This document discusses various types of refractive errors of the eye, including myopia, hyperopia, astigmatism, and presbyopia. Myopia occurs when light rays focus in front of the retina, causing distant objects to appear blurred. Hyperopia is the opposite, with light rays focusing behind the retina and near objects appearing blurred. Astigmatism causes different focal points for vertical and horizontal light rays. Presbyopia is an age-related condition where the eye loses its ability to focus on close objects. The document provides details on causes, symptoms, and corrections using lenses for each type of refractive error.

1. OPTICS OF EYES 2

2. CYLINDRICAL LENS BENDS LIGHT RAYS IN ONLY
ONE PLANE- COMPARISION WITH SPHERICAL
LENSES
• The following diagram shows both a convex
spherical lens and a convex cylindrical lens.
• The cylindrical lens bends light rays from the two
sides of the lens but not from the top or the
bottom; that is bending occurs in one plane but
not the other. Thus, parallel light rays are bent to
a focal line.
• Light rays that pass through the spherical lens are
refracted at all edges of the lens (in both planes)
towards the central ray and all the rays come to a
focal point.

3. A, point focus of parallel light rays by a spherical convex lens. B, line
focus of parallel light rays by a cylindrical convex lens.

4. COMBINATION OF TWO CYLINDRICAL LENSES
AT RIGHT ANGLES EQUALS A SPHERICAL LENS
• The following diagram shows two convex
cylindrical lenses at right angles to each other.
• The vertical cylindrical lens converges the light
rays that pass through the two sides of the lens,
and the horizontal lens converges the top and
bottom rays.
• Thus all the light rays come to a single point
focus. In other words, two cylindrical lenses cross
at right angles to each other perform the same
function as one spherical lens of the same
refractive power.

5. A, focusing of light from a point source to a line focus by a cylindrical lens.
B, Two cylindrical convex lenses at right angles to each other, demonstrating
that one lens converges light rays in one plane and the other lens converges
light rays in the plane at a right angle. The two lenses combined give the
same point focus as that obtained with a single spherical convex lens.

6. FOCAL LENGTH OF A LENS
The upper most diagram
• The distance beyond a convex lens at which
parallel rays converge to a common focal point
is called focal length of the lens. The following
diagram the top one demonstrates this
focusing of parallel light rays.

7. FOCAL LENGTH OF A LENS
Middle diagram
• The light rays that enter the convex lens are not
parallel but are diverging because the origin of
light is a point source not far away from the lens
itself.
• Because these rays are diverging outwards from
the point source, it can be seen from the diagram
that they do not focus at the same distance away
from the lens as do parallel rays.

8. Middle diagram
• In other words, when rays of light that are
already diverging enter a convex lens, the
distance of focus on the other side of lens is
farther from the lens than is the focal length
of the lens for parallel rays.
FOCAL LENGTH OF A LENS

9. FOCAL LENGTH OF A LENS
The bottom diagram
• This diagram shows light rays that are diverging
toward a convex lens that has far greater
curvature than that of the other two lenses in the
other two diagrams. In this diagram, the distance
from the lens at which the light rays come to
focus is exactly the same as that from the lens in
the first diagram in which the lens has less
curvature but the rays entering it are parallel.

10. FOCAL LENGTH OF A LENS
The bottom diagram
• This demonstrates that both parallel rays and
diverging rays can be focused at the same
distance beyond a lens, provided the lens
changes its convexity.

11. The two upper lenses of this figure have the same focal length, but the
light rays entering the top lens are parallel, whereas those entering the
middle lens are diverging ; the effect of parallel verses diverging rays on
the focal distance is shown. The bottom lens has far more refractive
power than either of the other two lenses (i.e., has a much shorter focal
length), demonstrating that the stronger the lens is, the nearer to the
lens the point focus is.

12. ERRORS OF REFRACTION
• These are conditions in which vision is
defective due to some fault in the; refractive
medium especially;
i. The cornea or
ii. The lens or
iii. When there is an abnormality of the
eyeball.

13. ERRORS OF REFRACTION
• Light rays from an object situated more than 6
meters (20 feet) are considered to be parallel.
• Light rays coming from an object closer than 6
meters are divergent.
• Emmetropic eye (the eye with normal
refractive power)
This means that with relaxed accommodation
parallel rays from infinity are focused on the
retina where an image is produced.

14. ERRORS OF REFRACTION
The image is f focused to b real, inverted and
diminished. The connections of retinal receptors
are such that although the image of the object
produced on the retina is inverted (upside down),
but the object is seen in its true form (upside
up).the mind is trained to consider an inverted
image as normal.
With relaxed accommodation light rays from an
object less than 6 meters from the eye will be
focused behind the retina and therefore the
object will be seen as a blurred object.

