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REFRACTIVE ERRORS SPS Y2-2012

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WHAT ARE WE GOING TO DO TODAY? Illustrate how light is brought into focus on the retina Associate refractive errors with poor vision Draw ray diagrams to show refractive errors (hyperopia, myopia, astigmatism) Describe Accommodation, Presbyopia and its correction Formulate a plan of vision correction (by spectacles) for refractive errors (distance & near) Demonstrate Snellens Visual Acuity test [Hands-on]

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WHAT ARE WE GOING TO DO TODAY? A little bit of optics Refractive media of the eye How the eye interacts with light Refractive Errors

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OPTICS 101 Incident ray Refracted ray Emergent ray REFRACTION

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OPTICS 101: REFRACTION

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OPTICS 101: ACCEPT IT! Light rays coming from infinity (e.g. sun, stars, galaxies) are parallel All other light rays that originate from an object (TV, bulb, book) are divergent Us ophthalmologists consider a distance of 6 meters or more as infinity All distances are measured in meters

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OPTICS 101: LENSES CONVEX LENS Parallel light rays (incident) Converging light rays (emergent) MORE CONVERGING POWER 3M Divergent light rays (incident) Converging light rays (emergent) 3M AMOUNT OF CONVERGENCE DEPENDENT ON NATURE OF INCIDENT RAYS

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SO FAR Convex Lenses converge Light The thicker the lens (= more power) the greater the convergence Amount of convergence depends on the type of incident rays

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OPTICS 101: LENSES CONCAVE LENS Parallel light rays (incident) Divergent light rays (emergent) Virtual Image MORE DIVERGING POWER Virtual Image 3M Divergent light rays (incident) AMOUNT OF DIVERGENCECE DEPENDENT ON NATURE OF INCIDENT RAYS

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Concave Lenses diverge Light The thicker (= more power) the lens the greater the divergence Amount of divergence depends on the type of incident rays

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RECAP Convex lenses converge Thicker the lens, the greater the power, more the convergence Amount of convergence depends on nature of incident rays

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RECAP Diverging lenses diverge Thicker the lens, the greater the power, more the divergence Amount of divergence depends on nature of incident rays

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Assimilate; Enjoy Ask Questions

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The Next few slides introduces a new term DIOPTER It will require some attention If you think its too difficult, dont worry. Most ophthalmologists are in the same boat

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DIOPTER Measures the divergence (spread) or convergence of light. The power of light Calculated (notation D) as: n/d n = refractive index of medium (1 = air) d = distance from object that measurements are taken (in meters) DO NOT CONFUSE D (DIOPTER) with d (distance) Minus for Diverging light Plus for Converging light

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DIOPTER The Diopter also signifies the power of lens Calculated just as before (n/f. f = focal length) The more the power the more the converging or diverging ability of the lens Minus for Diverging lens (just like light rays) Plus for Converging lens (just like light rays)

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DIOPTER POWER OF OBJECTS.5m1m2m - P = -1/ 0.5. P =-2D P = -1/ 1. P =-1D P = -1/ 2. P =-.5D * Power = n/d * n=1 * - sign for diverging light d As the distance increase. The spread of light also increases. The power contained in the light beam decreases as it is spread too thin

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DIOPTER POWER OF LENSES HOW WILL THE INCIDENT LIGHT RAYS INTERACT WITH THE LENS THE LIGHT RAYS WILL BE CONVERGED BY THE LENS THE AMOUNT OF CONVERGENCE IS THE POWER OF THE LENS FOCAL POINT THE POINT OF LIGHT RAYS CONVERGE IS CALLED THE FOCAL POINT FOCAL LENGTH DISTNACE BETWEEN THE LENS & FOCIAL POINT IS THE FOCAL LENGTH Focal Point Focal length f n/f POWER OF THE LENS IS CALCULATED AS: n/f (n= Refractive Index) If f= 1m; P lens ? P lens = 1/1 = +1D + (POSITIVE) IF A LENS CONVERGES LIGHT RAYS ITS POWER IS + (POSITIVE) Converges light rays

