SCN3230-ABED_1: Module 1 Lesson 2 - 7

Lesson 2 — Photoreception: The Eye

Conditions that Affect the Eyes

Read pages 412 – 413

As we grow older, the flexibility of our lenses decreases. As a result, many people must wear reading glasses to focus the details of nearby objects. Hyperopia describes the ability to see distant objects clearly and the inability to focus on objects that are close. You may have heard already the more common term for this condition, which is farsightedness. It is caused by a short eyeball or flat cornea, which results in images being focused behind the retina. Hyperopia can be corrected using convex lenses in eyeglasses.

Inversely, myopia is the condition of seeing clearly objects that are close but not being able to focus on objects farther away. The common term for myopia is nearsightedness. It is caused by a lengthened eyeball or increased curvature of the cornea, which results in images being focused in front of the retina. Myopia can be corrected using concave lenses in eyeglasses.

Another condition that affects the lens of the eye is a cataract. This is a clouding of the lens, and it results in cloudy or blurry vision. It can be corrected by replacing the lens of the eye.

Astigmatism is a condition that affects the cornea. An uneven curve of cornea or lens leads to astigmatism, and it can be corrected with eyeglasses or laser surgery.

Glaucoma is a condition that results from blocked ducts that drain aqueous humour. This increased in fluid leads to increased pressure on the retinal arteries and results in damage to the retina and the optic nerve. Vision loss is gradual, and it begins with the loss of peripheral vision.

Macular degeneration is commonly affects those people aged 50 and older. It is caused by damage to the macula, the area surrounding the fovea centralis that is needed for sharp, central vision. Macular degeneration results in a small blurred area in the central vision that grows larger into a blank spot.

Normal Vision. National Eye Institute, National Institutes of Health..CC BY 2.0

Myopia. National Eye Institute, National Institutes of Health.. CC BY 2.0

Cataract. National Eye Institute, National Institutes of Health.. CC BY 2.0

Glaucoma. National Eye Institute, National Institutes of Health.. CC BY 2.0

Macular Degeneration. National Eye Institute, National Institutes of Health.. CC BY 2.0

Try This

Determining your dominant eye

Find an object on which you can focus (such as a door knob). Stand as far from the object as possible.
Extend both arms in front of you, and place the hands together to make a small triangular opening between your thumbs and index fingers.
With both eyes open, look through the triangle and centre the object.
Close your left eye and observe what happens to the object.
Close your right eye and observe what happens to the object.
The eye that keeps the object in view is the dominant eye. If you were an archer, you would want to place the string of the bow in front of your dominant eye. If you played billiards or used a rifle, you would want to sight your target with your dominant eye, if possible.

Determining the blind spot

The eye has some interesting phenomena concerning the blind spot and its function (or its lack of function). Try to place your face about 30 cm from the screen with your nose centred on the crosshair. Close or cover your right eye. Start reading the letters in successionfocusing on each letter in turn.

At which letter does the crosshair disappear? The crosshair should disappear somewhere between J and N. This happens because the light from the crosshair is falling on the blind spot. Normally, you do not notice the blind spots on your retina because one eye compensates for the other eye by supplying the missing information.