What is the structure of human eyes? What are the parts of an eye and their functions? How do we see?
An eye is spherical in shape and consists of three layers.
1. The outer layer, the sclerotic layer, is a thick, opaque, white layer, except in the front. The transparerit layer in front of the eye is called the cornea. Light enters your eye through it. You cannot see the cornea because it is like a thin sheet of glass. Do you see the colored circle? The color of this circle is either blue, brown, gray, green or a combination of two colors. This is the iris. The iris is the colored part of the eye. It controls the amount of light that enters the eye through the pupil. What do you think about the black spot at the center of the iris? This is called the pupil. Light travels through the cornea and then passes through this opening. The iris is a ring of muscle. it can make the pupil bigger or smaller since it controls the size of the pupil. In bright light (sun light) the pupil becomes smaller. In dim light, the pupil enlarges, letting more light into the eye.
Light passes through the pupil and hits he lens which is behind the pupil. The lens is like the lens of a camera or a magnifying glass. But there is a very important difference between the lens of an eye and the lens of a camera or a magnifying glass. The lens in the eye can change its own shape. There are ciliary muscles in your eyes that change the shape of the eye lens. This makes it possible for you to see objects clearly when they are near or when they are distant. To change the shape of the lens to project an image on retina is called accomodation.
2. The middle layer, of the eye is called the choroid layer. This layer is rich in blood capillaries which supply food and oxygen to t e eye. At the front of the eye the choroid layer forms the iris.
3. The innermost layer of the eye is retina. It contains light receiver cells (receptors), the rods and cones. Cones are sensitiye to bright light, and collect color information for the eye. Rods are not sensitive to color so they collect Black and white information. They respond to dim light and help us to see in dark. The point where the optic nerve is attached to the eye is called the blind spot. The blind spot has no receptors, so it is unable to see things.
The eye is filled with liquid. The front chamber is filled with a watery, liquid the aquous humor. The back chamber is filled with a jelly like material, the vitreous humor.
HOW DO WE SEE?
Light enters the eye through the pupil. The lens of the eye focuses the image onto the retina. Nerve impulses are produced in the retina by the image. These impulses travel along the optic nerve to the brain. The impulses are interpreted by the brain and changed into image. There are no rods or cones at the point where the optic nerve leaves the retina. This point is called the blind spot and there can be no vision at this point.