The Human Eye and Vision: a Fascinating Phenomenon

The human eyes are organs which reacts to light and pressure to give us vision.

The eyes are at work as long as we are awake, taking in mega loads of information; the color, the shapes, movements, etc. These are sent to the brain for processing, so we could go about our normal businesses without bothering about bumping into that electric utility pole since we can see it!

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A Simplified Version of How The Human Vision Work

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1. The light enters the eye through the cornea (outermost part of the eye ) which refracts (bends it). See fig.2 above.
   
   
2. The refracted light is directed to the pupil, which is a small hole in the center of the iris. The iris can open or change the size of the pupil to allow more or less light, much the same way you can adjust a camera's aperture to allow more or less light to get to the film.
   
   
3. The light is redirected one more time to the eye's lens that points the light to the nerve cells at the back of the eye
   These nerve cells are of two types:
   i Cones: detects colors and other fine details in good light. It is positioned in the center of the back of the eye.
   ii Rods: detects the presence of objects in bad/poor light and is concentrated on the sides of the back part of the eye.
   
   
4. The nerve cells (cones and rods) then sends the light signal to the brain for interpretation.

Even though we see objects upside down the brain corrects it and shows it the right way. But the human brains is really fast, it interprets an image that the eye sees in just 13 milliseconds. The brain is capable of storing and processing an image that was presented to it in as little as 13 milliseconds. 1 milliseconds is a thousandth of a second or 1/1000 second.

We blink at an average of 17 times a minute, 14,280 times per day and 5.2 million times a year!

Every time you blink, your eyelids spread a cocktail of oils and mucous secretions across the surface of the eye to keep your globes from drying out. Blinking also keeps eyes safe from potentially damaging stimuli, such as bright lights and foreign bodies like dust.

20/20 Vision

You must have heard of someone being said to have a 20/20 vision. But what does that mean?

The standard eye chart used for visual acuity testing is known as a Snellen chart and was developed in 1862 by Hermann Snellen, although it has been modified several times since. The big E at the top measures 20/200 vision so reading that is no big deal. The chart has rows of letters which get progressively smaller. 20/20 vision is actually line 8 so you need to be able to read quite a few of the upper lines to have even reasonable eyesight. Eye exams test one eye at a time as our eyesight is different in each eye.

^{Source. Snellen Chart}


20/20 vision is from the visual acuity test measured with a Snellen chart. The visual acuity (VA) is the clarity of vision. The chart shows numbers in decreasing order of progress. The "normal" vision is 20/20 which means that a test person will see same letters standing at 20 feet away from the chart, that a person with a normal vision could see at 20 feet.

In Australia, this 20/20 vision is referred to as 6/6 vision due to the metric system in use there. So instead of 20 feet it is 6meters.

If someone has a 20/40 vision, this means the vision is half as good as a normal vision. You can see only half as good as a normal person can see at 40 feet away from an eye chart. So a normal person with 20/20 vision would see the same details from the Snellen chart standing 40 feet away as would a 20/40 vision person standing 20 feet away.

Levels of Vision

• 20/20 - Normal vision. A minimum requirement for fighter pilots.
• 20/40 - Able to get a driver's license in all 50 states of the US. Can read most printed materials
• 20/80 - Can read an alarm clock at 10 feet (Newspaper headline print size)
• 20/200 - In the US, the person is declared legally blind. Could read STOP signs.

How about 20/10 Vision?

That is a rare occurrence, though many healthy children could obtain a 20/15 vision.
The theoretical limit of human visual acuity is 20/5. So if you can obtain a 20/10 vision, you are amongst the lucky 1% of the population.

While a human with 20/10 vision is said to be a rare occurrence, the bird of prey, hawk beats human vision hands down. Hawks are considered to have an acuity of about 20/2, which is much better than human eyesight.



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Their large retinal cells contribute to this superior visual excellence. A hawk could see something at 20 feet, it would require a human standing 3 feet from the object to see the same thing.

Now, this simple fact explains how the hawk always gets the chicks no matter how the mother hen hides them!

Many humans have one eye that has superior visual acuity over the other.

How does the eye get its color?

^{Source: Peace Samuel Omana the 15-year old Nigerian girl with beautiful eyes}


Eye Color

How does the eye get its color?

The color of an eye is dependent on the melanin on the iris. The darker the eyes the more the pigment in the eyes. Grey, green and blue is as a result of lighter pigment due to less melanin presence.

The six main eye colors are brown, blue, gray, hazel (or red), amber, and green . Green is the least common eye color.

Newborn babies lack melanin, so they start life with almost colorless eyes. As they get older, melanin increases and by age three their eyes get their true real color.


Besides the melanin giving the eye color, it also protects it from the sun rays. Light eyes are more sensitive to the sun's ultraviolet UV rays than darker eyes. That is not to say dark-eyed peoples should go about without protecting their eyes outdoors from the sun with the recommended UV glasses.

Some good ol' superstition

Since eyes are windows to the soul, there has been some superstition surrounding eye colors.

• Green-eyed people are said to have sharp minds
• Blue-eyed people are said to have vivid imaginations
• Browd-eye individuals have an underlying passion but are cool-headed.
• Hazel eyes are said to be an indication of passionate souls.

Some eye colors are even attributed to witchcraft/ diabolic people, especially in Africa.

These are just superstitions, so whatever color your eyes are it is a great one.

I have brown eyes. What about you?

But this is how I'd feel if I could have green natural eye color for a year <3

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