The Human Eye

Luke Seavitt

Physics blog post

Wirth

The Eye:

Although the human eye is one of the smallest human organs it is one of the most complex. It is composed of several different parts that all work together to help us see the objects that we look at. In order for the eye to maintain its shape it has something called the sclera which is the outermost layer of the eye. The front sixth of the sclera is called the cornea and this part of the eye is also clear. All light that is seen by the human eye has to pass through the cornea. Also attached to the sclera are muscles that provide movement to the eye. These muscles are called extrocular muscles. Blood vessels that supply blood to the eye are located in the second layer of the eye which is called the choroid. The front part of the choroid is composed of two parts the ciliary body and the iris. The ciliary body contracts and relaxes to control the size of the lens for focusing. The iris is the colored part of your eye and is determined by the color of the tissue and pigment cells. The iris itself has two muscles. It has the dilator muscle which makes the pupil larger allowing more light through the eye and making the iris smaller. The sphincter does the opposite of the dilator and makes the pupil smaller by increasing the size of the iris allowing for less light to go through the eye. The intermost layer of the eye is the retina which is the light sensing part of the eye. It contains cells called rod cells that are in charge of vision in low light. It also has cone cells which are responsible for color vision and detail. Another part of the retina is located in the back called the macula. In the center of the retina is something called the fovea centrails which is responsible for seeing small details. Also in the retina is something called the rhodopsin that converts light into electrical impulses to send to the brain to interpret the visions. The retina also has nerve fibers at the back of the eye which form the optic nerve, which conducts the electrical impulses to the brain. The spot where the optic nerve and blood vessels exit the retina is called the optic 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. In order for the scela to move the eye it has six different muscles to move the eye. The medial rectus which moves the eye towards the nose and the lateral rectus which does the opposite of the medial rectus and moves the eye away from the nose. It also has the superior rectus which raises the eye and the inferior rectus which lowers the eye. Lastly it has the superior oblique and inferior oblique which both rotate the eye.

Light and Color:

The eye perceives light first through the cornea then the aqueous humor, lens and vitreous humor. Eventually the light reaches the retina which is the light sensing structure. It contains two cells which are rods and cones. The rods handle the vision in low light and the cones handle color vision and detail. When light contacts these two types of cells, a series of complex chemical reactions occurs. This creates electrical impulses that in the optic nerve. In order to see color the eye has color responsive chemicals in the cones called cone pigments and they are very similar to those found in the rods. The retinal portion of the chemical is the same, however the scotopsin is replaced with photopsins. There are three kinds of color-sensitive pigments, red, green, and blue sensitive pigments. Each cone has one of these pigments so it is sensitive to that color. The peak absorbancy of blue-sensitive pigment is 445 nanometers, for green-sensitive pigment it is 535 nanometers, and for red-sensitive pigment it is 570 nanometers.

Refraction:

The eye has multiple angulated surfaces that cause light to bend. These are the interface between the air and the front of the cornea, the interface between the back of the cornea and the aqueous humor. Also the interface between the aqueous humor and the front of the lens and the interface between the back of the lens and the vitreous humor. When every one of these are working perfectly, light makes it through these four interfaces and arrives at the retina in perfect focus.

Normal Vision and Vision Problems:

The normal vision of a human being is 20/20 vision. This is determined by looking at the Snellen eye chart at a distance of 20 feet. 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. However, not all people have perfect 20/20 vision. People can be nearsighted or farsighted meaning that they can either only see up close or they can only see from far away. To fix their vision many people wear eye glasses or contacts. There is also another problem that can occur with the eye called Astigmatism, which is an uneven curvature of the cornea and causes a distortion in vision. To correct this, a lens is shaped to correct the unevenness.

Bianco Md., Dr. Carl. "How Vision Works." How Stuff Works. 1998. 4 Mar. 2008 .

Myers, David G. Psycgolgy . VIII. New York: Worth Publishers, 2007.