Dr Md Ferdous IslamDepartment of 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