Eyes are believed by scientists to have evolved at about the same time as the first animals (during the Cambrian explosion) in one species and within a few million years had spread to most of the others. No other sense organ is more common among the animals, probably a measure of the eye's utility. With the eyes usefulness also comes it's vulnerability due to it being constructed of mostly soft tissue.
The human eye in particular is endowed with plenty of protection (eyelids, the eyeball lies within a protective shell made of bone, a membrane etc.) but people still manage to do damage to their eyes despite this. This is because, like any sensitive instrument, the eye needs extra care and protection in order for it to continue functioning optimally. That's why you need to take better care of your eyes if you want to continue enjoying the gift of sight.
Explaining the basic working principle of the eye is simple, despite this, human beings are yet to create a device that even comes close to matching the eye's function. This is because the light is not simply a light detector, but does what it does through a complex interaction with the brain. In fact, it would take a powerful super computer to model the visual cortex on it's own.
In favor of the claim that the eye is one of the most important sense organs in all organisms is the fact that there are more than 10 distinctive kinds of eyes present among various animal species. On top of that, it is believed that these evolved independently from each other. The fact that it is not just a few organisms that recognized the evolutionary importance of eyes, but ten different once, underlines the claim that eyes are of such great importance.
The human eye, in fact, is very limited, as it can only differentiate and perceive colors, depth and direction to a certain degree. Some animals have much greater sight, like some birds that can detect ultraviolet, or the mantis shrimp that has hyper-spectral vision. On the other hand, there are less evolved organisms whose eyesight is very basic. Some microorganisms' eyes only serve to differentiate between light and dark.
The human eye can easily be compared to a telescope, camera, microscope or other devices that use light focusing. Light first enters the iris, and is immediately focused onto a small patch of light-sensitive cells at the back. The cells transform the photons into neural signals. In order the change the quantity of light entering the eye, the iris changes size.
Things are relatively simple before the light reaches the rods and cones (the light sensitive cells) but afterwards become appreciably more complex and as yet no convincing explanation of how people actually see has been put forward by the scientific community. The eye is a fascinating instrument and more still remains to be known about it. This is one reason why everyone must take every precaution to keep his or hers healthy, imagine losing such a wonderful gift!
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