The human eye is like a camera that takes in light and forms an image on the retina. It has several main parts including the cornea, iris, pupil, lens, retina, and optic nerve. The cornea and lens work together to refract light and focus it onto the light-sensitive retina. The retina then converts the light into neural signals that are sent to the brain via the optic nerve. Common defects of the eye that affect vision include near-sightedness, far-sightedness, cataracts, and presbyopia. These defects can typically be corrected using lenses in glasses or contact lenses.

1. THE HUMAN EYE
Priyanka jakhar

3. THE HUMAN EYE
• The human eye is one of the most valuable and sensitive sense organs.
• It enables us to see the wonderful world and the colours around us.
• On closing the eyes, we can identify objects to some extent by their smell,
taste, sound they make or by touch.
• It is, however, impossible to identify colours while closing the eyes.
• Thus, of all the sense organs, the human eye is the most significant one as it
enables us to see the beautiful, colourful world around us.
• The human eye is like a camera.
• Its lens system forms an image on a light-sensitive screen called the retina.
• Light enters the eye through a thin membrane called the cornea. It forms the
transparent bulge on the front surface of the eyeball.ss
• It is like a camera which has a lens and screen system.
• The human eye are located in the specialized sockets carved out in
the human skull.
• Each human eye sizes for approximately 2.5 cm in diameter.

4. S.No. Human Eye Part Functions
1. Pupil
Opens and closes in order to regulate and control the amount of
light.
2. Iris Controls light level similar to the aperture of a camera.
3. Sclera Protects the outer coat.
4. Cornea
A thin membrane which provides 67% of the eye’s focusing
power.
5. Crystalline lens Helps to focus light into the retina.
6. Conjunctive Covers the outer surface (visible part) of the eye.
7. Aqueous humour Provides power to the cornea.
8. Vitreous humour Provides the eye with its form and shape.
9. Retina
Captures the light rays focussed by the lens and sends impulses
to the brain via the optic nerve.
10. Optic nerve Transmits electrical signals to the brain.
11. Ciliary muscles
Contracts and extends in order to change the lens shape for

5. Anterior Chambers
The anterior chamber is the front part of the eye between the cornea and the iris.
Posterior Chambers
The posterior chamber is between the iris and lens.
Posterior chamber is an important structure involved in production and circulation of a watery fluid
known as the aqueous humor, or aqueous.
WALL OF THE EYEBALL :
OUTER LAYER (FIBROUS COAT) : SCLERA AND CORNEA
MIDDLE LAYER (VASCULAR COAT) : CHOROID ,CILIARYBODY AND IRIS
INNER LAYER (NERVOUS COAT) : RETINA
The main parts of a human eye are:s
Sclera
• The white outer region of our eye which protects the internal parts of
our eyes is known as ‘sclera‘.
• Its made up of fibrous tissues.
• It’s continuous with the cornea.
• It resists intraocular pressure.

6. • It provides protection to the delicate structure within the eye.
• It maintains shape of the eyeball
• The smooth external surface allow easy eye movement
Cornea
The cornea is described as the “window of the eye”
• The anterior one sixth part of the sclera is transparent and is known as cornea.
• Light rays pass through the cornea to reach the retina.
• The transparent portion of our eye that allows the light to enter our
eye is known as the ‘cornea’ and is made up of transparent tissue.
• The cornea covers the pupil, anterior chamber, and the iris.
• Along with the anterior chamber and lens, cornea refracts light and
accounts for two-thirds of the eye’s total power.
• Usually, the refractive power of the cornea is approximately 43
dioptres.

8. Iris
A circular, thin structure made up of contracting and relaxing muscles
in the eye that controls the size of the pupil and the light reaching the
retina are known as the ‘iris’.
Iris defines a person’s eye color.
If we define the human eye as a camera then the iris becomes aperture
of the eye.
• Iris is the pigmented membrane surrounds the pupil
• It arises from the margin of ciliary body and forms a dark centered opening
called pupil
• The space between cornea (in front) and the lens (behind) is the anterior
segment
• It is again divided into two parts by the iris;
• Anterior chamber -The space between the iris and cornea is the anterior
chamber

9. • Posterior chamber -The space between iris and lens is posterior chamber
• They are filled with a clear fluid, the aqueous humor
Choroid
Choroid is a thin pigmented membrane, dark brown in color which is situated in
between sclera (externally) and retina (internally).
CILIARY BODY
Ciliary body is the continuation of choroid consisting of smooth muscle fibers,
i.e., the ciliary muscle.

