Pupil
That dark spot in the center of your eye is a hole - somewhat like the dark opening of a cave. It's through this hole, which is covered by your cornea, that light passes into your eye.
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Iris
Surrounding your pupil is the iris, the colored part of your eye. Its color comes from a pigment called melanin in your iris tissue. The more pigment, the darker the color. Brown eyes have a lot of pigment. Blue or green eyes have less pigment. As you get older, the color may change as your iris loses some of this pigment.
But the iris adds more than color to your eye. The iris contains a ring of muscle fibers that can expand or contract the size of the pupil, and thereby control the amount of light that gets inside the eyeball. It's a bit like adjusting blinds to control the amount of sunlight coming through a window. When the light is bright, the iris reacts quickly to reduce the size of the pupil. When the light is dim, the iris enlarges the size of the pupil.
The muscles of your iris can react to more than light. Your emotions affect the size of your pupils. Anger can make them smaller. Excitement and pleasure can open them wider. Certain drugs can open (dilate) the pupils. Eye doctors use dilating drugs to get a better look inside your eyes during an examination.
The space between your cornea and iris is called the anterior chamber. It's filled with a clear fluid called aqueous humor, which nourishes the cornea and the lens, washes away waste products, and plays an important role in maintaining pressure in the eye.
Lens
Behind the iris and pupil is the lens, a clear, elliptical structure about the size and shape of an M&M's candy. A circular muscle surrounds the lens. As the muscle relaxes or contracts, the lens curvature changes to sharpen the focus of whatever you're looking at.
When an object is nearby, the muscle contracts and the lens thickens by its own elasticity. When an object is far away, the muscle relaxes and the lens stretches thin. These adjustments, known as accommodation, allow the lens to change its focusing power and sharpen the definition of the objects you're looking at. This variable focusing power fine-tunes the fixed focusing power of your cornea. As you get older, your lens can lose its elasticity, and you may have difficulty focusing on objects close by.
20/20 vision
It's great when an eye doctor says you have 20/20 vision. But that doesn't mean you have perfect vision. It simply means that you can see objects clearly from 20 feet away that an average of normal-sighted people can see clearly from 20 feet away. In other words it's a measure of your visual acuity - how sharply or clearly you can see something at a distance.
If you're nearsighted and have 20/50 vision, that means distant objects are fuzzy. In fact they're so fuzzy that what you see from 20 feet away is what people with normal vision generally can see from 50 feet away. Some people have sharper vision than 20/20. Some have 20/15 vision, or even 20/10.
There is no such thing as perfect vision. That's because many factors other than visual acuity affect your ability to see well. Even if you can see what you should from 20 feet away, your doctor will want to check your depth perception, color vision, peripheral vision and ability to focus on close objects. Many of these indicators are tested in a routine eye examination.
Vitreous cavity
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The vitreous cavity extends from the back of the lens to the retina at the back of the eyeball. It is filled with a clear, gelatinous substance called the vitreous humor or, simply, the vitreous. Together with the aqueous humor in the anterior chamber, the vitreous helps maintain the pressure and shape of the eyeball.
In order for light to pass through it, the vitreous is clear. You may occasionally notice what look like tiny bits of string or lint darting through your vision. These are called floaters, tiny bits of material forming in the vitreous. A sudden onset of or increase in floaters, especially when associated with flashing lights or hazy vision, can be a sign of potentially serious eye problems. To find out more, you can check out How To Improve Your Eyesight Without Glasses.