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Optics Assignment
Opto 251
Dr. Safiah Mull
Presented by:
Hanan khaled Mofty
427200187
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Contents
page
 Part 1(optics and light)………………………………………………………...3
1. Part of the eye…………………………………………………………….....3
2. Layers of light passing…………….………………………………………..4
3. Calculation of light refraction………………………………………………5
4. Refractive power of the eye……………………………………………..….7
5. Focal length of the eye……………………………………………..……….7
 Part 2(refractive error in eye)………………………………………………….8
1. Myopic eye………………………………………………………………….8
2. Hyperopic eye………………………………………………………….….10
3. Astigmatism…………………………………………………………….....11
 Part3(refractive surgery for refractive correction)…………………………...12
 Reference……………………………………………………………………..13
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1- Optics and light
Introduction
When light pass through eye it will refract, scatter, absorb and transmit toward
the back of eye and form image. In this part we will talk about the characteristic of
each part of eye and there effect in refraction of light.
1. Part of the eye:
Figure (1) parts of eye
Reference: http://www.lpch.org/photos/greystone/ei_0330.gif
1. Light enters the eye through the cornea, the clear, dome-shaped surface that
covers the front of the eye.
2. From the cornea, the light passes through the pupil. The amount of light passing
through is regulated by the iris, or the colored part of your eye.
3. From there, the light then hits the lens, the transparent structure inside the eye
that focuses light rays onto the retina.
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4. Next, it passes through the vitreous humor, the clear, jelly-like substance that
fills the center of the eye and helps to keep the eye round in shape.
5. Finally, it reaches the retina, the light-sensitive nerve layer that lines the back of
the eye, where the image appears inverted.
6. The optic nerve carries signals of light, dark, and colors to the area of the brain
(the visual cortex), which assembles the signals into images (our vision).
2. Layers of light passing:
2.1 Tear film:
The tear film is the first layer that light hit it and its form of three layers that is the
majority function is protecting the eye from dehydrations. It’s not decided as a
primary refractive surface of the eye. Index of refraction: n= 1.33
2.2 Cornea:
The cornea is the clear bulging surface in front of the cornea2 eye and t is the main
refractive surface of the eye.
-Primary refractive surface of the eye
-Index of refraction: n = 1.37
-Normally transparent and uniformly thick
-Nearly avascular
-Richly supplied with nerve fibers
-Sensitive to foreign bodies, cold air, chemical irritation
-Nutrition from aqueous humor and
-Tears maintain oxygen exchange and water content
-Tears prevent scattering and improve optical quality
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2.3 Anterior and posterior chambers:
-The anterior chamber is between the cornea and the iris
-The posterior chamber is between the iris and the lens
-Contains the aqueous humor
-Index of refraction: n = 1.33
-Specific viscosity of the aqueous just over 1.0 (like water, hence the name)
-Pressure of 15-18 mm of mercury maintains shape of eye and spacing of the
elements
-Aqueous humor generated from blood plasma
-Renewal requires about an hour
-Glaucoma is a result of the increased fluid pressure in the eye due to the reduction
or blockage of aqueous from the anterior to posterior chambers.
2.4 Iris/Pupil:
-Iris is heavily pigmented
-Sphincter muscle to constrict or dilate the pupil
-Pupil is the hole through which light passes
-Pupil diameter ranges from about 3-7 mm
-Area of 7-38 square mm (factor of 5)
-Eye color (brown, green, blue, etc.) dependent on amount and distribution of the
pigment melanin
2.5 Lens:
-Transparent body enclosed in an elastic capsule
-Made up of proteins and water
-Consists of layers, like an onion, with firm nucleus, soft cortex
-Gradient refractive index (1.38 - 1.40)
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-Young person can change shape of the lens via ciliary muscles
-Contraction of muscle causes lens to bulge
-At roughly age 50, the lens can no longer change shape
-Becomes more yellow with age: Cataracts.
-The graph on the right shows the optical density (-log transmittance) of the lens as
a function of wavelength. The curves show the change in density with age. Shorter
wavelength light is blocked at increases ages.
2.6 Vitreous humor:
-Fills the space between lens and retina
-Transparent gelatinous body
-Specific viscosity of 1.8 - 2.0 (jelly-like consistency).
-Index of refraction, n=1.33.
-Nutrition from retinal vessels, ciliary body, aqueous.
-Floaters, shadows of sloughed off material/debris in the vitreous.
-Also maintains eye shape.
Figure (2) layers that light pass through it
Reference: http://www.cis.rit.edu/people/faculty/montag/vandplite/images/chapter_8/cornea2.jpg
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3. Calculation of light refraction:






pre-corneal tear film
cornea
aqueous humor
lens cortex
lens nucleus
vitreous humor
1.3375
1.3760
1.3360
1.3800
1.4100
1.3360
3.1 Tear film
Sin Q / sin Q = n2/n1
sin 90 / sin Q = 1.33/1
Q = sin .7519
sin Q =1/1.33
Q =48.75 >>>> bent=41 toward normal
3.2 Cornea:
Sin Q / sin Q = n2/n1
sin 48.75/ sin Q = 1.37/1.33
Sin Q * 1.37= sin 48.75 * 1.33
Sin Q = 1/1.37
sin Q * 1.37 = .7519 * 1.33
Q =sin .7299
Q =46.88
Bent = 1.87 toward normal.
