Human eye

Human eye is an organ of visual perception in human being.

Parts of Human Eye :

The eye ball is spherical in shape with a diameter of about 2.5cm.It consists of the
following parts:


  It is the outer most layer of eye.It is thick and opaque and impermeable
to light.


It is the front part of sclera.It is transparent and allows the light
to enter the eye.


  It is the pigmented middle layer of eye.It forms a colored curtain
in front of the lens.


  It is the colored curtain which is muscular and acts for varying the diameter
of central pupil.


The central hole or aperture of the iris is called automatically
controls the amount of light entering the eye.By this, eye is protected from
too bright light.

Convex lens

Behind the iris is a convex lens which is transparent and flexible.its focal
length can be changed by the pressure of ring shaped siliary muscles surrounding
it.It focuses and converges the rays of light entering the eye.

Ciliary Muscles :

They control the lens in its adjustments for seeing a far off ora near byobject.

Retina : It is the third layer inside.The choroid.The rays of light form 

and object are focused and converged by the lens on retina.

Optical Nerves

They carry impression to the brain.

Aqueous Humour

It is a transparent water like liquid between cornea and lens.It acts as
refracting component.The shape of eye is maintained by it.

Vitreous Humour

It is a transparent jlly like liquid filled between lens and retina.

Defects Of Eye :

Following are the main defects of eye :

  • Short sightedness (Myopia)

  • Long sightedness (Hypermetropia)

  • Astigmatism 

  • Presbyopia

Short Sightedness (Myopia)

Defects :

If a persons cannot see distance object but can see near objects then he
is suffering from short sightedness.

Cause :

The eyeball is too converging or too long as a result the image is formed
in front of retina and cannot be seen vlearly.

Remedy :

Use of concave lens of suitable focal length in front of the eye can focus
the image clearly on the retina.

Long Sightedness (Hypermetropia)

Defect :

If a person see near objects but can see distant object then he is suffering
from long sightedness.

Cause :

The eye ball is less converging or too short, thus the rays are focused
behind the retina.


A convex lens of suitable focal length is used to bring the image at retine.

Astigmatism :


It is a vision defect in which object is not seen clearly because it is 
not focused clearly on etine.


The cornea is not perfectly spherical the eye has different focal pionts.


It is corrected by asymmetrical lenses with different raddi of curvature,
that is cylinderical lenses.



A vision defect in which accomodation power is gradually lost.

Cause :

It is due to age.The lens loses its elasticity and ability to change its shape
and the ciliary muscles weaken, resulting in lack of accomodation.


Mainly this defect can be corrected by convex lens.But to see distant objects
one needs concave lens, therefore bi-focal lenses is normally used; i.e. 
convex part in the lower portion to see near objects and concave part in the
upper portion to see distant objects.

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