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Capt Md Ferdous IslamTrainee in Ophthalmology
CMH Dhaka
LIGHT
•Refraction is the change 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.
Refraction
Incident ray
Refracted ray
Emergent ray
Angle of incidence
Angle of refraction
normal
Angle of emergence
When light passes from vacuum (or air) into a given medium (eg. water), the constant ratio of is known as the refractive index, n, for that medium.
Refractive Index
ri
sinsin
rin
sinsin
Angle of incidence
Angle of refraction
Speed and Refractive Index Speed of light in vacuum = 3 x 108 ms-1
Light is found to move slower in optically denser mediums. (eg. glass and water)
vc n
mediumin light of speedin vacuumlight of speed n
LIGHT
The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.
For two given media, 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
LIGHT
The higher the optical 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 rayglass
i2
r2
Refracted ray
Refracted ray
LIGHT
When light travels from a less dense
medium to a denser medium…
n = sin isin r
i
r
air
water
When light travels from a denser medium
to a less dense medium…
n = sin rsin i
i
r air
water
LIGHT
Refractive index, n =
Speed of light in vacuum / air
Speed of light in medium
=cv
Refracting power or vergence power or surface power=(n2-n1)/r
Refraction Of Light At A Curved Interface
LIGHT
The critical angle is the angle of incidence in the optically denser medium for which the angle of refraction is 90o.
When i = critical angle,c r = 90o
LIGHT
We know that r = 90o…
Refractive
Index, n= sin r
sin i
n = sin csin 90o
= sin c1
LIGHT
n=sin c
=c
1
sin-1n1
http://www.oculist.net/downaton502/prof/ebook/duanes/graphics/figures/v1/0300/044f.gif
Mismatch between axial length and refractive power.
Parallel light rays don’t fall on the retina.
◦ Nearsightedness (Myopia)◦ Farsightedness (Hypermetropia)◦ Astigmatism◦ Presbyopia◦ Anisometropia
Ametropia (Refractive error)Gk, ametros, irregular, opsis, sight
Uncorrected, light focuses behind fovea
Corrected by convergent lens, light focuses on fovea
Procedure of DETERMINING and CORRECTING refractive error.
Two methods Objective and Subjective. Objective Retinoscopy Autorefractometry Photorefraction
Clinical Refraction
Pinhole glasses Autorefractor
View through an autorefractorPhoropter
• Objective method of 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
Retinoscopy
Begins with directing 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
Retinoscopy Procedure
Moving the light 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
Retinoscopy Procedure…
If fundal glow 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
Retinoscopy Procedure…
Two meridians (vertical & hori) checked to take care of astigmatism
Both eyes checked to take care of anisometropia
Retinoscopy Procedure…
Autorefractometer is an 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.
Autorefractometer
STEPS1. Monocular Subjective refraction2. Binocular balancing3. Correction for near vision
INSTRUMENTSa. Phoropterb. Trial Framec. Trial Box
Subjective Refraction
History Visual Acuity 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
Steps Of Clinical Refraction
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
Types Of Optical Correction
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