3.3 Aqueous humor:
sin Q / sin Q = n2/n1
sin 46.88/sin Q = 1.336/1.37
sin Q * 1.336=sin 46.88 * 1.37
sin Q
= 0.999/1.336
sin Q *1.336 = .7299 * 1.37
Q =sin .7485
Q =48.46
Bent = 1.58 away from normal.
3.4 Lens cortex:
sin Q /sin Q = n2/n1
sin 48.46 / sin Q = 1.38/1.336
sin Q* 1.38 = sin 48.46 * 1.336
sin Q * 1.38 = .7485* 1.336
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sin Q = 0.999/1.38
Q = sin .7246
Q = 46.44
Bent = 2.02 toward normal.
3.5 Lens nucleus:
sin Q /sin Q = n2/n1
sin46.44/ sin Q = 1.4/1.38
sin Q *1.4 =sin 46.44*1.38
sin Q =0.999/1.4
sin Q *1.4 = .7246 * 1.38
Q =sin .7142
Q =45.58
Bent = 0.86 toward normal
3.6 Vitreous humor:
sin Q /sin Q = n2/n1
sin45.58/sin Q = 1.33/1.4
sin Q *1.33=sin 45.58 * 1.4
sin Q = 0.999/1.33
sin Q *1.33= .7142 * 1.4
Q =sin .752
Q =48.75
Bent = 3.17 away from normal
Note that:
The primary refractive angle is going to be like in final (tear film refractive
angle=48.75 equal to refractive angle of vitreous humor=48.75) and that also closed
to the corneal refractive angle. So, that explain the curvature of cornea is one of the
factor that effect in light refraction.
4. Refractive power of the eye:
The eye manifests its refractive power via several curved surfaces, each
separated by media with different indices of refraction. The most significant refractive
surfaces are the anterior and posterior cornea and the anterior and posterior crystalline
lens.
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In the emmetropic eye (which has no refractive error), the range of corneal
refracting power is between 39 and 48 diopters, while the range of lenticular
refracting power is between 15 and 24 diopters and the total refractive power of eye is
60 diopters. In the emmetropic eye, the axial length (from the posterior corneal
surface to the retina) varies from 22 to 26 millimeters, or approximately an inch.
5. Focal length of the eye:
D=1/f
D for total eye refractive power is 60
F=1/60= 0.0166 m from the cornea to retina.
** Its shorter than the diameter of eye because it’s not in the fovea area, it’s near to it.
Figure (3) show as when light pass toward a certain point in retina
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2- Refractive error in eye
Introduction
In ammetropic eye, when light enter the eye the image will not form in the
normal place and that cause blurry image for the person.
This part include ammetropic eye like hyperopia, myopia, and astigmatism that can be
correct there refractive error by lenses.
1. Myopic eye:
In myopic eye when light ray entering, the eye focus in front of retina, rather than
directly on it. Three factors caused myopia:
-the curvature of cornea.
-the power of lens.
-the axial length of eye.
Dealing with these situations must be b concave lens description that makes light
more diverge before entering eye
Figure (3) the focus of light is front of retinaReference:
http://www.lpch.org/photos/greystone/ei_0323.gif
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Lens needed to allow a + 70 diopters myopic eye to see clearly. The zero vergence
rays enter the -10 diopters lens and the rays are diverged 10 diopters. These diverge
rays enter the + 70 diopters myopic eye.
The first 10 diopters of convergence are used in diverging rays to zero vergence.
Which leave + 60 diopters for converging. The rays will now be focused on the
retina1.66cm away, and the retina will receive a clear image from the cornea. If the
eye is large than normal without a compensating change in the power of the optical
system (+60) myopia can also occur. Shown in figure (5).
Figure (5) how minus lens diverge light before converge inside eye
(Reference: myopia effect lens clinical optics –Goggle books search page 198)
For elongated eye, -10 diopters lens combined with the +60 diopters eyes leave a total
bending power of + 50 diopters and this will focus the light .2cm long from the
cornea or exactly on the retina. And that apply in this example:
For a person of -2D lens effect for 6m by; m = 1/D =1/50= 0.02
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U+D=V
0.02-2=-1.98
M=1/D=1/-1.98= -0.50 m
Shown in figure (6)…
Figure (6) how concave lens return the image back toward retina
Reference: http://www.lpch.org/photos/greystone/ei_0323.gif
2. Hyperopic eye
This vision problem occurs when light rays entering the eye focus behind the retina,
rather than directly on it. Shown in figure (7).
