I. Introduction The human eye is one of the most important things to a person and thus has many forms and protection. Do you know that there is also physics in your eyes. The human eye is a complex anatomical device that remarkably demonstrates the architectural wonders of the human body. Like a camera, the eye is able to refract light and produce a focused image that can stimulate neural responses and enable the ability to see. The human eye is an adjustable lens system consisting out of two focussing elements and a light receptor sysytem.
In this term paper, we will focus on the physics of sight. We will use our understanding of refraction and image formation to understand the means by which the human eye produces images of distant and nearby objects. Additionally, we will investigate some of the common vision problems that plague humans and the customary solutions to those problems.
Each part of the eye plays a distinct part in enabling humans to see. In the body of my term paper, we will know the important parts of our eye. And how they are related with each other. We will also know the relation of the human eye with the camera.

II. Body There are imporant parts of the eye, if they will be damaged, your eyesight will not function. The eye is essentially an opaque eyeball filled with a water-like fluid. These are the parts that can be located in our eyes: the cornea, the iris and pupil, the lens, the retina, the fovea centralis, and the optic nerve. In the front of the eyeball is a transparent opening known as the cornea. The cornea is a thin membrane that has an index of refraction of approximately 1.38. The cornea has the dual purpose of protecting the eye and refracting light as it enters the eye. The radius of the cornea is 7.5 mm. this is close to the typical diameter of the eyeball which is 25 mm. Focussing are reduced, main focussing is done by second lens. The dioptric strenght of the eye D=1/f[m]; as larger focal length as smaller the dioptric strenght. The main focussing is done by the cornea, the lens does the fine tuning to obtain sharp focussing short image distance. After light passes through the cornea, a portion of it passes through an opening known as the pupil. Rather than being an actual part of the eye's anatomy, the pupil is merely an opening. The pupil is the black portion in the middle of the eyeball. Its black appearance is attributed to the fact that the light that the pupil allows to enter the eye is absorbed on the retina (and elsewhere) and does not exit the eye. Thus, as you sight at another person's pupil opening, no light is exiting their pupil and coming to your eye; subsequently, the pupil appears black. Like the aperture of a camera, the size of the pupil opening can be adjusted by the dilation of the iris. The iris is the colored part of the eye - being blue for some people and brown for others (and so forth); it is a diaphragm that is capable of stretching and reducing the size of the opening. In bright-light situations, the iris adjusts its size to reduce the pupil opening and limit the amount of light that enters the eye. And in dim-light situations, the iris adjusts so as to maximize the size of the pupil opening and increase the amount of light that enters the eye. The most commonly known parts of the eye are the iris and the pupil. The iris is made of connective tissue and muscle that contains a hole in the midst of them. The iris is the part of the eye that is colored, either brown, green, blue, hazel, etc. The color is a result of the amount of pigment that is found in the eye, brown eyes have a lot of pigment, and light eyes, such as blue, have little pigment. The pupil is the hole in the eye that allows light to reach the retina. The pupil widens in a darker room to allow more light in, and narrows when in the light to restrict the amount of light to reach the retina. The pupil is adjusted by the iris muscles. The aperture area of the pupil can vary between 1.8mm2 and 51mm2. The response time of the iris with respect to changes in light intensity is =0.3-0.8s. The lens is located in front of the iris. It is an elastic body, it stretches and returns to its former place. It is biconvex, and almost transparent. Directly in front of the lens is the aqueous, and directly behind it, the vitreous. Light that passes through the pupil opening, will enter the crystalline lens. The crystalline lens is made of layers of a fibrous material that has an index of refraction of roughly 1.40. Unlike the lens on a camera, the lens of the eye is able to change its shape and thus serves to fine-tune the vision process. The lens is attached to the ciliary muscles. These muscles relax and contract in order to change the shape of the lens. By carefully adjusting the lenses shape, the ciliary muscles assist the eye in the critical task of producing an image on the back of the eyeball. The lens is one of the items in a persons body that deteriates with age, like many other body parts. The lens will lose its elasticity, which reduces the ability for it to change shape easily and results in a person needing a type of aid for nearby objects, often in the form of reading glasses.

