Vision is the dominant sense for most people with sight.
The organ of sight is the eye.
Think of it as a slightly irregular,
hollow sphere that takes in light and translates it into images.
If we enlarge the eye and look inside it, we can discover how that's done.
Inside the eye are various structures working together
to create an image the brain can understand.
Among these are the cornea -- a clear dome-like structure
covering the iris or colored part of the eye -- the lens
directly below it, and the retina, which lines the back of the eye.
The retina consist of thin layers of light-sensitive tissue.
This candle can help us understand
how the eye captures images
and then sends them to the brain.
First, the candlelight passes through the cornea.
As it does, it's bent, or refracted, onto the lens.
As the light passes through the lens, it's bent a second time.
Finally, it arrives at the retina where an image is formed.
This double bending, though, has reversed the image and turned it upside down.
If that was the end of the story, the world would always appear upside down.
Fortunately, the image is turned right side up in the brain.
Before that can happen, the image needs to travel
as impulses along the optic nerve and enter
the brain's occipital lobe. When the image
forms there, it regains its proper perspective.
Now let's consider two common conditions that cause blurry vision.
The eye's shape is important for keeping things in focus.
With normal vision, light focuses precisely on
the retina at a location called the focal point.
But what happens if the eye
is longer than normal?
The longer the eye, the more distance there
is between the lens and retina.
But the cornea and lens still
bend light the same way.
That means the focal point will be somewhere
in front of the retina rather than on it.
This makes it difficult to see
things that are far away.
A person with a "long" eye is said
to be nearsighted.
Glasses with concave lenses
can correct nearsightedness.
The lens widens the plain of light
coming through the cornea.
That pushes the focal point back onto the retina.
Farsightedness is just the opposite.
The eye's length is too short.
When that happens,
the focal point is behind the retina.
So it's difficult to see things that are up close.
Glasses with convex lenses narrow
the plain of light.
Narrowing the light passing through the cornea
moves the focal point back onto the retina
and can correct farsightedness.