15. ERRORS OF REFRACTION
Normally the lens becomes more globular
under these conditions by the process called
accommodation. This results in a raised
diopteric power of the eye and the image of
the object can be produced on the retina.
• The non- emmetropic eye is called
ammetropic eye and this condition lead to
various errors of refraction which are
described below;

16. ERRORS OF REFRACTION
MYOPIA, (NEAR SIGHTEDNESS)
• The patient complains of difficulty in seeing
distant objects which appear blurred.
• In this case the image of a distant object is
focused in front of the retina. It results;
i. Either from an increased anterior posterior axial
length of the eye (axial myopia) or
ii. The eyeball is normal but the refractive power
of the eye is increased (refractive myopia). The
refractive power of the eye may be due to an
increased curvature of cornea or lens.

17. MYOPIA, (NEAR SIGHTEDNESS)
iii. In some cases the lens is present anterior to
the normal position. The result is that a point
will focus on the retina not as a point but as a
round spot.
• Near vision is not as defective as distant
vision. This is because the diverging rays from
a nearby object are focused behind the focus
of parallel rays from a distant object. Myopia
is therefore also called near sightedness.

18. Clinical types of myopia
1. Congenital (present at birth)
2. Developmental (precocious neurological
growth, nutritional disorders and excessive
dose work involved in activities such as
studies during childhood).
3. Drugs including sulfonamides and steroids.
4. It is also seen in diabetes mellitus and in
children who are made to sleep in lighted rooms
before the age of two.

19. Treatment of myopia
• Myopia is treated with concave lenses of
suitable strength which diverge light rays
before they strike the cornea; this
enables the light rays to focus on the
retina. Contact lens is an ideal substitute
in high myopia.

20. Hypermetropia (hyperopia, far
sightedness).
• In this case the subject’s distance vision
is better than near vision. However, the
distant vision of a hypermetrope is
poorer than a normal person.

21. Hypermetropia (hyperopia, far sightedness).
• The image of the near object is focused
somewhere behind the retina.
• Hypermetropia may be axial (the eyeball is
smaller resulting in decreased axial length) or
refractive (decreased refractive power of the
eye).
• If hypermetropia is slight, it may be
compensated by accommodation which by
increasing the diopteric power of the lens may
result in focusing the rays of light on the
retina.

22. Hypermetropia (hyperopia, far sightedness).
• However, even accommodation may not
succeed in compensating the visual defect for
close vision as the image of diverging light rays
from a close object is produced too much
behind the retina.
• The constant accommodation may lead to
hypertrophy of ciliary body and the subject
may complain of headache.

23. Hypermetropia (hyperopia, far sightedness).
• The prolonged convergence of a visual axes
associated with accommodation may lead to
strabismus (squint).
• All new born infants have hypermetropia of +4
diopter but the eyeball grows rapidly after
birth resulting in disappearance of this defect
in most children

24. Hypermetropia (hyperopia, far sightedness).
• Delayed development of eyeball will result in
the persistence of hypermetropia. The child
complains of frontal and occipital headache.
• Aphakia (removal of the lens) also results in a
decreased diopteric power of eye.

25. Treatment of hypermetropia
• It is treated with suitably powered convex
lenses placed extraocularly (spectacles) or
intraocularly.

27. ASTIGMATISM( A= not ; stigma= Point)
As the name suggests this is a refractive
error of vision.
In Astigmatism the light rays are not
brought to a point of focus on the retina.
The focus for horizontal rays different from that
for vertical rays.
Hence an astigmatic person looking at a piece of
graph paper may focus on the vertical lines and
may fail to focus the horizontal line and vice versa.
The defect is mostly due to a difference in
the horizontal and vertical curvatures of the cornea.,
occasionally the same abnormality affects the lens.

28. Astigmatism is of two types
(i) With the rule : the curvature is greater
in vertical meridian it is more
common.
(ii) Against the rule . The curvature is
greater in horizontal meridian
Treatment ( Correction)
It is corrected with cylindrical glasses
which have the property of bending the
light rays in one plane more than in the
other so that refraction in all meridians
is equalized.

31. PRESBYOPIA
( Eyesight of old age)
> It is not an error of refraction but
condition of physiological
insufficiency of accommodation
leading to failing vision for near.
> Since, we usually keep the book at about
25cm, so we can read comfortably up
to the age of 40 years.
> after the age of 40 years, the near point
of accommodation recedes beyond
the normal reading or working range
> This condition of failing near vision due
to age related decrease in the
amplitude of accommodation or increase
in punctum proximum is called
persbyopia.

32. Presbyopia accurs due to;
1. Decrease in the elasticity and
plasticity of the crystalline lens
( which results from age- related
sclerosis)
2. Age- related decrease in the power of
ciliary muscles
Symptoms.
Difficulty in near vision ( to start with
in the evening & in dim light and later
even in good light )
Treatment. ( Correction)
Convex glasses (plus) for near vision.

33. THE END