For most of us, vision - the act of being able to see an object - is the beginning of understanding. That's why when we understand something we often replace the phrase "I understand" with "I see." No other sense of perception comes close to eyesight, in terms of affecting our lives, and our ability to perform in the world we live in. Of course, all our senses are continually functioning, but if we don't hear anything for a few minutes, or don't smell, taste or feel anything, we are fine. The fact that we are not receiving any sensory input from the relevant organs does not worry us - when there is something to smell, touch or feel, we will know. But, if we are deprived of our vision for even a few minutes, we tend to feel insecure and cut off from the rest of the world.
The eyes are not only one of the most complex organs in our bodies; they are also one of the most expressive. Along with the mouth they are the organs by which the world can tell how we are feeling and what our emotions are. In the case of the eye, most of the emotion is conveyed by the eyelids - they narrow when we concentrate and open wide when we are amazed. The eye itself has only one part that changes in a manner that is visible to others and that is the pupil.
The pupil is the circular opening in the middle of the iris. It opens and closes to control the amount of light that is allowed to enter the eye. This light is then focused on the retina, which is the layer of light sensitive cells at the back of the eye.
To better understand the pupil, we should look at it in terms of the way a camera is made. Light enters the lens of the camera and passes through the shutter when it opens and reaches a sensor when the image conveyed by the light is recorded. The shutter opening or aperture is increased or reduced to control the amount of light that enters the camera. When external conditions are bright, the aperture is small since too wide an opening would allow excess light to enter which could affect the image. Similarly when external light is low, the aperture opens to the maximum to allow the largest amount of light to enter the camera so as to produce as bright an image as possible.
This is exactly the role that the pupil plays in the eye. The pupil reacts to external light and changes its size accordingly. That is why when we go into bright sunlight we involuntarily squint - we almost close our eyes to limit the sudden excess of light entering. Once the pupils have adjusted to the light by becoming smaller, we no longer need to squint. Similarly when we go into a dark room it takes a few moments for our pupils to open wide so that the eyes can receive the maximum possible light so as to be able to see as much as possible.
There are times when the external light is so bright that even when the pupils are at their smallest, the amount of light entering the eye is still too much for the retina to accept comfortably. That's why wearing dark glasses, which reduce the amount of light, brings us such relief.
The pupils also react to certain emotions and this can make them change their size. They tend to become small when a person is angry or doubtful, and open wider when a person is pleased or surprised. These emotion caused changes in the size of the pupils are seen by others and are interpreted as conveying these emotions.
No other part of the eye undergoes any observable change, which is why the size of the pupil is so noticeable by others.
The cornea is a clear cover that envelopes the eye to protect it from external damage. But excess strong light can also hurt the eye by damaging the retina which can be scarred by and excess of light hitting it. It is the pupil that acts as the guardian and controls the amount of light hitting the retina.
Those who suffer from sensitivity to strong light either have pupils that are slow to react to bright lights and reduce their size or have had their retinas scarred by a sudden burst of bright light that the pupils could not control.
Anyone who has ever gone for an eye examination and has had his eyes dilated, and has gone into bright light knows how painful it can be when the pupils are unable to respond to the change in light.
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