10. • Ciliary body contains suspensory ligament for attaching the lens in position
• The ciliary muscles help in accommodation by adjusting the thickness of lens
Pupil
• The part of the eye located in the center of the iris allowing light to
reach the retina.
• The pupil appears black in color since the eye tissues absorb or
diffusely reflect the light entering the pupil. Iris controls the pupil.
Lens
• The lens is a biconvex, transparent structure present in the eye
behind the pupil.
• The lens along with the cornea refracts the light, so as to focus it on
the retina. • By changing its shape, the lens is capable of changing the
focal distance of the eye.
Retina
• Retina is the inner most layer of the eyeball
• It is a thin delicate layer continuous posteriorly with optic nerve

11. • The outer surface of the retina, formed by pigment cells, is attached to choroid.
• Its inner surface is in contact with the hyaloid membrane of the vitreous.
• The small area of retina where the optic nerve leaves the eye is the optic disc
or the blind spot.
• It has no light sensitive cells (Rods or Cones).
• The retina is a light-sensitive tissue in the inner coat of the eye that
sends electrical signals after converting them from light to the brain
for processing.
• When light strikes the retina, two types of cells are activated.
• Rods detect light and dark and help form images under dim conditions.
• Cones are responsible for color vision.
• The three types of cones are called red, green, and blue, but each actually
detects a range of wavelengths and not these specific colors.
• When you focus clearly on an object, light strikes a region called the fovea.
• The fovea is packed with cones and allows sharp vision.
• Rods outside the fovea are largely responsible for peripheral vision.
• Rods and cones convert light into an electric signal that is carried from the

12. • The brain translates nerve impulses. to form an image.
• Three-dimensional information comes from comparing the differences
between the images formed by each eye.
Rods and cones are the two light-sensitive types of cells present in
the retina.
Rods help us for night-time vision and cones help us see colors.
The rods and cones are the receptors of light and sight
These cells contains photosensitive pigments (Rods Rhodopsin, Cones – Iodopsin)
involved in the conversion of light rays into nerve impulses
Optic Nerve
The optic nerve sends electrical impulses from the retina, at the back
of the eyes to the brain.
• LIGHT TRANSMITTING MEDIA (OR) REFRACTIVE MEDIA
•Aqueous humor
•Vitreous humor
•Lens

13. • Aqueous humor is a clear fluid fills the space between cornea and lens
• It is secreted by capillaries of ciliary process
• From here the fluid reaches to the anterior chamber which finally reaches to
the canal of Schlemm.
Interference with drainage of aqueous humor results in an increase of intraocular
pressure (glaucoma) (Normal IOP 10 to 20 mmHg)
This leads to atrophy of the retina, leading to blindness
FUNCTIONS
• It helps to maintain intraocular pressure and thus maintains the shape of
eyeball
• It is rich in ascorbic acid, glucose and amino acids and nourishes the cornea and
lens
VITREOUS HUMOR
Vitreous humor or vitreous body is a colorless, transparent, jelly-like substance
which fills the posterior segment of the eye (i.e., behind the lens).
It is enclosed in a delicate hyaloid membrane

14. FUNCTIONS
• It helps to preserve the spherical shape of the eyeball and to support the
retina.
Lens .
• The lens of the eyeball is crystalline in nature .
• It is situated behind the pupil .
• It is biconvex, transparent, and elastic in structure.
• Lens refracts light rays and helps to focus the image of the object on retina.
• Lens is supported by suspensory ligaments (Zonular fibers) which are attached
with ciliary bodies.
• The lens obtains nutrients from the fluid, aqueous humour because it does not have any blood
supply.
• Waste products are removed through these fluids as well.
Macula
The macula, a small part which is located in the center of the retina that gives central
vision.

15. Conjunctiva
The conjunctiva is the clear, thin membrane that covers part of the front surface of
the eye and the inner surface of the eyelids . It has two segments:
• Bulbar conjunctiva
 This portion of the conjunctiva covers the anterior part of the sclera.
 The bulbar conjunctiva stops at the junction between the sclera and cornea; it
does not cover the cornea.
• Palpebral conjunctiva.
 This portion covers the inner surface of both the upper and lower eyelids.
(Another term for the palpebral conjunctiva is tarsal conjunctiva.)
The bulbar and palpebral conjunctiva are continuous (see illustration).
This feature makes it impossible for a contact lens (or anything else) to get lost
behind your eye.
Conjunctiva Function
The primary functions of the conjunctiva are:
•Keep the front surface of the eye moist and lubricated.

16. •Keep the inner surface of the eyelids moist and lubricated so they open and close
easily without friction or causing eye irritation.
•Protect the eye from dust, debris and infection-causing microorganisms.
•The conjunctiva has many small blood vessels that provide nutrients to the eye
and lids.
•It also contains special cells that secrete a component of the tear film to help
prevent dry eye syndrome.

