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Refraction.Dr Ferdous
- 2. Dr Md FerdousIslam Department of Ophthalmology CMH Dhaka
- 3. LIGHT •Refraction is thechange in direction of light when it passes from one medium to another. •The greater the optical density of the medium, the slower the speed of light.
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- 5. When lightpasses from vacuum (or air) into a given medium (eg. water), the constant ratio of is known as the refractive index, n, for that medium. r i sin sin r i n sin sin Angle of incidence Angle of refraction
- 6. Speed oflight in vacuum = 3 x 108 ms-1 Light is found to move slower in optically denser mediums. (eg. glass and water) v c n mediuminlightofspeed in vacuumlightofspeed n
- 7. LIGHT The incident ray,the refracted ray and the normal at the point of incidence all lie in the same plane.
- 8. For two givenmedia, the ratio sin i ÷ sin r is a constant, where i is the angle of incidence and r is the angle of refraction LIGHT i r air water Incident ray Refracted ray normal Refractive Index, n = sin i sin r
- 9. LIGHT The higher theoptical density, the greater the refractive index. The greater the refractive index, the greater the bending of light towards the normal. air water i1 r1 Incident ray Refracted ray glass i2 r2 Refracted ray air water i1 r1 Incident ray glass i2 r2 Refracted ray Refracted ray
- 10. LIGHT When light travelsfrom a less dense medium to a denser medium… n = sin i sin r i r air water When light travels from a denser medium to a less dense medium… n = sin r sin i i r air water
- 11. LIGHT Refractive index, n = Speedof light in vacuum / air Speed of light in medium = c v
- 12. Refracting poweror vergence power or surface power=(n2-n1)/r
- 13. LIGHT The critical angleis the angle of incidence in the optically denser medium for which the angle of refraction is 90o. When i = critical angle,c r = 90o
- 15. LIGHT We know thatr = 90o… Refractive Index, n = sin r sin i n = sin c sin 90o = sin c 1
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- 18. Mismatch betweenaxial length and refractive power. Parallel light rays don’t fall on the retina. ◦ Nearsightedness (Myopia) ◦ Farsightedness (Hypermetropia) ◦ Astigmatism ◦ Presbyopia ◦ Anisometropia
- 19. Uncorrected, light focusesbehind fovea Corrected by convergent lens, light focuses on fovea
- 20. Procedure ofDETERMINING and CORRECTING refractive error. Two methods Objective and Subjective. Objective Retinoscopy Autorefractometry Photorefraction
- 21. Pinhole glasses Autorefractor Viewthrough an autorefractorPhoropter
- 22. • Objective methodof estimation of ref status • Precise assessment of astigmatism & axes difficult • Difficult in children, uncooperative patients & small pupils(old & DM) opacities in Media • Cycloplegic refraction essential in children, strabismus
- 23. Begins withdirecting light into pts’ eye and illuminating area of retina Emergent rays from pts eye forms an image It is referred to as red fundal glow By convention referred to patients’ pupillary area
- 24. Moving thelight across the pts’ retina & observing the movement of the fundal glow- ref status is assessed With suitable lenses, movement of fundal glow is neutralized & error estimated
- 25. If fundalglow moves with the mirror(plane), neutralized with plus lenses If fundal glow moves against the mirror – neutralized with minus lenses Point of neutralization – no movement of fundal glow will be seen, cross checked with concave mirror Ultimately pt of neutralization is to achieve 1.0D myopia using suitable lenses
- 26. Two meridians(vertical & hori) checked to take care of astigmatism Both eyes checked to take care of anisometropia
- 27. Autorefractometer isan optical instrument that can automatically determine the refractive measurement of the eye. It has a chin rest, forehead rest, viewing screen for the observer to monitor fixation.
- 28. STEPS 1. MonocularSubjective refraction 2. Binocular balancing 3. Correction for near vision INSTRUMENTS a. Phoropter b. Trial Frame c. Trial Box
- 29. History VisualAcuity Ext Exam Ophthalmoscopic exam Cover test to detect latent and manifest deviation Retinoscopy Subjective refraction Subjective refinement and finalization of lens Binocular balancing Near vision correction
- 30. Spectacle lenses ◦Monofocal lenses : spherical lenses , cylindrical lenses ◦ Multifocal lenses Contact lenses: ◦ higher quality of optical image and less influence on the size of retinal image than spectacle lenses ◦ indication : cosmetic , athletic activities , occupational , irregular corneal astigmatism , high anisometropia , corneal disease. ◦ soft, hard, gas-permeable
- 31. CONTACT LENSES ◦Disadvantages : careful daily cleaning and disinfection , expense ◦ Complication : infectious keratitis , giant papillary conjunctivitis , corneal vascularization , severe chronic conjunctivitis INTRAOCULAR LENSES ◦ replacement of cataract crystalline lens ◦ give best optical correction for aphakia , avoid significant magnification and distortion caused by spectacle lenses