6. Specialized Areas of the Retina
• The posterior area of the retina where the
optic nerve leaves the retina is devoid of
photoreceptors and is known as the blind spot
of the retina, or the optic disc.
• On the temporal side of the optic disc, at the
posterior pole of the optical axis, lies a
specialized area of the retina called the fovea
centralis (0.3 mm)
11. Rods
• 120 million
• 30 to 300 times more
sensitive to light
• Dim light vision (Scotopic
vision)
• Absent in fovea
• Night blindness
• Rhodopsin
• One type of rod
Cones
• 3 million
• Less sensitive than rods
• Bright light vision (photopic
vision)
• Concentrated at fovea
• Decrease visual acuity &
color vision
• Color pigment
• Three types of cones
12.
13. FOVEA CENTRALIS
• Cones are thin and long
• Light falls directly on
photoreceptors
• 1:1 connection with
bipolar and ganglionic
cells
• Cones- Bipolar-
Ganglionic cells
PERIPHERAL RETINA
• Cones are fatter
• Light does not fall
directly
• Many rods and cones
converge on bipolar and
ganglionic cells
• Rods & cones- Bipolar-
Amacrine- Ganglionic
cells
14.
15. Nodal point (Point at the center of lens
where light rays pass un deviated)
20. Refractive Index of a Transparent
Substance
• The refractive index of a transparent
substance is the ratio of the velocity of light in
air to the velocity in the substance.
• The refractive index of air itself is 1.00.
21. • Speed of light in air is 300,000 Km/sec
• Thus, if light travels through a particular type
of glass at a velocity of 200,000 km/sec, the
refractive index of this glass is 300,000 divided
by 200,000, or 1.50.
24. Focal Length of a Lens
• The distance beyond a convex lens at which
parallel rays converge to a common focal
point is called the focal length of the lens.
25. Relation of focal length of the lens
• 1/f = 1/a + 1/b
• in which f is the focal length of the lens for
parallel rays,
• a is the distance of the point source of light
from the lens,
• b is the distance of focus on the other side of
the lens.
26.
27. Measurement of the Refractive
Power of a Lens—“Diopter”
• The more a lens bends light rays, the greater is
its “refractive power.”
• This refractive power is measured in terms of
diopters.
• The refractive power in diopters of a
convex lens is equal to 1 meter divided by its
focal length.
28. Optics of the Eye
The lens system of the eye is composed of four
refractive interfaces:
• (1) the interface between air and the anterior
surface of the cornea,
• (2) the interface between the posterior surface of
the cornea and the aqueous humor,
• (3) the interface between the aqueous humor
and the anterior surface of the lens of the eye
• (4) the interface between the posterior surface
of the lens and the vitreous humor.
29. • The internal index of air is 1;
• the cornea, 1.38;
• the aqueous humor, 1.33;
• the crystalline lens (on average), 1.40; and
• the vitreous humor, 1.34.
30.
31. Reduced Eye
• If all the refractive surfaces of the eye are
algebraically added together and then considered
to be one single lens in the eye. This is called as
Reduced eye.
• This surface has power of 59 Diopters and lies 17
mm in front of eye
• Refractive power of cornea (39 D) and lens (20 D)
32. Formation of image on the retina
• Image on retina is inverted and reversed with
respect to the object
• Mind perceives this object in upright position
33. Mechanism of “Accommodation”
• The shape of the lens is changed from a
moderately convex lens to that of a very
convex lens.
• Constriction of pupil.
• Convergence of the eyeball.
36. • Accommodation Is Controlled by
Parasympathetic Nerves.
• Through 3rd cranial nerve from 3rd nerve
nucleus in brain stem
37. Presbyopia
• The lens remains almost totally non
accommodating, a condition known as
“presbyopia.”
