The Human Eye and Vision Optics of the eye (lenses, focusing, etc.) Physiology of the eye (detection of light)

Summary of Lecture The human eye can be modeled as a simple optical system consisting of two lenses and a projection screen for images. The cornea is the main refractive element of the eye, with the crystalline lens assisting to form an image on the retina. Rods and cones in the retina are the sensors that detect light. Common defects in the visual system are myopia, hyperopia and presbyopia. All can be corrected either with eyeglasses or, more recently for the first two, with surgical procedures.

Structure of the Eye Diagram from National Eye Institute Website

Human Eye Cornea – most refraction Crystalline lens – fine tuning Ciliary muscles – flex the lens Retina - extension of the brain Fovea – center of retina Blind spot - Where the optic nerve leaves the eyeball

Human Eye - Vision Receptors “Rods” - more sensitive to low light levels; fewer in vicinity of fovea. “Cones” - bunched together at fovea; color sensitive; used for sharp images

Human Eye - Simple Model

What type of image is formed? Real, inverted and demagnified. Inverted ??? The brain takes care of interpreting the information received at the retina. It “compensates” for the contradictory sensory information (e.g. inversion).

Human Eye - Cornea Most refraction occurs at the cornea (n cornea = 1.376) Index of refraction change is greatest at that interface.

Human Eye - Goggles Question: Why is it that you can see better underwater when wearing goggles? Answer: Without goggles, there is very little refractive index change between the water and your cornea. You effectively become very far-sighted.

Human Eye - Crystalline Lens Crystalline lens fine tunes the focus (we call this “accomodation”) Cornea-to-retina distance is fixed What change is needed in the lens to allow focusing of an object as it moves closer? The lens must take on a shorter focal length (i.e. become more converging)

“Eye Optics of Marine Mammals” 1. What first led Heather Zorn to think about marine mammal eyes? 2. What is the major difference between the conditions under which our eyes (or those of a fish) operate, and those of marine mammals? 3. When you open your eyes under water without using goggles, do you become near-sighted or far-sighted? Why is there a change at all? 4. How might the cornea play a role in marine mammal vision? The pupil? The crystalline lens? Which is most important?

Human Eye - Abnormalities Myopia - Near-sightedness Hyperopia - Far-sightedness Presbyopia - Age-related decrease in accomodation ability In all cases, the lens cannot compensate for all object distances.

“Normal” Eye Retina ("screen") Object at “infinity”

Myopia: Near-sightedness Retina("screen") Diverging lens

Correcting Myopia How to determine your prescription If you are near-sighted: 1. Find your “far-point” distance 2. Convert this to meters 3. Take the inverse of this number 4. The result is your prescription, if you put a “-” in front.

Correcting Myopia Another possibility for correcting myopia? Radial keratotomy (RK) Photorefractive keratectomy (PRK) LASIK (Laser-assisted in situ keratomileusis)

Hyperopia: Far-sightedness Retina ("screen") Near point Normal “near point” is 25cm (10")

Hyperopia: Far-sightedness Near point Object gives “fuzzy” image because it is too close.Retina ("screen") New image is at the near point

Correcting Hyperopia Are there surgical options for correcting hyperopia? Yes, LASIK can help out here as well.

Presbyopia Age-related lack of ability to focus on nearby objects Crystalline lens becomes less flexible Not surgically correctable by changing the cornea. Why?

Astigmatism Cornea is not “spherical” Vertical and horizontal objects focus at different distances from the cornea.

Eyeglasses? When do eyeglasses first appear? Roger Bacon? (13th century) Umberto Eco’s “The Name of the Rose” Principles were not understood, however.

Kepler (ca. 1600) Solves the Problem He was the first to theoretically understand how images are formed by lenses. Kepler still did not have an exact mathematical formula to describe the phenomenon, however. Museum der Sternwarte Kremsmünster, Austria