How Vision Works

Although small in size, the eye is a very complex organ. The eye is approximately 1 inch (2.54 cm) wide, 1 inch deep and 0.9 inches (2.3 cm) tall.

The tough, outermost layer of the eye is called thesclera. It maintains theshape of the eye. The front sixth of this layer is clear and is called thecornea. Alllightmust first pass through the cornea when it enters the eye. Attached to the sclera are themusclesthat move the eye, called theextraocular muscles.

Thechoroid(or uveal tract) is the second layer of the eye. It contains the blood vessels that supply blood to structures of the eye. The front part of the choroid contains two structures:

The iris has two muscles: Thedilatormuscle makes the iris smaller and therefore the pupil larger, allowing more light into the eye; thesphincter肌肉使虹膜大,瞳孔变小, allowing less light into the eye. Pupil size can change from 2 millimeters to 8 millimeters. This means that by changing the size of the pupil, the eye can change the amount of light that enters it by 30 times.

The innermost layer is theretina-- the light-sensing portion of the eye. It containsrod cells, which are responsible for vision in low light, andcone cells, which are responsible for color vision and detail. In the back of the eye, in the center of the retina, is themacula. In the center of the macula is an area called thefovea centralis. This area contains only cones and is responsible for seeing fine detail clearly.

The retina contains a chemical calledrhodopsin, or "visual purple." This is the chemical that converts light into electrical impulses that the brain interprets as vision. The retinal nerve fibers collect at the back of the eye and form theoptic nerve, which conducts the electrical impulses to the brain. The spot where the optic nerve and blood vessels exit the retina is called theoptic disk. This area is a blind spot on the retina because there are no rods or cones at that location. However, you are not aware of this blind spot because each eye covers for the blind spot of the other eye.

When a doctor looks at the back of your eye through an ophthalmoscope, here's the view:

Inside the eyeball there are two fluid-filled sections separated by the lens. The larger, back section contains a clear, gel-like material calledvitreous humor. The smaller, front section contains a clear, watery material calledaqueous humor. The aqueous humor is divided into two sections called the anterior chamber (in front of the iris) and the posterior chamber (behind the iris). The aqueous humor is produced in the ciliary body and is drained through thecanal of Schlemm. When this drainage is blocked, a disease calledglaucomacan result.

Thelensis a clear, bi-convex structure about 10 mm (0.4 inches) in diameter. The lens changes shape because it is attached to muscles in the ciliary body. The lens is used to fine-tune vision.

Covering the inside surface of the eyelids and sclera is a mucous membrane called theconjunctiva, which helps to keep the eye moist. An infection of this area is calledconjunctivitis(also called pink eye).

The eye is unique in that it is able to move in many directions to maximize the field of vision, yet is protected from injury by a bony cavity called theorbital cavity. The eye is embedded in fat, which provides some cushioning. The eyelids protect the eye by blinking. This also keeps the surface of the eye moist by spreading tears over the eyes. Eyelashes and eyebrows protect the eye from particles that may injure it.

Tears are produced in thelacrimal glands, which are located above the outer segment of each eye. The tears eventually drain into the inner corner of the eye, into the lacrimal sac, then through the nasal duct and into the nose. That is why your nose runs when you cry.

There are six muscles attached to the sclera that control the movements of the eye. They are shown here:

Primary muscles and functions:

In the next section, you'll learn how the eye perceives light.

When light enters the eye, it first passes through the cornea, then the aqueous humor, lens and vitreous humor. Ultimately it reaches theretina,这是light-sensing structure of the eye. The retina contains two types of cells, called rods and cones.Rodshandle vision in low light, andconeshandle color vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. The chemical that is formed (activated rhodopsin) creates electrical impulses in the optic nerve. Generally, the outer segment of rods are long and thin, whereas the outer segment of cones are more, well, cone shaped. Below is an example of a rod and a cone:

The outer segment of a rod or a cone contains the photosensitive chemicals. In rods, this chemical is calledrhodopsin; in cones, these chemicals are calledcolor pigments. The retina contains 100 million rods and 7 million cones. The retina is lined with black pigment calledmelanin-- just as the inside of acamerais black -- to lessen the amount of reflection. The retina has a central area, called themacula, that contains a high concentration of only cones. This area is responsible for sharp, detailed vision.

When light enters the eye, it comes in contact with the photosensitive chemical rhodopsin (also calledvisual purple). Rhodopsin is a mixture of a protein calledscotopsinand11-cis-retinal-- the latter is derived fromvitamin A(which is why a lack of vitamin A causes vision problems). Rhodopsin decomposes when it is exposed to light because light causes a physical change in the 11-cis-retinal portion of the rhodopsin, changing it toall-trans retinal. This first reaction takes only a few trillionths of a second. The 11-cis-retinal is an angulated molecule, while all-trans retinal is a straight molecule. This makes the chemical unstable. Rhodopsin breaks down into several intermediate compounds, but eventually (in less than a second) formsmetarhodopsin II(activated rhodopsin). This chemical causes electrical impulses that are transmitted to thebrainand interpreted aslight. Here is a diagram of the chemical reaction we just discussed:

Activated rhodopsincauses electrical impulses in the following way:

Eventually, rhodopsin needs to be re-formed so that the process can recur. The all-trans retinal is converted to 11-cis-retinal, which then recombines with scotopsin to form rhodopsin to begin the process again when exposed to light.