The purpose of the lens is to refract light that enters the eye through the pupil and gives it the ability to focus on the retina. The lens also contains the ability to absorb ultraviolet radiation and is thus kept from reaching the retina. This absorption of ultraviolet radiation is actually what leads to cataracts--a lack of transparency of the lens--usually found in older people. The back side of the eyeball is the retina(area=50-60cm2). The retina is coveres with light sensitive photo receptor systems which convert by photo induced chemical reactions the light into electrical energy. The retina contains the rods and cones that serve the task of detecting the intensity and the frequency of the incoming light. An adult eye is typically equipped with up to 120 million rods that detect the intensity of light and about 6 million cones that detect the frequency of light. These rods and cones send nerve impulses to the brain. The nerve impulses travel through a network of nerve cells. There are as many as one million neural pathways from the rods and cones to the brain. This network of nerve cells is bundled together to form the optic nerve on the very back of the eyeball. Cones are responsible for daylight absorption, sensitive to fine detail and color, cones have direct nerve connection to the brain. Rods are responsible for night and peripheral vision-nerve connection per multitude of rods, therefore sensitive at low intensity. Cones and rods are covered by nerve tissue except in small center area =0.3mm2(diameter 3 mm), the fovea centralis(yellow spot) area of high sensitivity. Fovea centralis determines the size for sharp images and therefore limits the view to only small objects; to see a 2m man well focused requires an object distance of 16.6m. Spherical aberrations are due to differences in focal length for light rays at the outer edges of the lens and rightrays at the center of the lens. In human eye, this effect is corrected by small pupil size which cuts out light from outer edges of the cornea. Chromatical aberrations are due to wavelenght dependence of the index of refraction ; light with different wavelenght has different focal length. As a consequence at identical distance, blue objects appear to be further away than red objects. Normal eye is emmentropic eye. Relaxed (focus on far distance object);D=50. Focused at near point: d=54. Near point=25 cm. far point=infinity. Any defects from normal sight are called ametopia. Near sightedness is also known as myopia. Axial length of the eye is to long – deformation along optical axis. Focus point is in front of retina. Far point is closer than normal. Requires correction with a diverging lens. Far sightedness is also known as hyperopia. Axial length is too short, focus point is behind retina. Near point is further away than normal. Requires correction with converging lens. Astigmatism is when the light rays do not all come to a single focal point on the retina, instead some focus on the retina and some focus in front of or behind it. This is usually caused by a non-uniform curvature of the cornea. A typical symptom of astigmatism is if you are looking at a pattern of lines placed at various angles and the lines running in one direction appear sharp whilst those in other directions appear blurred. Astigmatism can usually be corrected by using a special spherical cylindrical lens; this is placed in the out-of-focus axis.

III. Conclusion The eye is a complex organ of the body, and all of the aforementioned parts of the eye work together to give a person sight. The cornea, the pupil and iris, the lens, the retina, the fovea centralis and the optic nerve are the important parts of the human eye.

The most common way of describing how the eye works is to describe it as a camera. The pupil is the aperture through which light enters, and the retina is the film which records and sends images to the brain via the optic nerve.

Light enters the eye through objects around it. "Light is energy emitted in waves or rays. If an eye is to get a sharp image, the light rays must be bent when they enter the eye, which the cornea and the lens help to do, and thus provide proper focus for the retina. "Light rays can either be divergent, convergent or parallel. A light source always emits diverging rays. A convergent or parallel wave results when diverging rays reach a limiting aperture or opening, such as the pupil. Light is converged by a convex lens, and is diverged by a concave lens.

Each color of light has a different wavelength and energy. Red light has the longest wavelength and the least energy while violet has the shortest wavelength and the highest energy.

When a person sees something, light bounced off the object, and refracted from the cornea and the lens and an image is focused on the retina. The image is actually seen on the retina upside down, and is reversed when it is sent to the brain. Recall that the left side of the brain controls the right side of the body, and in remembering this, the fact that the left half of the visual field in each eye is sent to the right half of the brain.

Although an eye is likened to a camera, "the one major difference is that the focus of the eye is altered by changing the focal length (or power, P=1/f) of the lens, rather than by changing the distance between the lens and the retina. It is a surprise to most people to find out that the lens of the eye is not where all the refraction of incoming light rays takes place. Most of the refraction occurs at the cornea. The cornea is the outer membrane of the eyeball that has an index of refraction of 1.38. The index of refraction of the cornea is significantly greater than the index of refraction of the surrounding air. This difference in optical density between the air the corneal material combined with the fact that the cornea has the shape of a converging lens is what explains the ability of the cornea to do most of the refracting of incoming light rays. The crystalline lens is able to alter its shape due to the action of the ciliary muscles. This serves to induce small alterations in the amount of corneal bulge as well as to fine-tune some of the additional refraction that occurs as light passes through the lens material. There are also many defects of the eye: the myopia(near sightedness), the hyperopia(far sightedness) and the astigmatism.

IV. Bibliography

1. http://www.chm.bris.ac.uk/webprojects2002/upton/defects_of_the_eye .html
2. http://nd.edu/~nsl/Lectures/mphysics/Medical%20Physics/Part%20I.%20Physics%20of%20the%20Body/Chapter%205.%20Optical%20System%20of%20the%20Body/Chapter%205.%20Optical%20System%20of%20the%20Body.pdf
3. http://www.physicsclassroom.com/class/refrn/u14l6a.cfm
4. http://www.physicsclassroom.com/class/refrn/u14l6a.cfm
5. http://hyperphysics.phy-astr.gsu.edu/hbase/vision/eyedef.html

Physics of the Eye
Submitted to: mr. jose paulo dela cruz

Submitted by: stephen s. sabido IV-shockley