1. The Cornea Light enters the eye through the cornea
The cornea is transparent so that light can travel through it.
Seventy-five percent of the eye’s power rests in the cornea, although it is very small. It is only about half a millimeter thick.
The cornea is connected to the sclera, the tough white part of the eye.
The cornea acts as a lens, bending rays of light as they enter the eye
Each time you blink, your eyelids cleanse and moisten the cornea

2. The Iris
The iris is a ring of muscle that contracts and expands to change the amount of light that enters the eye
The iris contracts when light coming in is bright:
The iris expands when the light coming in is dim:
Iris gives your eye color

3. The pupil The pupil is the part of the eye that looks black
It is actually a hole covered by the clear cornea
The pupil looks black because it is an opening into the dark inside of the eye
The iris determines how large the pupil is depending on the amount of light entering the eye.

4. The Lens Directly behind the pupil is the lens
The lens of your eye is convex, however, muscles attached to the lens will change shape to help focus the light entering the eye (this process begins with the cornea).
The lens refracts light, forming an image on the lining of your eyeball
When you focus on a distant object, the ciliary muscles holding the lens contract, making the lens longer and thinner
When you focus on a nearby object, the muscles relax and the lens becomes shorter and fatter
Behind the lens is the vitreous humor. This is a thick, white fluid that gives the eyes their firm, rubbery texture. Most of your eye is filled with vitreous humor fluid.

5. The Retina
The layer of the cells lining the inside of the eyeball is the retina
As the cornea and lens refract light, an upside-down image is formed on the retina
The retina is made up of light sensitive cells called rods and cones
Rods contain pigment that reacts to small amounts of light and distinguish between black, white, and shades of gray. Rods are responsible for night vision
Cones respond to color.
Three types of cones: red, green, and blue
Cone cells only function in bright light (this is why it is difficult to see colored in dim lighting)

7. Optic Nerve and the brain
Signals generated by the rods and cones travel to your brain along a short, thick nerve call the optic nerve.
When the signals reach your brain, it automatically turns the image right-side-up
Your brain combines both images (one from each eye) into a single three-dimensional image
Your optic nerve creates a blind spot- one spot of the retina where there are no cones or rods and where the optic nerve begins

9. Diagram

10. Correcting Vision
Some lenses in eyeglasses are convex and some are concave . The type of lens used depends on whether the eyeball is too long or too short
If the eyeball is too long it results in nearsightedness
If the eyeball is too short, it results in farsightedness

11. Nearsightedness
A nearsighted person can see nearby things clearly, but objects at a distance appear blurry
Lens focuses the image in front of the retina
Concave lenses can help correct nearsightedness because the lens spreads out the rays a little before they enter the lens of the eye

12. Farsightedness
A farsighted person can see distant objects, but nearby objects appear blurry
The lens focuses the rays of light so that they would meet behind the retina, the image that falls on the retina is out of focus
Convex lenses are used to help correct farsightedness because they make the rays bend toward each other a little before they enter the eye