Sense Organs – THE EYE

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Alan Stevens

From the Exam Guideline Structure and functions of the different parts of the human eye

Functioning of the human eye in terms of:

• Formation of an image (including role of rods and cones)• Accommodation• Binocular vision• Pupil reflex/pupillary mechanism

Adaptations of the various parts of the eye for their functions

Nature and treatment (glasses and surgery) of the following visual defects:

• Short-sightedness• Long-sightedness• Astigmatism• Cataracts

INVESTIGATIONS• Model of the eye• Observe pupillary mechanism• Dissect and draw a diagram of the mammalian eye• Demonstration of the blind spot

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Some Tips

• MODELS & APPARATUS even from the physical sciences, ARE CRUCIAL !

• The arrows in the previous slide show how various aspects from the Exam Guideline can be linked to save time

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Structure and functions of the different parts of the human eye

MODELS & APPARATUS (even physical sciences) ARE CRUCIAL ! ! !

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Another View, with a Cut-out Section

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The Ciliary body and Suspensory ligaments filled in

Important to understand:Ciliary body is CIRCULAR, AROUND LENS

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A ray box is essential, the added circle represents the eye

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The optical bench

The optical bench could simply be 3 holders positioned as above, along a meter stick. Its purpose is to show that :•when the object moves, the lens has to move in order to form a sharp image on the screen•The image is always inverted (upside down)

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Lenses which are less / moreball-shaped

Start with a flat biconvex lens in a ray box. Then change to a more ball-shaped one. See how the focal length changes. Then use a biconcave lens. See what happens to the emerging rays. This is important in understanding how far- and near-sightedness, and astigmatism are corrected Alan Stevens

Another way to show the inverted image

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Accommodation

When the ciliary body (which is circular) CONTRACTS, the suspensory ligaments become SLACK, and the lens becomes BALL-SHAPED

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The Retina – RODS & CONES

Rod cell

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Packing the rodsand cones

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Common Vision Defects

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Hyperopia (Farsightedness)

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Myopia (Nearsightedness)

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Astigmatism

Aspherical cornea, corneal profile slopes. Refractive power in horizontal axis (plane) is less than that of the perpendicular axis. Results in more than one “focal point”, thus difficulties in seeing fine detail.Astigmatism can be often corrected by glasses with a lens that has different radii of curvature in different planes (a cylindrical lens), contact lenses, or refractive surgery.

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CataractsCataracts = cloudy lens, due to buildup of protein in the lens. Prevents light

from passing clearly through, causing some loss of vision.

New lens cells form on the outside of the lens, compacting all the older cells into the center of the lens resulting in the cataract.

Types of cataracts include:Age-related cataracts. Develops as a result of aging.Congenital cataracts. Babies are sometimes born with cataracts as a result of an infection, injury, or poor development before they were born, or they may develop during childhood.Secondary cataracts. These develop as a result of other medical conditions, like diabetes, or exposure to toxic substances, certain drugs (such as corticosteroids or diuretics), ultraviolet light, or radiation.Traumatic cataracts. These form after injury to the eye.

Other Risk Factors : cigarette smoke, air pollution, and heavy alcohol consumption

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Binocular VisionEach eye sees own image

Brain combines images, creates sense of depth

• Primates & Predators eyes in FRONT of head– Large binocular field (humans 140°)– Small monocular field (humans 20°)– Large blind area (humans 180°)

• In Prey, eyes on SIDE of head– Small binocular field (zebra 65°)– Large monocular field (zebra 146°)– Small blind area (zebra 3°)

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Bibliography• STEREOSCOPIC VISION

http://education.vetmed.vt.edu/Curriculum/VM8054/EYE/BINOCS.HTM, and

• What is stereoscopic visionhttp://www.wisegeek.com/what-is-stereoscopic-vision.htm

• STEREOSCOPIC VISION, Elementary Binocular Physiology, John Wattie http://nzphoto.tripod.com/sterea/3dvision.htm

• Binocular vision from Wikipedia, the free encyclopedia• Image Formation by Lenses and the Eye

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html ORhttp://hyperphysics.phy-astr.gsu.edu/

Alan Stevens