separated and still be perceived as two lines. Determined using Snellen charts. An emmetropic eye has VA of 20/20 or 6/6. The design of the VA chart is based on the subtended visual angle. For example, the letters of the 20/20 line of a projected Snellen chart is usually E V O T Z 2. Look at the E Each "stroke" of the E subtends an angle of 1 min, or an arc on a circle with a radius of 20 ft (6m). This one-min arc seems the minimum for the retinal photoreceptors to resolve. Visual Acuity 14

15. Visual acuity – determined using Snellen’s chart – V=d/D d=distance at which characters are read D=distance at which characters should be read Subject 6 meters from test chart. Test one eye at a time. Normally should be 6/6 The average diameter of the cones in the fovea of the retina—the central part of the retina, where visionm is most highly developed—is about 1.5 micrometers 15 Visual acuity – how to measure

16. Visual Acuity When visual acuity is below the largest letter on the chart, the reading distance is reduced until the patient can read it. Once the patient is able to read the chart, the letter size and test distance are noted. If the patient is unable to read the chart at any distance, he or she is tested as follows: 16 NameAbbreviationDefinition Counting FingersCFAbility to count fingers at a given distance. Hand MotionHM Ability to distinguish a hand if it is moving or not in front of the patient's face. Light PerceptionLPAbility to perceive any light. No Light PerceptionNLPInability to see any light. Total blindness.

17. Refractive Errors Physiological – Spherical abberration – Chromatic abberration – Diffraction Pathological – Myopia or near sightedness – Hypermetropia or far sightedness – Astigmatism 17

18. Refractive Errors Physiological – Spherical Abberration Parallel rays of light passing through the peripheral portion of the eye lens do not focus at the same point where rays passing through central portion of the lens focus – Chromatic Abberration Refractive power of the lens of the eye is different for different colors, there fore light rays of differen colors get focused at different distances behind the lens – Diffraction Bending of the light rays as they pass over the sharp borders of the pupil. It interferes with the image formation when pupilary size is small(2.5 -1.5 mm) 18

19. Errors of refraction Common defects of the optical system of the eye. In hyperopia, the eyeball is too short and light rays come to a focus behind the retina. A biconvex lens corrects this by adding to the refractive power of the lens of the eye. In myopia, the eyeball is too long and light rays focus in front of the retina. Placing a biconcave lens in front of the eye causes the light rays to diverge slightly before striking the eye, so that they are brought to a focus on the retina. 19

20. Presbyopia With aging, the elasticity of the lens gradually declines Accommodation of the lens for near vision becomes progressively less effective A young person can change power of lens as much as 14 D By age 40 years accommodation become half By age of 50 it decreases to 2D or less The lens become almost non-elastic solid mass and remains focused permanently at constant distance It becomes non accommodative for near and far vision It can be corrected by bifocal glasses with upper segment focused for far objects and lower segment for near objects 20

21. The nearest point to the eye to which an object can be brought to clear focus by accommodation. Near Point Of Vision (D) Changes in the ability of the lens to round up (accommodate) with age. The graph also shows how the near point (the closest point to the eye that can be brought into focus) changes. Accommodation, which is an optical measurement of the refractive power of the lens, is given in diopters. (After Westheimer, 1974.) 21

22. Astigmatism Defect in the refractive power of a lens only in one plane of the lens The curvature of the cornea is different in one plane from that in another plane giving different degree of refraction in different plane Person is asked to look at the chart with one eye normally bars in all directions will be seen clearly 22

23. Astigmatism Due to uneven curvature of the cornea, causing the visual image in one plane to focus at a different distance from that of the plane at right angles. Corrected using cylindrical lenses. n Astigmatism testing chart - 2 meters away n Are any of the lines clearer than others? 23

24. Aqueous Humour  clear liquid  nourishes the cornea and lens,  produced in the ciliary body by diffusion and active transport from plasma  flows through the pupil and fills the anterior chamber of the eye  normally reabsorbed through a network of trabeculae into the canal of Schlemm, a venous channel at the junction between the iris and the cornea (anterior chamber angle). 24

25. Glaucoma Degeneration of optic nerve associated with increased intraoccular pressure (IOP) in majority of cases Degeneration of optic nerve associated with increased intraoccular pressure (IOP) in majority of cases Normal intraocular pressure 12 – 20 mmHg with average about 15 mmHg Normal intraocular pressure 12 – 20 mmHg with average about 15 mmHg Intraocular pressure is measured using a tonometer Intraocular pressure is measured using a tonometer 25

26. References Guyton and Hall Textbook of Medical Physiology 12 th Edition Ganong's Review of Medical Physiology, 24th Edition Berne & Levy Physiology, 6th Updated Edition Thank you 26