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

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 refractionCrystalline lens – fine tuningCiliary muscles – flex the lens

Retina - extension of the brainFovea – center of retinaBlind spot - Where the optic nerve

leaves the eyeball

Human Eye - Vision Receptors

“Rods” - more sensitive to low lightlevels; fewer in vicinity of fovea.

“Cones” - bunched together at fovea;color sensitive;used for sharp images

Human Eye - Simple Model

RetinaLens

Cornea (Fine tuning)

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 contradictorysensory information (e.g. inversion).

Human Eye - Cornea

Most refraction occurs at the cornea(ncornea = 1.376)

Index of refraction change is greatestat 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 veryfar-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 lensto allow focusing of an objectas 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-sightednessHyperopia - Far-sightednessPresbyopia - 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” distance2. Convert this to meters3. Take the inverse of this number4. 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-sightednessNear 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 correctinghyperopia?

Yes, LASIK can help out here as well.

Presbyopia

Age-related lack of ability to focuson nearby objects

Crystalline lens becomes less flexible

Not surgically correctable by changingthe cornea. Why?

Astigmatism

Cornea is not “spherical”Vertical and horizontal objectsfocus 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 imagesare formed by lenses.Kepler still did not have an exact mathematical formulato describe the phenomenon, however.

Museum der Sternwarte Kremsmünster, Austria