1) Refraction is the bending of light when passing from one medium to another, with the angle of refraction determined by the refractive indices.
2) Refractive errors occur when the eye is too long or short for its refractive power, causing light to focus in front of or behind the retina.
3) Determining refractive error involves objective tests like retinoscopy and subjective refinement through refraction. Proper correction maintains light focus on the retina.
3. 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.
5. 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.
r
i
sin
sin
r
i
n
sin
sin
Angle of
incidence
Angle of
refraction
6. Speed of light 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 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
9. 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 ray
glass
i2
r2
Refracted ray
Refracted ray
10. LIGHT
When light travels from 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
13. 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
14.
15. LIGHT
We know that r =
90o…
Refractive
Index, n =
sin r
sin i
n =
sin c
sin 90o
=
sin c
1
20. Procedure of DETERMINING and CORRECTING
refractive error.
Two methods Objective and Subjective.
Objective
Retinoscopy
Autorefractometry
Photorefraction
22. • 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
23. 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
24. 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
25. 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
26. Two meridians (vertical & hori) checked to
take care of astigmatism
Both eyes checked to take care of
anisometropia
27. 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.
28. STEPS
1. Monocular Subjective refraction
2. Binocular balancing
3. Correction for near vision
INSTRUMENTS
a. Phoropter
b. Trial Frame
c. Trial Box
29. 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
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