How do the lenses of eyeglasses function differently from properly functioning lenses of the eye?Give at least 5 reasons.

The lens of the human eye is called the crystalline lens. 
It is located in the anterior (front) segment of the eye, behind the cornea and in front
of the vitreous body (clear gel which fills most of the eyeball).  It is composed of
lens fibers contained in a capsule, and has epithelium (lining cells) on its surface. 
The eye lens is biconcave, meaning that it is curved outward on both the front and the
back.  It acts in concert with the cornea to refract light waves (bend them) in order to
focus the light on the retina in the back of the eye.

The
eye lens can change its shape and therefore change its focal length.  This is called
accommodation.  Accommodation is necessary to allow a person to see distant objects as
well as nearby objects.  The eye lens increases its curvature during reading in order to
keep the words in focus.

A person who is nearsighted has an
eyeball that is too long for the focal length of the combined cornea and lens.  This can
be corrected with eyeglasses fitted with biconcave lenses in order to extend the focus
to reach the retina. A person who is farsighted needs biconvex
lenses.

Vision correction is accomplished with eyeglasses,
contact lenses or LASIK surgery.  LASIK surgery involves permanently changing the shape
of the patient’s cornea using a laser machine.

Here are
five ways in which eye lenses differ from eyeglass
lenses.

1. The eye lens is organic, and is made up
   of lens fibers, a capsule and epithelium.  Eyeglass lenses are inorganic, made entirely
   of glass or plastic.


2. The eye lens can accommodate (change
   its shape and focal length).  Eyeglass lenses have fixed focal lengths, although they
   can be manufactured with more than one focal length in different parts of the lens
   (bifocals, for example).


3. Eye lenses are biconvex, whereas
   eyeglass lenses can be either biconvex or biconcave.


4. Eye
   lenses are subject to degeneration and diseases (opacification from cataract, for
   example), whereas eyeglass lenses are not.


5. The eye lens
   works in concert with the cornea, whereas eyeglass lenses work alone.