17. Blind Spot
• This is a small area of the retina where the optic nerve connects.
• In this area there are no light-sensitive cells i.e. no rods and cones.
• Due to which the retina can not see at that spot.
• This is called a Blind Spot.
Tear Glands
• The tear gland located above each eyeball and inside your upper
eyelid.
• This gland is responsible for making a fluid that is mostly salt and
water (tears) to keep the surface of your eyeball clean and moist.
• sIt also protects your eye from damage.
Yellow spot
• The yellow spot, also known as macula, is the centre of the eye and sharpest sight place.
• In fact, it’s the centre of our eye placed on the background of the eye and it’s around 5 milli
metres big.
• Yellow spot is a part of inner layer of the eye called the retina.

18. • The nerves inside the macula are rich with lutein and zeaxanthin pigment, which makes it look
yellow.
• It’s also the reason it is called the yellow spot.
• In order to provide clear vision, the yellow spot must be very efficiently organized.
• There are millions of tiny nerves converting light to electric impulse in a very small area of only a
few milli metres.
• This process requires a lot of energy and oxygen.

19. Colour Blindness:
• A person having defective cone cells is not able to distinguish between the different colours.
• This defect is known as Colour Blindness.
Defects of The Eye and Their Corrections
As perfect the human eye may seem; it’s not.
If the human eye isn’t perfect, which means it has its share of defects of the
human eye.
Here are few common defects of the human eye:
a. Myopia or Near-Sightedness
Myopia is a defect of vision where in far-off objects appear blurred and objects
near are seen clearly.
Since the eyeball is too long or the eye lens’s refractive power is too high; the
image forms in front of the retina rather than forming on it.
Myopia is a defect of vision in which a person can see nearby objects clearly but
cannot see distant objects clearly because the image is formed in front of the
retina.
This may be due to:-
i) Increase in curvature of the eye lens

21. ii) Increase in the length of the eye ball
Correction of myopia can happen by wearing glasses/contacts made
of concave lenses to help focus the image on the retina.

22. b. Hypermetropia or Longsightedness
Hypermetropia is a defect of vision wherein there is difficulty in viewing objects
that are near but one can view far objects easily.
Since the eyeball is too short or eye lens’s refractive power is too weak hence the
image instead is of being forming upon the retina, its forms behind the retina.
Hypermetropia is a defect of vision in which a person can see distant objects
clearly but cannot see nearby objects clearly because the image is formed behind
the retina.
This may be due to:-
i) Decrease in curvature of eye lens
ii) Decrease in the length of the eye ball
Correction of hypermetropia can happen by wearing glasses/contacts containing
convex lenses.

25. c. Cataract
Cataract is the clouding of the lens, that prevents the formation of a clear, sharp
image.
A cataract forms when old cells after they die, stick in a capsule wherein with time
a clouding over lens happens. Because of this clouding blurred images are
formed.
Other factors that may increase the risk of developing cataracts :
•A family history of cataracts
•Diabetes
•High blood pressure
•Other eye conditions such as uveitis
•Previous eye surgery, injury or inflammation
•Long-term use of corticosteroid medication (eg : prednisone, prednisolone)
•Excessive exposure to sunlight
•Smoking
•Drinking too much alcohol
•Poor diet.
Correction of cataract can happen through a surgery. An artificial lens in place of

26. d. Presbyopia or Old-age Longsightedness
• Presbyopia is a natural defect that occurs with the age.
• In presbyopia, the ciliary muscles become weak and are no longer able to adjust
the eye lens.
• The eye muscles become so weak that no longer can a person see nearby
objects clearly.
• The near point of a person with presbyopia is more than 25cm.
• Presbyopia is a defect of vision in old people in which they are not able to see
nearby objects clearly due to the increase in the distance of near point.
• This is due to the weakening of the ciliary muscles and decrease in the flexibility
of the eye lens. It can be
• corrected by using suitable convex lens.
• Sometimes they are not able to see both nearby and distant objects clearly.
• It can be corrected by using bifocal lenses consisting of both concave and
convex lenses.
• The upper part is concave for correction of distant vision and the lower part is
convex for correction of near vision.
• Correction of presbyopia can happen by wearing bifocal glasses or Progressive

27. portion contains a convex lens.
• A person with presbyopia can also have just myopia or just
hypermetropia.

28. e. Astigmatism
Astigmatism is a defect where in the light rays entering the eye do not focus light
evenly to a single focal point on the retina but instead scatter away.
The light rays in a way where some focus on the retina and some focus in front of
or behind it. This happens because of non-uniform curvature of the cornea;
resulting in a distorted or blurry vision at any distance.
Correction of astigmatism can happen by using a special spherical cylindrical
lens.

30. Power of Accommodation
Power of accommodation is the process by which ciliary muscles
function, to adjust the focal length of the eyes so that clear image
forms on the retina.
This varies far or nearby objects. For a normal eyesight, the power of
accommodation is 4 dioptre.
Near point :-
The minimum distance at wh ich the eye can see objects clearly is called the near
point or least distance of distinct vision.
For a normal eye it is 25cm.
Far point :-
The farthest distance up to which the eye can see objects clearly is