Eyes and Ears

Amblyopia

Commonly known as lazy eye. Amblyopia is a term used to describe reduced or dimness of vision in a otherwise normal eye that, because of muscle imbalance, cannot focus in sync with the other eye.

Hyperopia

Eye focuses an image "behind" the retina, which cannot happen because light rays cannot pass through the choroids layer and sclera. Caused by the eyeball being too short or by the refractive power being too weak due to the lens being too thin. (hypermetropia or farsightedness)

Myopia

Eye focuses an image in front of the retina because the eyeball is too long or the refractive power is too great, which is caused by the lens being too thick. (nearsightedness)

Using a convex lens will cause light rays to converge.

How can Hyperopia be corrected and will they cause?

Using a convex lens and will cause light rays to diverge.

How can Myopia be corrected and what will they cause?

Using reading glasses, bifocals, or trifocals.

How can Presbyopia be corrected?

Inverted images

Images are focused upside down on the retina. They also undergo mirror reversal; that is, light reflected from the right side of an object hits the left side of the retina and vice versa.

Emmetropic Eye (Emmetropia)

This normal refraction involves a what?

Normal refraction

When an object is 20 feet or more away from your eye, the light rays reflected from the object are nearly parallel to one another ( 20/20 )

B. ACCOMMODATION OF THE LENS

1. Changes in the lens curvature to facilitate for near or far vision. This process will also involve constriction of the pupils and convergence. 2. Light rays reflected from an object 20 feet away are nearly parallel to one another. At this distance, the lens will have an oval shape. At distances less than 20 ft diverging light rays will strike the lens of the eye and the lens curvature must change, that is the lens must bulge (become spherical) in order to see a focuses image. 3. In near vision, the ciliary muscle contracts, pulling the remainder of the ciliary body (which is known as the ciliary process) and the choroids forward towards the lens. This action releases the tension on the lens and suspensory ligaments. Because of its elasticity, the lens shortens, and becomes almost spherical. The thicker the lens, the more light rays will be bent to be brought to focus on the retina. If the lens did not become almost spherical, the light rays would strike the periphery of the retina and result in a blurred image. 4. In far vision, the ciliary muscle is relaxed and the lens is flatter. 5. Constant accommodation of the lens results in eyestrain because of the continual ciliary muscle contraction.

CONTRICTION OF THE PUPIL

1. In constriction of the pupil, the circular smooth muscle fibers of the iris are stimulated to contract, this the pupil size becomes smaller. Constriction of the pupil occurs simultaneously with accommodation of the lens. This action prevents light rays from entering on the periphery of the cornea and lens. Light rays that enter on the periphery of the cornea and lens are not brought to focus on the retina, resulting in blurred vision. 2. If the radial smooth muscle fibers of the iris are stimulated, the pupil will dilate. 3. Accommodation Reflex- This reflex action occurs due to looking at a near object and then looking at a far away object or vise versa. When you look at a near object, the pupil constricts in order to prevent diverging light rays from hitting on the periphery of the retina and resulting in blurred image. You also need to remember that as this is happening, the lens is becoming spherical to facilitate for diverging light rays. When you look at an object that is far away, parallel light rays are entering the eye and the pupil will dilate. You also need to remember that as this is happening, the lens is becoming flattened. 4. Photopupil Reflex- This reflex action will occur due to bright light or absence of light. 5. Argyll Robertson pupil- This results from syphilis of the CNS. The photopupil reflex is lost, but the accommodation reflex is retained.

CONVERGENCE

1. In order to see a single binocular image, light rays reflected off of the object must strike on corresponding points on the two retinas. As you stare at a distant object the light rays are aimed at identical spots on the two retinas. Proper refraction of light rays, accommodation of the lens, and constriction of the pupil must occur if the light rays are brought to focus on the two retinas. 2. As you move closer to the object being viewed, your eyes must rotate medially for the light rays from the object to hit the same points on both retinas. 3. Convergence refers to this media movement of the two eyeballs so they are both directed toward the object being viewed. The nearer the object, the greater the degree of convergence necessary to maintain a singular binocular image. 4. Convergence is brought about by the coordinated action of the extrinsic eye muscles

RODS

1. Rods are not present in the macula lutea 2. Rods have the ability to manufacture a photopigment called rhodopsin. 3. Rhodopsin forms from the combination of opsin, a protein, and retinene (retinal) which is a derivative of vitamin A. The combination of opsin and retinene must occur in the presence of darkness. A lack of vitamin A results in night blindness (nyctalopia) because there is an absence of retinene (more noticeable at night). Scotopic- night vision 4. Rhodopsin is highly light sensitive. Even light rays from the moon or candle will break down some of it and thereby allow you to see. Therefore, the rods are specialized for night vision. In bright light, the rhodopsin is broken down faster than it can be manufactured. In dim light, production is able to keep pace with a slower rate of breakdown. These characteristics of rhodopsin are responsible for the experience of having to adjust to a dark room after walking in from the sunshine. It takes several minutes (20- 30 min for maximum sensitivity to darkness) to get complete rhodopsin formation. 5. Rods are black and white vision, not color- that's why in poor light colors are less vivid or black and white. (about 120 million per retina)

CONES

1. They have their greatest concentration in the macula lutea, with the greatest number located within the fovea centralis (center of macula lutea) 2. Cones are the receptors for bright light, color, and visual acuity (sharpness). Photopic- day vision 3. The photopigments in cones (photopsin) are similar to those in rods. In addition to retinene, cones contain opsins, which are different from the opsins found in rods. The pigments in cones require bright light for their breakdown and they re-form quickly. 4. There are three different types of cones, each containing a different combination of retinene and opsin. The different pigments are: Erythrolabe- red, Chlorolabe- green, and Cyanolabe- blue. 5. Each of the three different types of cones has a different maximum absorption of light of different wavelengths, and thus each responds best to light of a given color. The color a person senses depends upon which set of cones or combination of cones is stimulated by the light in a given image. If all three sets of cones are stimulated, the person senses the light as white, and if none are stimulated, the person senses black. 6. If a single group of color receptive cones is missing from the retina, an individual cannot distinguish some colors from others and is said to be colorblind. The common type is red-green color blindness. This person cannot distinguish between red and green. Color blindness is a sex linked recessive trait that affects about 8% to 10% of males and only 0.4% of females. (about 6 million per retina)

Refraction of Light Rays, Accommodation of the Lens, Constriction of the Pupil and Convergence

4 processes that involve the formation retinal image.

Presbyopia

A "farsighted" condition due to the lenses losing their elasticity or due to weakness of the ciliary muscle and the lenses lose their ability to bulge (become spherical) and to accommodate for near vision. This condition may be caused by aging.

Strabismus

A condition in which the visual axes of the two eyes differ, so that they do not fix on the same object. This may be caused by a lack of coordination of the extrinsic eye muscles. The person does not see double because the brain suppresses one of the images. This condition used to be referred to as "cross eyes."

Astigmatism

A condition is caused by an irregular curvature in the cornea, the most frequent cause, or by an irregular curvature of the lens. This condition prevents images form being focuses on the retina. This results in blurred or distorted vision.

Definition of Refraction of Light Rays

The bending or deflection of light rays as they pass through the various media of the eye.

conjunctiva, cornea, aqueous humor, lens, and vitreous body.

The media involved with the Refraction of Light Rays will be the:

Fovea Centralis

The parallel light rays are then bent sufficiently to fall exactly on what? (where vision is the sharpest)