CHAP 11 : HUMAN EYE AND COLOURFUL WORLD – NOTES

The Human Eye:

The Human Eye's Structure

• The eye is spherical in shape and measures 2.3 cm in diameter on average.

• It is made up of many types of muscles and tissues.

• It is a natural optical instrument which is used to see the objects by human beings.

• It is like a camera which has a lens and screen system.

• It consists of a convex lens made up of living tissues.

• Hence, human lenses are living organs contrary to the simple optical lenses.

The various parts of eye and their functions :

Parts of Human Eye:

Sclera:
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 CHAP 11 : HUMAN EYE AND COLOURFUL WORLD – NOTES

• It is a white layer of eyes. It is a fabrication. It is a supporting wall of eyes ball. The

interior parts are protected by strong connective tissue.

Conjunctiva:

• It is a loose tissue that covers the surface of the eye’s ball. It is a clear and thin
membrane. It lubricates in nature and maintains the moisture of the eyes.

Cornea:

• The cornea is transparent in nature. It is a front section of our eye that covers the
pupil and iris. It allows the light to enter the eyes.

• It is a thin membrane which covers the eye trail. It acts like a lens which refracts the
light entering the eye.

• Most of the refraction for the light rays entering the eye occurs at the outer surface
of the cornea

Iris:

• Dark and a colourful muscular diaphragm is called iris. It is responsible for colour of
the eye.

• When a large amount of light enters our eyes then it contrasts and when a small
amount of light enters our eyes then it expands.

• It controls the amount of light entering the eye by changing the size of the pupil.

Pupil:

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• It is a small circular hole in the middle of iris through which light enters the eye. It
appears black because light falling on it goes into the eye and does not come back.

• It regulates the amount of light entering the eye by adjusting the size of the iris.

How Pupil Works?

• When the light is bright: Iris contracts the pupil, so that less light enters the eye.

• When the light is dim: Iris expands the pupil, so that more light enters the eye.
Pupil opens completely when iris is relaxed.

Eye Lens :

• The eye lens is a convex lens made by the transparent and flexible jelly like material.
• Light is refracted by the lens and the cornea to focus on the retina.

Aqueous Humour:

• It is present between the cornea and the eyes lenses. It maintains the moisture of the
cornea and the shape of the eyeball.

Vitreous humour:

• The space between eye lens and retina is filled with another liquid called vitreous
humour.

• It is a jelly-like translucent material found between the lens and the retina. Water
(99%).

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• Vitreous humour protects the eyes and maintains the spherical shape of the eyes.

Ciliary Muscles:

• These are the muscles which are attached to eye lens.

• It can modify(changes) the shape of eye lens which leads to the variation in focal
lengths.

• It holds the eye lens at its proper position.

Retina :

• The back surface of the eye. It is the screen of the eye.

• It is a light sensitive screen inside the eye on which image is formed. It contains rods
and cones.

• A real and inverted image form on the retina.

Rods and Cones:

• These are colour sensitive rods and cones shaped cells. Rods are responsible for the
vision in dim light while cones are responsible for colour.

Blind Spot:

• The junction of the optic nerve and retina, where no rods and cones cells are present
is called the blind spot.
• The point at which the optic nerve leaves the eye. It is insensitive to light.
• An image formed at this point is not sent to the brain.

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Optic Nerve:

• The optic nerve is found in the back of the eyes

• It converts information of the image into a corresponding electric signal and take the
image to the brain in the form of electrical signals.

How the Eye Works:

• Light from objects passes through cornea, pupil, and lens.

• Lens changes shape (accommodation) to focus light correctly on the retina.

• Retina converts light into electrical signals.

• Signals travel through the optic nerve to the brain, which interprets them as images.

Colour Blindness:

• A person having defective cone cells is not able to distinguish between the different
colours. This defect is known as Colour Blindness.

Power of Accommodation:
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• The ability of the eye lens to adjust its focal length to focus both near and distant
objects.

• Focal length can be changed with the help of ciliary muscles.

• For near objects: Lens becomes thick and more convex. Focal length decreases when
Ciliary muscles get contract

• For distant objects: Lens becomes thinner. Focal length increases when Ciliary
muscles get relaxed.

• The maximum power of accommodation of a normal eye is reached when an object

is at a distance of about 25 cm away from the eye. After this, the ciliary muscles
cannot make the eye lens thicker (or we can say more convex).