• Occurs in old age
• Use convex lens for reading
• wear bifocal lenses if lens is focused at a
constant distance
38. Depth of focus
• Ability to maintain the focus with change in
lens power and position of retina
• Depth of focus is more with small pupilary
aperture
• 1.5 mm to 8mm in dm (variation in pupil
aperture)
46. Nodal point (Point at the center of lens
where light rays pass un deviated)
47. • Visual acuity is defined as the shortest distance
by which two lines can be separated and
visualized as two lines.
• If two black objects on a white background are
separated by a space subtending an angle of less
than 1 minute two objects will be identified as
one.
• Visual acuity is the ability of the eye to
discriminate between two points or to resolve
two points separately when they are placed close
to each other.
• Visual acuity is the function of cones. Cones are
most concentrated at the fovea centralis. Outside
this foveal area, the visual acuity becomes
progressively poor..
48.
49.
50.
51. Clinical Method for Stating Visual
Acuity
Distant vision Snellen Chart
Near Vision Jageur Chart
Children Tumbling E chart
52.
53. Principle of the Snellen chart
• Snellen chart consists of series of letters arranged
in lines. Height of each letter subtends an angle
of 5 minutes and each component of the letter
subtends an angle of 1 minute at nodal point of
the eye. Results are conventionally expressed as
ratio
• Visual Acuity = d/D
• d = Distance at which the subject is reading the
letter
• D = Distance at which the letter should be read by
normal eye
54. Clinical Method for Stating Visual
Acuity.
• The chart for testing eyes usually consists of
letters of different sizes placed 20 feet away from
the person being tested.
• If the person can see well the letters of a size
that he or she should be able to see at 20 feet,
the person is said to have 20/20 vision—that is,
normal vision.
• If the person can see only letters that he or she
should be able to see at 200 feet, the person is
said to have 20/200 vision.
60. FUNCTIONS OF AQUEOUS HUMOR
• Nutrition to lens and cornea
• Removes metabolites like lactates
• Maintains intraocular pressure
61. Intraocular Pressure
• The average normal intraocular pressure is
about 15 mm Hg, with a range from 12 to 20
mm Hg.
• “Glaucoma,” a Principal Cause of Blindness.
62.
63. DEPTH PERCEPTION
• Depth perception is the visual ability to
perceive the world in three dimensions (3D)
and the distance of an object.
64. Determination of Distance
of an Object from the Eye—
“Depth Perception”
• A person normally perceives distance by three
major means:
• (1) the sizes of the images of known objects
on the retina,
• (2) the phenomenon of moving parallax,
(Change in position of object by change in
observer’s eye position)
• (3) the phenomenon of stereopsis (depth
perception by binocular vision).
65. The sizes of the images of known
objects on the retina
• If two objects are known to be the same size
(e.g., two trees) but their absolute size is
unknown, relative size cues can provide
information about the relative depth of the
two objects.
• If one subtends a larger visual angle on the
retina than the other, the object which
subtends the larger visual angle appears
closer.
66.
67. The phenomenon of moving parallax
• Nearby things pass quickly, while far off
objects appear stationary.
68.
69. STEREOPSIS
• Perception of depth and three dimensional structure on
basis of binocular vision
• Each eye captures its own view and the two separate
images are sent on to the brain for processing.
• When the two images arrive simultaneously in the back of
the brain, they are united into one picture.
• The mind combines the two images by matching up the
similarities and adding in the small differences.
• The combined image has a three-dimensional stereo
picture.
• Stereoscopic vision makes it possible to appreciate depth
by superimposition of two entirely different images
70. OPHTHALMOSCOPE
• The ophthalmoscope is an instrument through
which an observer can look into another person's
eye and see the retina with clarity.
PRINCIPLE OF OPHTHALMOSCOPE
• if the retina of one person is made to emit light,
the image of his or her retina will be focused on
the retina of the observer, provided the two eyes
are emmetropic and are simply looking into each
other.
Editor's Notes
Antero-posterior diameter of eyeball is 24 mm. volume of 6 cu cm. mass of 7.5 gm.
Rate of formation = 2 to 3 microliters per minute
Process of ultrafiltration. Active secretion of ions. Water osmosis