The color-responsive chemicals in the cones are calledcone pigmentsand are very similar to the chemicals in the rods. The retinal portion of the chemical is the same, however the scotopsin is replaced with photopsins. Therefore, the color-responsive pigments are made of retinal and photopsins. There are three kinds of color-sensitive pigments:

Each cone cell has one of these pigments so that it is sensitive to that color. The human eye can sense almost any gradation of color when red, green and blue are mixed.

In the diagram above, the wavelengths of the three types of cones (red, green and blue) are shown. Thepeak absorbancyof blue-sensitive pigment is 445 nanometers, for green-sensitive pigment it is 535 nanometers, and for red-sensitive pigment it is 570 nanometers.

Color blindness is the inability to differentiate between different colors. The most common type is red-green color blindness. This occurs in 8 percent of males and 0.4 percent of females. It occurs when either the red or green cones are not present or not functioning properly. People with this problem are not completely unable to see red or green, but often confuse the two colors.

This is aninherited disorderand affects men more commonly since the capacity for color vision is located on theX chromosome. (Women have two X chromosomes, so the probability of inheriting at least one X with normal color vision is high; men have only one X chromosome to work with.Click herefor more on chromosomes.). The inability to see any color, or seeing only in different shades of gray, is very rare.

For more on color blindness,click here.

When severe vitamin A deficiency is present, thennight blindnessoccurs.

Vitamin A is necessary to formretinal, which is part of the rhodopsin molecule. When the levels of light-sensitive molecules are low due to vitamin A deficiency, there may not be enough light at night to permit vision. During daylight, there is enough light stimulation to produce vision despite low levels of retinal.

When light rays reach an angulated surface of a different material, it causes the light rays to bend. This is calledrefraction. When light reaches a convex lens, the light rays bend toward the center:

When light rays reach a concave lens, the light rays bend away from the center:

The eye has multiple angulated surfaces that cause light to bend. These are:

When everything is working correctly, light makes it through these four interfaces and arrives at the retina in perfect focus.

Vision orvisual acuityis tested by reading a Snellen eye chart at a distance of 20 feet. By looking at lots of people, eye doctors have decided what a "normal" human being should be able to see when standing 20 feet away from an eye chart. If you have 20/20 vision, it means that when you stand 20 feet away from the chart you can see what a "normal" human being can see. (In metric, the standard is 6 meters and it's called 6/6 vision). In other words, if you have 20/20 vision your vision is "normal" -- a majority of people in the population can see what you can see at 20 feet.

If you have 20/40 vision, it means that when you stand 20 feet away from the chart you can only see what a normal human can see when standing 40 feet from the chart. That is, if there is a "normal" person standing 40 feet away from the chart, and you are standing only 20 feet away from the chart, you and the normal person can see the same detail. 20/100 means that when you stand 20 feet from the chart you can only see what a normal person standing 100 feet away can see. 20/200 is the cutoff for legal blindness in the United States.

You can also have vision that is better than the norm. A person with 20/10 vision can see at 20 feet what a normal person can see when standing 10 feet away from the chart.

Hawks, owls and other birds of prey have much more acute vision than humans. A hawk has a much smaller eye than a human being but has lots of sensors (cones) packed into that space. This gives a hawk vision that is eight times more acute than a human's. A hawk might have 20/2 vision!

Normally, your eye can focus an image exactly on the retina:

Nearsightedness and farsightedness occur when the focusing is not perfect.

Whennearsightedness(myopia) is present, a person is able to see near objects well and has difficulty seeing objects that are far away. Light rays become focused in front of the retina. This is caused by an eyeball that is too long, or a lens system that has too much power to focus. Nearsightedness is corrected with aconcave lens. This lens causes the light to diverge slightly before it reaches the eye, as seen here:

Whenfarsightedness(hyperopia) is present, a person is able to see distant objects well and has difficulty seeing objects that are near. Light rays become focused behind the retina. This is caused by an eyeball that is too short, or by a lens system that has too little focusing power. This is corrected with aconvex lens, as seen here:

SeeHow Refractive Vision Problems WorkandHow Corrective Lenses Workfor details.

Astigmatismis an uneven curvature of the cornea and causes a distortion in vision. To correct this, a lens is shaped to correct the unevenness.

Why does vision worsen as we age?

As we grow older, the lens becomesless elastic. It loses its ability to change shape. This is calledpresbyopia比较明显,当我们试图看事情that are close up, because the ciliary body must contract to make the lens thicker. The loss of elasticity prevents the lens from becoming thicker. As a result, we lose the ability to focus on close objects.

At first, people begin holding things farther away in order to see them in focus. This usually becomes noticeable when we reach our mid-forties. Eventually, the lens is unable to move and becomes more or less permanently focused at a fixed distance (which is different for each person).

To correct this,bifocalsare required. Bifocals are a combination of a lower lens for close vision (reading) and an upper lens for distance vision.

The eye uses three methods to determine distance:

Legal blindness is usually defined as visual acuityless than 20/200with corrective lenses. Now that you have learned some anatomy of the eye and how it functions, it becomes easier to understand how the following conditions can lead to blindness:

There are many other causes of blindness, such as vitamin A deficiency, tumors, strokes, neurological diseases, other infections, hereditary diseases and toxins.