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Beauty of the World Experienced Only Through Sight
Visual reception involves the picking up of electromagnetic energy by receptors. Light waves stimulate these receptors to send impulses to the brain where they are interpreted as visual images. Visible light waves vary from 430nm to 750nm. Shorter wavelengths like x-rays and gamma rays are undetectable by receptors of any animal. Higher wavelengths like radio waves are also undetectable by animals. However, several animals can detect infrared or heat.
Humans are highly visual species. We, like many other animals, rely on sight to hunt for food and to escape from danger. It is also important for the mere co-ordination of movement and maintaining balance. The human eye is a very simple yet miraculously efficient tool for sight. It consists of mainly three layers.
The sclera is a tough, white layer on the outermost of the eye. It forms the transparent cornea at the front bulge of the eye. The sclera supports the shape of the eye and is on which the voluntary muscles are attached which in turn moves the eye. The cornea allows light to enter the eye and aids in focusing the light to the retina. The cornea has no blood capillaries and thus relies on diffusion for supply of oxygen.
The choroid layer lies immediately under the sclera. It forms the iris behind the cornea. The iris is a thin diaphragm of muscles which are able to contract and dilate antagonistically to change the size of the pupil, which the opening which allows light to enter the eye. Immediately behind the iris is the lens.
Held by another set of muscles, the lens is able to change shape accordingly to focus the bean of light onto the retina. The shape of the lens is controlled by the muscles which pull the suspensory ligaments which connect the muscles to the lens.
Lastly, there is the retina. The retina is the light sensitive area where the receptors are and where light in absorbed and impulses are stimulated and sent via the optic nerve as the end of the eye to the brain.
The retina consists mainly of 4 layers of cells. The inner most layer is pigmented the absorb light, hence preventing reflection of light within the eye and causing multiple, confusing images to be formed. Just above the pigmented layer are the rods and cones. As their name suggests, they are in the shape of rods and cones. They are light sensitive cells which when stimulated, activate the bipolar layer of cells above them and they in turn stimulate the ganglion cells layer above them. The ganglion cells would then form together into the optic nerve which would then bring the impulses to the brain.
Rods are most active in dim light. They are responsible for black, white and grey images and not coloured images. The light detecting regions on the rods are stacked disks containing rhodopsin, a photoactive pigment that breaks down in the presence of light. The chemical reaction stimulated the creation of neural impulses in the bipolar and ganglion cells. The rods outnumber the cones 18 to 1. They provide what we call night vision.
Cones are found in three types, the red sensitive, blue sensitive and the green sensitive. The reason why we can see much more colours then these three is due to the fact that the colour sensitivities of these cones overlap. Proteins attached to these cones determine the colour sensitivity of the cone.
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