REFRACTIVE ERRORS
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REFRACTIVE ERRORS
REFRACTIVE ERRORSSPS Y2-2012
WHAT ARE WE GOING TO DO TODAY?Illustrate how light is brought into focus on the retinaAssociate refractive errors with poor visionDraw ray diagrams to show refractive errors (hyperopia, myopia, astigmatism)Describe Accommodation, Presbyopia and its correctionFormulate a plan of vision correction (by spectacles) for refractive errors (distance & near)Demonstrate Snellens Visual Acuity test [Hands-on]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
OPTICS 101
Incident rayRefracted rayEmergent rayREFRACTIONOPTICS 101: REFRACTION
REFRACTIVE INDEXHOW LIGHT TRAVELS THROUGH MEDIUMOPTICS 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 metersOPTICS 101: LENSESCONVEX LENS
Parallel light rays (incident)
Converging light rays (emergent)MORE CONVERGING POWER
3MDivergent light rays (incident)Converging light rays (emergent)
3MAMOUNT OF CONVERGENCE DEPENDENT ON NATURE OF INCIDENT RAYSSO FARConvex Lenses converge Light
The thicker the lens (= more power) the greater the convergence
Amount of convergence depends on the type of incident raysOPTICS 101: LENSESCONCAVE LENS
Parallel light rays (incident)Divergent light rays (emergent)Virtual Image
MORE DIVERGING POWERVirtual Image
3M
3MDivergent light rays (incident)AMOUNT OF DIVERGENCECE DEPENDENT ON NATURE OF INCIDENT RAYSConcave Lenses diverge Light
The thicker (= more power) the lens the greater the divergence
Amount of divergence depends on the type of incident raysRECAP
Convex lenses converge
Thicker the lens, the greater the power, more the convergence
Amount of convergence depends on nature of incident rays
RECAP
Diverging lenses diverge
Thicker the lens, the greater the power, more the divergence
Amount of divergence depends on nature of incident rays
Assimilate; EnjoyAsk Questions
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
DIOPTERMeasures the divergence (spread) or convergence of light. The power of light
Calculated (notation D) as: n/dn = 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 lightPlus for Converging lightDIOPTERThe 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)
DIOPTER POWER OF OBJECTS
.5m1m2mP = -1/ 0.5. P =-2DP = -1/ 1. P =-1DP = -1/ 2. P =-.5D* Power = n/d* n=1* - sign for diverging lightdAs the distance increase. The spread of light also increases. The power contained in the light beam decreases as it is spread too thinDIOPTER POWER OF LENSES
HOW WILL THE INCIDENT LIGHT RAYS INTERACT WITH THE LENSTHE LIGHT RAYS WILL BE CONVERGED BY THE LENSTHE AMOUNT OF CONVERGENCE IS THE POWER OF THE LENSTHE POINT OF LIGHT RAYS CONVERGE IS CALLED THE FOCAL POINTDISTNACE BETWEEN THE LENS & FOCIAL POINT IS THE FOCAL LENGTHFocal PointFocal length fPOWER OF THE LENS IS CALCULATED AS: n/f (n= Refractive Index)If f= 1m; Plens ?Plens = 1/1 = +1D
IF A LENS CONVERGES LIGHT RAYS ITS POWER IS + (POSITIVE)Converges light raysDIOPTER POWER OF LENSES
Focal PointFocal length fIF A LENS DIVERGES LIGHT RAYS ITS POWER IS - (NEGATIVE)
If f= 1m; Plens ?Plens = 1/1 = -1D
Diverges light raysOBJECT LENS INTERACTIONSIncident 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:Pimage_rays = Pobj_rays + Plens (P = Power)
INTERACTION
OBJECTINCIDENT RAYS(from object)LENSEMERGENT RAYS(to image)IMAGE Pobj_rays + Plens = Pimage_rays n/d + Plens = Pimage_rays RI Location of image (d) =n/Pimage_rays
INTERACTION
2m+2D
Pobj_rays + Plens = Pimage_rays -.5 + 2 = +1.5 D (Diopters)(Pobj_rays = -1/2)n/d (Location of image (d) =n/Pimage_rays ) Location of image (d) = 1/1.5 = .67m.67m
INTERACTION
-2D.40m2m Pobj_rays + Plens = Pimage_rays (Pobj_rays = -1/2)n/d -.5 + (-)2 = -2.5D (Diopters) Location of image (d) = -1/2.5 = -.40m (Location of image (d) =n/Pimage_rays ) minus (-) sign: Image on same side as objectDIOPTER
*Remember: 1/f = 1/p + 1/q
*This is the same as: Plens =Pobj + Pimage* Flipping: Pimage =Pobj + Plens
pqfFocal Length
QUICK TEST4m1D?USE BOTH FORMULAE TO FIND IMAGE POSITION1/f = 1/p + 1/qPimage =Pobj + PlensPower = 1/d1/f = 1/p+ 1/qf (focal length)=1/power of lens =11/1 =1/4 + 1/q1/1 1/4 = 1/q1-.25 = 1/q.75 = 1/qq = 1/.75 = 1.33mPobj_rays + Plens = Pimage_rays Pobj_rays = -1/4 = -.25DPlens= 1DPimage_rays = -.25+1 =.75dimage= 1/.75 = 1.3m1.3m
1DDIOPTERMeasures 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
Assimilate; EnjoyAsk QuestionsTHE OPTICS OF EYETRANSPARENT MEDIA ARE RESPONSIBLE FOR REFRACTION AS THEY ALLOW LIGHT TO PASS THROUGH TO THE RETINA
CorneaAqueous humorLENSVitreous 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)VISION
FOVEAMACULALIGHT RAYS ARE FOCUSED ON THE FOVEA
OPTICS OF THE EYEMore Refraction occurs at Cornea-air interfaceDifference in refractive indexAir = 1.00Cornea = 1.337Lens = 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 *This is called accommodation
ACCOMODATION
ACCOMODATION
ACCOMODATION
ACCOMMODATIONCiliary 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
NEAR REFLEXWhen a person focuses for near
Three distinct changes occurAccommodationPupillary constrictionEyes converging (moving inwards)
These three together are called the near reflex
The accommodation reflex sometimes is used synonymously with near reflex
RECAPLight rays coming from infinity (>6m) are focused by a resting (non-accommodating) eye on the retina
Light rays coming from a finite distance (6m)objects
As well as for near (finite;