REFRACTIVE ERRORS

1. REFRACTIVE ERRORS SPS Y2-2012

4. WHAT ARE WE GOING TO DO TODAY?

5. 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

6. OPTICS 101 “REFRACTION” Emergent ray Refracted ray Incident ray

7. OPTICS 101: REFRACTION REFRACTIVE INDEX HOW LIGHT TRAVELS THROUGH MEDIUM

8. 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

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

10. 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

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

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

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

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

15. Assimilate; Enjoy Ask Questions

17. The Next few slides introduces a new term “DIOPTER” It will require some attention If you think its too difficult, don’t worry. Most ophthalmologists are in the same boat 

18. 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

19. 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)

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

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

22. DIOPTER –POWER OF LENSES IF A LENS DIVERGES LIGHT RAYS ITS POWER IS - (NEGATIVE) Focal length ‘f’ Focal Point If f= 1m; Plens ? Plens = 1/1 = -1D Diverges light rays

23. OBJECT – LENS INTERACTIONS • Incident lightrays 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)

24. INTERACTION LENS EMERGENT RAYS (to image) INCIDENT RAYS (from object) OBJECT IMAGE Pobj_rays + Plens= Pimage_rays RI n/d + Plens= Pimage_rays Location of image (d) =n/Pimage_rays

25. INTERACTION +2D 2m .67m n/d Pobj_rays + Plens= Pimage_rays (Pobj_rays= -1/2) -.5 + 2 = +1.5 D (Diopters) Location of image (d) = 1/1.5 = .67m (Location of image (d) =n/Pimage_rays)

26. INTERACTION -2D 2m .40m Pobj_rays + Plens= Pimage_rays n/d (Pobj_rays= -1/2) -.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 object

27. DIOPTER Focal Length f p q *Remember: 1/f = 1/p + 1/q *This is the same as: Plens =Pobj +Pimage * Flipping: Pimage=Pobj + Plens

28. QUICK TEST Power = 1/d USE BOTH FORMULAE TO FIND IMAGE POSITION 1/f = 1/p + 1/q Pimage =Pobj + Plens 1D 1D ? 4m 1.3m 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 Pobj_rays+ Plens= Pimage_rays Pobj_rays=-1/4 = -.25D Plens=1D Pimage_rays= -.25+1 =.75 dimage= 1/.75 = 1.3m

29. 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

30. Assimilate; Enjoy Ask Questions

31. THE OPTICS OF EYE Vitreous humor Cornea Aqueous humor LENS *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) TRANSPARENT MEDIA ARE RESPONSIBLE FOR REFRACTION AS THEY ALLOW LIGHT TO PASS THROUGH TO THE RETINA

32. VISION FOVEA MACULA LIGHT RAYS ARE FOCUSED ON THE FOVEA

33. OPTICS OF THE EYE *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 Difference in refractive index Air = 1.00 Cornea = 1.337 Lens = 1.38 More Refraction occurs at Cornea-air interface

34. ACCOMODATION

35. ACCOMODATION

36. ACCOMODATION

37. 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

38. 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

40. 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 (<6m) produce a blur image on the retina • This blur image forces the eye to accommodate • Accommodation involves constriction of ciliary muscles to make the lens thicker (more power) • Thicker lens then focuses the light rays back onto the retina • Amount of accommodation depends on the distance of the near object • The eye will always try to focus a blurred image on the retina • Remembering this simple fact will help clear A LOT of confusion

41. EMMTEROPIA EMMTEROPIA = Equal Measure = No Refractive Error = Desirable optical system RESTING EYE ACCOMMODATING EYE

42. 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; <6m) objects • AM-METROPIA = Not Equal Measure

43. NEAR VISION ERROR(READING ERRORS)

44. PRESBYOPIA • Presby = Old + Hard (Rigid) • Opia = sight • An age related phenomena (40 Years) • Weakness in ciliary muscles • Can’t contract as much as before • Lens can’t accommodate as well • Thus can’t focus for near objects • Becomes worse with advancing age

45. PRESBYOPIA What do old people do to read without glasses *The eye will try to accommodate to nullify the blur *It will fail as there is no more accommodative effort left (weak muscle) *As the object is moved further *The light rays spread more decreasing their power (Diopter; Power = n/d) *Light striking the cornea will have lower power *Less accommodative effort then can help focus the light rays

46. CORRECTING PRESBYOPIA WITH LENSES *A Convex lens corrects presbyopic error. *As muscle weakens with increasing age so does the power of correcting lens *A lens that corrects a refractive error is called a correcting lens WHAT TYPE OF LENS CAN HELP BRING THE LIGHT TO FOCUS ON THE RETINA?

47. DISTANT VISION ERRORS(infinity; 6m)

48. DISTANT VISION ERRORS: RULES • ALL ERRORS ARE DEFINED WITH THE FOLLOWING CONDITIONS • Light rays are parallel (coming from distance) • The eye is at rest (not accommodating) • The error is defined based on where the light focuses • Hyperopia/ Hypermetropia (Long measure) • Myopia (Short measure)

49. DISTANT VISION ERROR I: HYPEROPIA(also known as hypermetropia)

50. HYPERMETROPIA How does eye react to ‘blur’ *The eye voluntarily (unknowing to the patient) accommodates to the blurred image *Most hyperopes don’t even know they have a refractive error *They are latent hyperopes *Only when they start getting older and they loose their accommodative power *Their hyperopia then becomes manifest For a box of candy: Help fix this hyperopic patient’s vision without using lenses What type of lens can be used to correct this error? A common cause of hypermetropia is a short eye ball Convex Lens It accommodates! …focusing behind the retina Parallel light rays… …while eye is at rest… (Not Accommodating)