Eye defects and lenses The eye is considered normal, or emmetropic, if parallel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed The normal eye contracts its ciliary muscle to provide for accomodation. When the eye cannot function properly, the light entering the eye is not brought into sharp focus on the retina.

1. Eye defects and lenses
By: Ellen Pintang

2.  The eye is considered normal, or
emmetropic, if parallel light rays
from distant objects are in sharp
focus on the retina when the
ciliary muscle is completely
relaxed

3. Focusing an object at close range
The normal eye contracts its ciliary
muscle to provide for accomodation.
When the eye cannot function
properly, the light entering the eye is
not brought into sharp focus on the
retina.

4. Refractiv
e errors:
It is the
common
errors of
refraction
in the
eyes
Causes of Refractive
Errors:
Irregularities in the
shape of cornea
Abnormal size or shape
of eyeball itself
inability of the lens to
focus

5. How can this be corrected?
 These refractive errors can be fully
corrected with eyeglasses or contact
lenses

6. Common Eye Defects

7. Hyperopia or Farsightedness
This condition is due to either the eyeball
is too short or, sometimes, a lens that is too
weak.
How does hyperopia
happen?
It happens because parallel light rays are
not bent sufficiently by the relaxed lens
system to come focus by the time they
reach the retina. The ciliary muscles of
the eye are not strong enough to contract,
resulting to the lens not being convex
enough to allow for sharp focus.

8. Myopia or Nearsightedness
Caused by having an eyeball that is too
long , or having too much refractive power in
the lens system of the eye.
In myopia, the ciliary muscle is completely
relaxed and the light rays coming from
distant objects are focused in front of the
retina. A myopic person has no
mechanism to focus distant objects
sharply on the retina.

9. Astigmatism
Caused by irregularly curved cornea that
distorts the focus entering the eye. This defect
causes the visual image in one plane to focus
at a different distance from that plane at right
angles.
Because of this condition, the light rays
passing through an astigmatic lens do not
all come to a common focal point. Thus
blurred image is perceived by an
astigmatic person.

10. Correcting Common Eye Defects

11. This condition can be corrected by adding
refractive power with a convex lens in front of the
eye.
How to correct Hyperopia or
Farsightedness
Convex lens are converging
lenses. If the light rays
converge to a focal point on the
retinal surface it creates a
focused image.

12. In myopia, the eyes of a myopic person has
an excessive refractive power that can only be
neutralized by concave lenses.
How to correct Myopia or Nearsightedness
Placing a diverging lens in front of
the eye diverges the light rays just
enough such that the rays
converge on the retinal surface
where a focused image is created.

13.  Find a spherical lens by trial and error
that corrects the focus in one of the
two planes of the astigmatic lens of
the eye.
 After the correct spherical lens has
been determined, an additional
cylindrical lens is used to correct in
the remaining plane.
How to correct Astigmatism

15. Contact Lenses:
Are transparent materials
that are fitted against the
interior surface of the
cornea. This lenses are held
in place by a thin layer of
liquid (tears) that fills the
space between the contact
lens and anterior surface of
the eye.

16. How does it works?
 The refraction of the cornea-lens
system of the eye is neutralized.
 The refraction is done by the
anterior surface of the contact
lens.

17. Advantages
 It turns with the eye and thus gives a
broader field of vision
 Has very little effect on the size of the
object the user sees. Because the
glasses are placed several
centimeters in front of the eye, it
affects the size of he image, resulting
in slightly bigger or smaller images