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;