Structure of Eyeball
Eye is one of the receptor organs of the body which respond to a stimuli. The reason for it being a receptor organ is that it senses light and images and responds by letting you see the images. Before understanding how it works first you need to understand its Structure for ease of understanding.
The eyeball is located in the vortex of our skull. It is attached to the vortex by the rectus muscles which help it to move around as well. The eyeball has three layers. The outer layer being the sclera. It is the white layer which continues around the eyeball and is identified as the white of the eye on the exposed part. The sclera is risen in front of the pupil making the cornea. Here the sclera turns transparent which helps the light to pass through the cornea and onto the pupil.
The exposed part of the eyeball is covered with a layer called the conjunctiva which protects the eyeball from dust. The inner layer of the eyeball is called the choroid which continues along the sclera. It is pigmented black to prevent reflection. The choroid forms the ciliary body which is connected to the eye lens with the suspensory ligaments, it also forms the iris in front of the lens which forms the pupil of an eye. The iris may differ in color from brown to green to blue to grey. The inner most layer of the eye ball is called the retina which is light sensitive. It has many cones and rods in it which are photo-receptor cells. When light falls onto it images are formed.
There are two chambers in a cell which are divided by the eye lens. The chamber in front of it which is the cornea has aquas humor in it which is a watery substance while the chamber behind the lens has vitreous humor in it which is gel like substance. This is why when the light enters in the eye refraction takes place.
The retina is connected to the optic nerve which sends impulses to the brain about the images formed in the eye. The retina has two main spots. One is the fovea or the yellow spot which is rich in cones. Here images are formed clearly in bright light. The second spot is the blind spot found directly on the optic nerve which has neither rods nor cones. When light touches this spot you cannot see a thing.
There are tear glands found behind the eyeball also called the lachrymal gland. It releases tears according to the emotion of a human being. The gland also releases tears when the eyelids close to clear of the dust on the eyeballs and to lubricate the eye so that there is no friction. The excess tears drain off to the side of the eyes and then through the tear duct. This then flows through tear sac and then on to the nasolacrimal duct.
To fully understand how eyes work you should know that the retina has rods and cones in its layers. The rods have visual purple in it which is why we can see well enough in dark or dim light. When we see a bright light the visual purple is bleached. When we enter a dark place again the visual purple is formed again which is why it is hard to see in dark for a few seconds. Vitamin A is required to create the visual purple. If someone is deficient in vitamin A he cannot see in dim light or dark which is why we say that he is suffering from night blindness.
The cones sense light. This helps us to see in bright places. The cones are rich in the fovea which is why we can see clearly in bright light. The cones are of red, blue and green color which forms a complete spectrum. Any person deficient in any of these color cones is said to be suffering from color blindness. When light touches the blind spot we cannot see a thing.
Controlling Amount Of Light entering our Eyes
The amount of light entering our eyes is controlled by our eyelids and eyelashes. When there is bright light then eyelids squint causing less light entering our eyes.
The iris is also involved in this. The iris has circular muscles and the radial muscles which helps in controlling the size of the pupil. When there is bright light the circular muscles contract and radial muscles relax the pupils become smaller but when the radial muscles contract and the circular muscles relax the pupils become larger in dim light.
How Eyes work
Light first interacts on an object and then reflects. Some of the reflected light enters the eye. When it passes through the cornea it is refracted, and then when it passes through the iris and then through the lens refraction takes place again. This light then enters the vitreous humor and then touches the retina. On the retina the images formed is upside down, smaller in size than the original object and is also back to front.
The impulses formed is send to the brain by the optic nerve. The brain then interprets the images so that we see the object clearly and the right way. In case of vision the distance of the object also counts. We can see a distant and a close object clearly because of the ciliary muscles action.
The ciliary muscles relax which is why the suspensory ligaments pulls the lens which is flexible. The lens tightens which helps in refraction even with a distant object. When we see a close object the ciliary muscles contract and the lens loosens which helps in seeing the close object. However if the object comes too close to the eye the vision is blurred.
Humans and predator animals have eyes set together straight. Each eye sees an object differently. The brain interprets these images and forms a 3 dimensional image. This is called the stereoscopic vision.