Evolution of the

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

George Silk 1 2

Parts of the eye The Eye as a Camera • A human eyeball is like a simple camera. – : outer walls, hard, like a light-tight box. – and crystalline (eyelens): the two lens system. – : at the back of eyeball, like the CCD. – : like diaphragms or stop in a camera. – : camera aperture. – : lens cover. 3 4

1 Humor Humor Functions

• The Aqueous Humor is the clear liquid • Provides nourishment to the eyelens and between the cornea and the lens. cornea. – The space that it inhabits is called the anterior chamber. Cannot use the blood vessels: • The Vitreous Humor is the clear liquid – Will block the light. between the lens and the retina. – This makes corneal transplants easy - no – The space that it fills is called the vitreous body. rejection problems. – Also aqueous humor in this region • Hold the shape of the eyeball.

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Vitreous Humor (Body) The Iris

Vitreous humor has the following composition: 1. water (99%) 2. a network of fibrils 3. large molecules of • When it is full open, it is about f/2 and f/3. This 4. peripheral cells (hyalocytes) happens at low light level. 5. inorganic salts • When the iris has a small opening, it can cut down 6. sugar the light intensity by a factor of 20 (to f/8-f/10) 7. ascorbic acid • However, the main function of stopping down the iris is to increase the depth of field. (remember aperture “jelly-like” stops on a camera) - removal of the vitreous humor

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2 Indices of Refraction The Lens System

• The cornea and eyelens form a compound lens Cornea n=1.376 Lens n=1.40 system, producing a real inverted image on the retina. – From air to cornea (n=1.376): large bending, the main focusing. Air n=1.000 – From cornea to eyelens (n=1.406), less focusing power. Vitreous and aqueous humor n=1.336 – The eye has a limited depth of field. We cannot see things close and far at the same time. Recall Snell’s Law: Cornea: ~ 40 D (25 mm f.l.) Lens ~ 15-24 D (40 - 70 mm f.l.) Aqueous humor n=1.336 ni Sinθi = nf Sinθf Combined ~ 60 D (15 mm) Distance from cornea to retina - 22-26 mm 9 10

Accommodation

• Focusing is done by changing the focal length of the eyelens Ciliary muscles help to change the shape of the lens: accommodation. – Muscles relax, long focal length, see objects far way; Muscles tense, short focal length see objects close. – Accommodation deteriorates with age: Age Range Nearpoint 10 14 D 7 cm Reshape the lens Move the lens Presbyopia 20 10 D 10 cm 40 4.5 D 22 cm 60 1.0 D 1 m 11 12

3 Visual Acuity Clicker survey

average “good” vision - can read a letter that subtends 5/60 degree (8ft at one mile) Do you need to wear corrective lenses? .08 degrees 1. No, my are just fine. 2. Yes, I am near-sighted Eyechart at 20 ft. - if you see what the 3. Yes, I am far-sighted person with average good vision can see at 40 ft. - you have 20/40 vision. 4. Yes, but I don’t know which I am…

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Correcting Problems Astigmatism

Different focal lengths horizontal and vertical (nearsighted) Hyperopia (farsighted)

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4 LASIK Laser-Assisted In Situ Keratomileusis • reshape the cornea to correct vision

1. Cut flap in outer layer of cornea 2. Sculpt cornea with pulsed Excimer (UV) laser that vaporizes cornea

3. put flap back (reattaches on its own) 17 18

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