Four factors effect in hyperopic eye
-the curvature of cornea
-the power of lens
-the axial length of eye
-the accommodation of lens.
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Dealing with this situation must be by convex lens descriptions to make light more
converge before entering eye.
Figure (7) when light ray enter the eye the focus behind retina
Reference: http://www.lpch.org/photos/greystone/ei_0246.gif
Lens needed to allow a +50 diopters hyperopic eye to see clearly. The zero vergence
ray enter the +10 diopters lens and the ray are converge 10 diopters. These converge
rays enter the +50 diopters hyperopic eye.
The first 10 diopters of convergence are used in converging rays to zero vengeances.
Which leave + 60 diopters for converging. The rays will now be focused on the retina.
1.66cm away and the retina will receive a clear image from the cornea.
If the eye is short than normal without a compensating change in the power of the
optical system (+60) hyperopia can also occur.
For shortness eye, +10 diopters lens combined with the + 60 diopters eye leave a total
bending power of + 70 diopters and this will focus the light .26cm short from the
cornea or exactly on the retina.
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For a person of +2D lens it effect by:
U+D=V
-0.014+2= +1.98
-m=1/D = 1/70= -0.014
M=1/D=1/1.98=+0.5 m. shown in figure (8)
Figure (8) how convex lens return the light toward retina
Reference: http://www.lpch.org/photos/greystone/ei_0247.gif
3. Astigmatism:
One axis of the cornea is steeper than the other.
Spherical correction alone will not work for these eyes. For astigmatism, we need to
add a cylindrical shaped lens to correct the refractive aberration along one axis. When
we check for glasses, we determine the amount of cylinder power, and the exact angle
axis this cylinder needs to be oriented to work. To measure this we use the phoropter
Use it by:
1. Figure out the overall spherical error
2. Figure out the extra cylinder to correct for any astigmatism
2. 3. Tweak the angle of the cylinder correction
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Figure (8) how light take two places in astigmatism.
Reference: http://www.lpch.org/photos/greystone/ei_0032.gif
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3 - Refractive surgery for refractive correction
Introduction
Another way for correct refractive error in ammetropic eye (myopia, hyperopia,
astigmatism) is refractive surgery. In this part we will know how refractive surgery
corrects these cases. In general, thicker corneas permit treatment of larger amounts of
nearsightedness and farsightedness. Corneas that are too thin may only allow small
amounts of nearsightedness and farsightedness to be corrected by Lasik. Shown in
figure (9).
In nearsightedness the cornea will be more flat than farsightedness that makes it
more successful. If you are having wave front-guided LASIK, your ophthalmologist
maps both of your eyes using a wave front scanner, called an analyzer or aberrometer. The aberro-meter produces a very precise, detailed map of light rays as they
travel through your eye, highlighting imperfections in your vision.
A targeted beam of light will be sent through your eye and focused on the retina. A
wave of light rays is reflected back from the retina through the eye's lens, pupil and
cornea. A sensor will measure the irregularities in the wave front pattern of light as it
emerges from your eye. Using this measurement, the wave front computer creates an
accurate, three-dimensional map of the light rays created by your eye’s optical system
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Figure (9) change the shape of cornea to correct refraction
Reference: www.cvs.rochester.edu/.../images/r_conv4.jpg
Figure (10) reflection of wave ray used for measurement
Reference: http://medicineworld.org/news/ophthalmology-news.html
In PRK, the epithelium, the layer of cells covering the cornea, is removed and the
excimer laser sculpts the cornea to correct refractive error. A bandage contact lens is
usually placed on the eye following the procedure to speed the epithelial healing
process, which usually takes three to four days.
Because PRK sculpts the outer surface of the cornea, patients experience some
discomfort after surgery and recovery time lasts for a period of several weeks. PRK
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has been largely displaced by LASIK because LASIK provides less discomfort, faster
vision recovery, and the ability to enhance or refine the outcome easily in as little as
three months following the initial surgery.
The ablation profile of an excimer laser counteracts the spherical and cylindrical
portions of the refractive error .To correct myopia; an excimer laser is used to remove
a round lenticule (piece of tissue) from the center of the cornea. Ablation to correct
hyperopia is performed in the corneal periphery, so that the curvature of the central
portion of the cornea is increased and the refractive index of the cornea is made
higher.
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Reference:
http://www.lpch.org
http://www.cis.rit.edu/people/faculty/montag/vandplite/images/chapter_8/cornea2.jpg
Optical measurement of glucose content of the aqueous humor presented by Dan Daly and
Graeme Clark
http://www.cvs.rochester.edu/williamslab/r_conventional.html
http://medicineworld.org/news/ophthalmology-news.html
www.emedicine.com
http://www.aao.org/isrs/patients/ref_procedures.cfm
http://www.eyesurgerybelgium.com/refractivetreatments_bottom.html
Clinical optics – Google books search page196/197/198
http://www.aerzteblatt.de/int/references.asp
http://www.tedmontgomery.com/the_eye/glossary/P.html
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