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1George Silk

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Evolution of the eye

http://www.pbs.org/media/evolution/library/01/1/l_011_01_ref.mov

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Parts of the eye

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The Eye as a Camera• A human eyeball is like a simple

camera.– Sclera: outer walls, hard, like a light-tight

box.– Cornea and crystalline lens (eyelens):

the two lens system.– Retina: at the back of eyeball, like the

CCD.– Iris: like diaphragms or stop in a camera.– Pupil: camera aperture.– Eyelid: lens cover.

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Humor

• The Aqueous Humor is the clear liquidbetween the cornea and the lens.– The space that it inhabits is called the anterior

chamber.• The Vitreous Humor is the clear liquid

between the lens and the retina.– The space that it fills is called the vitreous body.– Also aqueous humor in this region

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• Provides nourishment to the eyelens andcornea.

Cannot use the blood vessels:– Will block the light.– This makes corneal transplants easy - no

rejection problems.• Hold the shape of the eyeball.

Humor Functions

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Vitreous humor has the following composition:    1.  water (99%)  2.  a network of collagen fibrils    3.  large molecules of hyaluronic acid  4.  peripheral cells (hyalocytes)    5.  inorganic salts    6.  sugar    7.  ascorbic acid

Vitreous Humor (Body)

“jelly-like”

Vitrectomy - removal of the vitreous humor

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The Iris

• When it is full open, it is about f/2 and f/3. Thishappens at low light level.

• When the iris has a small opening, it can cut downthe light intensity by a factor of 20 (to f/8-f/10)

• However, the main function of stopping down the irisis to increase the depth of field. (remember aperturestops on a camera)

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Indices of Refraction

Air n=1.000

Lens n=1.40

Aqueous humor n=1.336

Cornea n=1.376

Vitreous and aqueous humor n=1.336

ni Sinθi = nf Sinθf

Recall Snell’s Law:

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The Lens System

• The cornea and eyelens form a compound lenssystem, producing a real inverted image on the retina.– From air to cornea (n=1.376): large bending, the main

focusing.– From cornea to eyelens (n=1.406), less focusing power.

– The eye has a limited depth of field. We cannot see thingsclose and far at the same time.

Cornea: ~ 40 D (25 mm f.l.)Lens ~ 15-24 D (40 - 70 mm f.l.)

Distance from cornea to retina - 22-26 mm

Combined ~ 60 D (15 mm)

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Accommodation• Focusing is done by changing the focal length

of the eyelens Ciliary muscles help tochange the shape of the lens:accommodation.– Muscles relax, long focal length, see objects far

way; Muscles tense, short focal length seeobjects close.

– Accommodation deteriorates with age:

1 m1.0 D60

22 cm4.5 D40

10 cm10 D20

7 cm14 D10

NearpointRangeAge

Presbyopia

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Accommodation

Reshape the lens Move the lens

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Visual Acuity average “good” vision - can read a letter thatsubtends 5/60 degree (8ft at one mile)

.08 degrees

Eyechart at 20 ft. - if you see what theperson with average good vision can seeat 40 ft. - you have 20/40 vision.

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Clicker survey

Do you need to wear corrective lenses?

1. No, my eyes are just fine.2. Yes, I am near-sighted3. Yes, I am far-sighted4. Yes, but I don’t know which I am…

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Correcting Problems

Myopia (nearsighted) Hyperopia (farsighted)

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AstigmatismDifferent focal lengths horizontal and vertical

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LASIKLaser-Assisted In Situ Keratomileusis

• reshape the cornea to correct vision

1. Cut flap in outer layer of cornea2. Sculpt cornea with pulsed

Excimer (UV) laser that vaporizes cornea

3. put flap back (reattaches on its own)18

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