HOW WE SEE
The human eye is the organ of sight in the human body.
In order for humans to see light must be present.
This along with the brain helps us to see images.
At the front of the eye is a transparent protective covering called
the cornea. It is transparent so that light will be able to pass through
the cornea covers the white part that we see. The white part is called
In the middle of the eye is an opening or a hole. This hole is the pupil.
This is where the light enters the eye. Surrounding the pupil is a
coloured membrane known as the iris. It has muscles which helps it
to contract or expand and control the amount of light entering the eye.
In bright light the muscles of the iris will contract and make the pupil smaller. In dim light the muscles of the iris expand. The pigment in the iris will determine its colour.
Behind the pupil and iris is the lens. The lens in the eye is a convex lens which helps to focus or bring light rays together when they enter the eye. The light rays are focused to the back of the eye onto the retina. The retina contains light sensitive cells called rods and cones. Rods allow us to see in black and cones allow us to see in colour.
An inverted image is formed on the retina, attached to the retina is the opic nerve. The optic nerve sends a signal to the brain which corrects the inverted image and we are able to see the image correctly.
Inside the eyeball is a jelly like substance called vitreous humor which keeps the eyeball in place and gives its shape.
A clear image must be formed on the retina
in order for us to see.
When light is reflected from the object, it passes through the pupil to the lens. The lens bends the light rays so that they come together at a point of focus and forms an upside down image on the retina.
The optic nerve which is attached to the retina then sends a signal to the brain so that the brain will then interpret the image and turn it right side up, allowing us to see the object clearly.
Two of the most common vision problems are Near-Sightedness (Myopia) and Far Sightedness (Hyperopia).
A person who is near-sighted is able to see near objects clearly but unable to see far objects clearly.
Persons with near-sightedness have long eyeballs and hence when the the light rays come together at the focus it falls in front of the retina. This problem may be corrected with a concave lens.
A person who is far-sighted is able to see distant objects clearly but not able to see close objects clearly.
In persons with far-sightedness the eyeballs are short and when the light rays come together at the focus they fall behind the retina.
This problem may be corrected with a convex lens.