Transcript of REFRACTIVE ERRORS SPS Y2-2012. WHAT ARE WE GOING TO DO TODAY? Illustrate how light is brought into...
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- REFRACTIVE ERRORS SPS Y2-2012
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- WHAT ARE WE GOING TO DO TODAY? Illustrate how light is brought
into focus on the retina Associate refractive errors with poor
vision Draw ray diagrams to show refractive errors (hyperopia,
myopia, astigmatism) Describe Accommodation, Presbyopia and its
correction Formulate a plan of vision correction (by spectacles)
for refractive errors (distance & near) Demonstrate Snellens
Visual Acuity test [Hands-on]
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- 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
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- OPTICS 101 Incident ray Refracted ray Emergent ray
REFRACTION
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- OPTICS 101: REFRACTION
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- 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
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- OPTICS 101: LENSES CONVEX LENS Parallel light rays (incident)
Converging light rays (emergent) MORE CONVERGING POWER 3M Divergent
light rays (incident) Converging light rays (emergent) 3M AMOUNT OF
CONVERGENCE DEPENDENT ON NATURE OF INCIDENT RAYS
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- 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
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- OPTICS 101: LENSES CONCAVE LENS Parallel light rays (incident)
Divergent light rays (emergent) Virtual Image MORE DIVERGING POWER
Virtual Image 3M Divergent light rays (incident) AMOUNT OF
DIVERGENCECE DEPENDENT ON NATURE OF INCIDENT RAYS
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- Concave Lenses diverge Light The thicker (= more power) the
lens the greater the divergence Amount of divergence depends on the
type of incident rays
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- RECAP Convex lenses converge Thicker the lens, the greater the
power, more the convergence Amount of convergence depends on nature
of incident rays
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- RECAP Diverging lenses diverge Thicker the lens, the greater
the power, more the divergence Amount of divergence depends on
nature of incident rays
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- Assimilate; Enjoy Ask Questions
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- The Next few slides introduces a new term DIOPTER It will
require some attention If you think its too difficult, dont worry.
Most ophthalmologists are in the same boat
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- 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
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- 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)
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- DIOPTER POWER OF OBJECTS.5m1m2m - P = -1/ 0.5. P =-2D P = -1/
1. P =-1D P = -1/ 2. P =-.5D * Power = n/d * n=1 * - sign for
diverging light d As the distance increase. The spread of light
also increases. The power contained in the light beam decreases as
it is spread too thin
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- DIOPTER POWER OF LENSES HOW WILL THE INCIDENT LIGHT RAYS
INTERACT WITH THE LENS THE LIGHT RAYS WILL BE CONVERGED BY THE LENS
THE AMOUNT OF CONVERGENCE IS THE POWER OF THE LENS FOCAL POINT THE
POINT OF LIGHT RAYS CONVERGE IS CALLED THE FOCAL POINT FOCAL LENGTH
DISTNACE BETWEEN THE LENS & FOCIAL POINT IS THE FOCAL LENGTH
Focal Point Focal length f n/f POWER OF THE LENS IS CALCULATED AS:
n/f (n= Refractive Index) If f= 1m; P lens ? P lens = 1/1 = +1D +
(POSITIVE) IF A LENS CONVERGES LIGHT RAYS ITS POWER IS + (POSITIVE)
Converges light rays
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- DIOPTER POWER OF LENSES Focal Point Focal length f - (NEGATIVE)
IF A LENS DIVERGES LIGHT RAYS ITS POWER IS - (NEGATIVE) If f= 1m; P
lens ? P lens = 1/1 = -1D Diverges light rays
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- OBJECT LENS INTERACTIONS Incident light rays 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: P
image_rays = P obj_rays + P lens (P = Power)
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- INTERACTION OBJECT INCIDENT RAYS (from object) LENS EMERGENT
RAYS ( to image ) IMAGE P obj_rays + P lens = P image_rays n/d + P
lens = P image_rays RI Location of image (d) =n/P image_rays
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- INTERACTION 2m +2D P obj_rays + P lens = P image_rays -.5 + 2 =
+1.5 D (Diopters) (P obj_rays = -1/2) n/d (Location of image (d)
=n/P image_rays ) Location of image (d) = 1/1.5 =.67m.67m
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- INTERACTION -2D.40m2m P obj_rays + P lens = P image_rays (P
obj_rays = -1/2) n/d -.5 + (-)2 = -2.5D (Diopters) Location of
image (d) = -1/2.5 = -.40m (Location of image (d) =n/P image_rays )
minus (-) sign: Image on same side as object
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- DIOPTER *Remember: 1/f = 1/p + 1/q *This is the same as: P lens
=P obj + P image * Flipping: P image =P obj + P lens p q f Focal
Length
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- QUICK TEST 4m 1D ? USE BOTH FORMULAE TO FIND IMAGE POSITION 1/f
= 1/p + 1/q P image =P obj + P lens Power = 1/d 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 P obj_rays + P lens = P
image_rays P obj_rays = -1/4 = -.25D P lens = 1D P image_rays =
-.25+1 =.75 d image = 1/.75 = 1.3m 1.3m 1D
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- 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
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- Assimilate; Enjoy Ask Questions
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- THE OPTICS OF EYE TRANSPARENT MEDIA ARE RESPONSIBLE FOR
REFRACTION AS THEY ALLOW LIGHT TO PASS THROUGH TO THE RETINA Cornea
Aqueous humor LENS Vitreous humor * 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)
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- VISION FOVEA MACULA LIGHT RAYS ARE FOCUSED ON THE FOVEA
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- OPTICS OF THE EYE More Refraction occurs at Cornea-air
interface Difference in refractive index Air = 1.00 Cornea = 1.337
Lens = 1.38 *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 accommodation *This is called accommodation
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- ACCOMODATION
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- 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
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- 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
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- 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 (
- 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;