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DIOPTER POWER OF LENSES Focal Point Focal length f - (NEGATIVE) IF A LENS DIVERGES LIGHT RAYS ITS POWER IS - (NEGATIVE) If f= 1m; P lens ? P lens = 1/1 = -1D Diverges light rays

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OBJECT LENS INTERACTIONS Incident light rays from objects (aka object light rays) interact with lenses to form images Light rays that exit the lens are emergent light rays (aka image light rays) The point where image light rays meet is the where image is formed The location of Image is determined using this simple formula: P image_rays = P obj_rays + P lens (P = Power)

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INTERACTION OBJECT INCIDENT RAYS (from object) LENS EMERGENT RAYS ( to image ) IMAGE P obj_rays + P lens = P image_rays n/d + P lens = P image_rays RI Location of image (d) =n/P image_rays

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INTERACTION 2m +2D P obj_rays + P lens = P image_rays -.5 + 2 = +1.5 D (Diopters) (P obj_rays = -1/2) n/d (Location of image (d) =n/P image_rays ) Location of image (d) = 1/1.5 =.67m.67m

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INTERACTION -2D.40m2m P obj_rays + P lens = P image_rays (P obj_rays = -1/2) n/d -.5 + (-)2 = -2.5D (Diopters) Location of image (d) = -1/2.5 = -.40m (Location of image (d) =n/P image_rays ) minus (-) sign: Image on same side as object

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DIOPTER *Remember: 1/f = 1/p + 1/q *This is the same as: P lens =P obj + P image * Flipping: P image =P obj + P lens p q f Focal Length

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QUICK TEST 4m 1D ? USE BOTH FORMULAE TO FIND IMAGE POSITION 1/f = 1/p + 1/q P image =P obj + P lens Power = 1/d 1/f = 1/p+ 1/q f (focal length)=1/power of lens =1 1/1 =1/4 + 1/q 1/1 1/4 = 1/q 1-.25 = 1/q.75 = 1/q q = 1/.75 = 1.33m P obj_rays + P lens = P image_rays P obj_rays = -1/4 = -.25D P lens = 1D P image_rays = -.25+1 =.75 d image = 1/.75 = 1.3m 1.3m 1D

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DIOPTER Measures power of light rays as well as lenses Is + for Converging light rays and lenses Is - for Diverging light rays and lenses Light ray lens interaction is calculated algebraically

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Assimilate; Enjoy Ask Questions

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THE OPTICS OF EYE TRANSPARENT MEDIA ARE RESPONSIBLE FOR REFRACTION AS THEY ALLOW LIGHT TO PASS THROUGH TO THE RETINA Cornea Aqueous humor LENS Vitreous humor * We only consider Cornea and Lens as refractive media *Total Power of the eye ~ 60D (54D) Cornea = 40D (36D) [Greater difference in refractive index) Lens = 20D (18D)

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VISION FOVEA MACULA LIGHT RAYS ARE FOCUSED ON THE FOVEA

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OPTICS OF THE EYE More Refraction occurs at Cornea-air interface Difference in refractive index Air = 1.00 Cornea = 1.337 Lens = 1.38 *The lens is able to change its shape *The stimulus is a blurred image *The lens tries to bring the image into focus just like a camera *For near tasks (reading) the lens thickens increasing power accommodation *This is called accommodation

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ACCOMODATION

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ACCOMMODATION Ciliary Muscles strength largely determine amount of accomodation At Birth it is about 14 Diopters (i.e. the lens power can be increased by 14D by accommodation to about 34D). At 40 this is effectively reduced to 2 Diopters

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NEAR REFLEX When a person focuses for near Three distinct changes occur Accommodation Pupillary constriction Eyes converging (moving inwards) These three together are called the near reflex The accommodation reflex sometimes is used synonymously with near reflex

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RECAP Light rays coming from infinity (>6m) are focused by a resting (non- accommodating) eye on the retina Light rays coming from a finite distance (

REFRACTIVE ERRORS A fault in the mechanism of Refraction Produces a blur image on the retina Can be for far (infinite; >6m)objects As well as for near (finite;