• Thus, a normal eye has a power of accommodation which enables objects even
situated at infinity and as close as 25 cm to be focused on the retina (screen of the
eye).

• The power of accommodation of the eye for a person having normal vision (normal
eye-sight) is about 4 dioptres.

Near Point (Least distance of distinct vision):

• The nearest point from eye at which the eye can see clearly without strain is called
near point.

• The minimum distance upto which an eye can see clearly is called the least distance
of distinct vision.

• The least distance of distinct vision is equal to the distance between the eye and its
near point.

• For a normal human eye, of an adult, the near point is about 25 cm from the eye.

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Far Point:

• The farthest point, upto which the eye can see the object clearly is called far point.

• For a normal eye, the far point is infinity.

The range of Vision:

• Distance between near point and far point of eye is called range of vision. (From 25
cm to infinity)

Persistence of Vision:

• It is the time for which the sensation of an object continue in the eye.

• The image of an object persists on the retina for 1/16 second, even after the removal
of the object.

• It is about 1/16th of a second.

• The sequence of still pictures taken by a movie camera is projected on a screen at a

rate of about 24 images or more per second. The successive impressions of images on
the screen appear to merge smoothly into one another to give us the feeling of
moving images.

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 CHAP 11 : HUMAN EYE AND COLOURFUL WORLD – NOTES

Defects of Vision and their Correction :

There are commonly three types of defects in human eyes. It is caused by many factors.

Myopia (Near sightedness) or short sightedness :

• A person with myopia can see nearby objects clearly but cannot see distant objects
distinctly(clearly).

• In a myopic eye, the image of a distant object is formed in front of the retina and not
at the retina itself.

• A person with this defect has the far point nearer than infinity.

This defect may arise due to (Reason) :

• excessive curvature of the eye lens, or

• elongation of the eyeball.

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• Due to increase in power (or decrease in focal length) of eye lens.

Correction:

• Since a concave lens has an ability to diverge incoming rays, it is used to correct this
defect of vision.

• The image is allowed to form at the retina by using a concave lens of suitable power
as shown in the given figure.

Hypermetropia (Far sightedness) or long sightedness:

• In this type of defect, the person can see the farthest object clearly but can’t see the
nearer object clearly(distinctly).
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• The near point, for the person, is farther away from the normal near point (25 cm).

• Such a person has to keep reading material much beyond 25 cm from the eye for
comfortable reading.

• This is because the light rays from a close by object are focussed at a point behind
the retina.

This defect arises either because (Reason) :

• The eyeball has become too small.

• Decrease in the power of eye lens i.e., increase in focal length of eye lens.

• The ciliary muscle is unable to change the shape of the lens enough to properly focus
the image.

• The eye loses its power of accommodation at old age.

Correction:

• Since a convex lens has the ability to converge incoming rays, it can be used to
correct this defect of vision.

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• The ray diagram for the corrective measure for a hypermetropic eye is shown in the
given figure.

Presbyopia (Old age Hypermetropia) :

• It is the defect of vision due to which an old person cannot see the nearby objects
clearly due to loss of power of accomodation of the eye.

• The near-point of the old person having presbyopia gradually recedes and becomes
much more than 25 cm away.

• In this type of defect the person cannot see the nearest and farther object clearly.

• This defect is a special kind of Hypermetropia. Person may suffer from both myopia
and hypermetropia.

Reason (cause) of defect :

• Gradual weakening of ciliary muscles.

• Diminishing (decreasing) flexibility of eye lens.

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Correction :

• Such people often require bi-focal lenses. A common type of bi-focal lens consists of
both concave and convex lenses.

• The upper portion consists of a concave lens. It facilitates distant vision. The lower
part is a convex lens. It facilitates near vision.

Cataract:

• Sometimes, the crystalline lens of people in old age becomes milky and cloudy.

• Due to the membrane growth over eye lens, the eye lens becomes hazy or even
opaque.

• This condition is called cataract. This causes a partial or complete loss of vision

• The image can not be seen distinctly,it can cause complete or partial loss of vision.

Correction :

• This can be corrected by surgical removal of extra growth (cataract surgery) to

restore vision.

Astigmatism:

• It is a kind of defect in human eye due to which a person cannot see (focus)
simultaneously horizontal and vertical lines both.

Cause (reason) :

• This condition arises either when the cornea or the crystalline lens or both are not
perfectly spherical.

Correction:

• By using a cylindrical lens.

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