The Human Eye

• Is an optical device able to respond to an enormous range of light brightness

• It is able to focus on objects from billions of kilometers away to those a few

centimeters away. It can also detect colour (See the fig. below)

Parts of the human Eye 1. Cornea

 The cornea is the transparent outer 'window' of the eye, which covers the iris

and the pupil at the front of the eye. The cornea, together with the lens,

refracts or changes the direction of light to focus it on the retina.

2.. Iris

• The iris is the coloured part of the eye which surrounds the pupil.

• It controls the amount of light going into the eye by changing the size of the

pupil using tiny muscles to contract and dilate it. (It acts like diaphragm of the

camera)

3. Pupil

• Your pupil is the black circle in the center of your iris. It regulates how much

light enters your eye. Interestingly, the pupil appears black because most of

the light entering the pupil is absorbed by the tissues inside the eye. When it

is very light the pupil is small and when it is dark the pupil gets bigger to allow

more light into the eye.

• The pupil functions in the same way as the aperture of a camera. The size of

the pupil determines the amount of light entering the eye.

4. Lens.

• The lens is a transparent part of the eye behind the iris. The purpose of the

lens is to focus light onto the retina at the back of the eye by bending light

rays so that they form a clear image on the retina. The lens is elastic, so it is

able to change shape to focus on to the retina. Its shape gets fatter to focus

close objects and thinner to focus distant objects on the retina.

• In persons under 40 years of age, the lens is soft and flexible, allowing for fine

focusing from a wide variety of distance

5. Retina

• The retina is a thin layer of nerve cells which contains millions of

photoreceptors that form the light-sensitive inner lining at the back of the eye.

Rays of light enter the eye and are focused

on the retina by the cornea and lens. The retina converts light into electrical

impulses to produce an image which is sent along the optic nerve to the brain

to interpret.

• There are two types of photoreceptors: rods work in low light and are

responsible for night vision and cones work best in bright light and provide

perceptions of colour and fine detail.

• The retina compares to the film in a lens camera.

6. Optic nerve

 This nerve carries electrical impulses from the rods and cones in the retina to

the visual cortex in your brain. Without the optic nerve, your other eye

components cannot send images to your brain and produce your sense of

sight.

7. Sclera

 The sclera is more commonly known as the whites of your eyes. This fibrous

layer contains collagen and protects the inner components of your eye from

damage.

8. Vitreous humour

• The vitreous humour is a jelly-like substance that fills the body of the eye. It is

normally clear

• Vitreous humour helps to maintain eye pressure and also helps in focusing

light rays

9. The ciliary muscles:

• These control the thickness of the lens during focusing. By contracting or

squeezing the lens, they make it thicker and vice versa. Because the power

of the lens is directly related to its thickness, the ciliary muscles change the

power of the lens by their movement.

10.

Aqueous Humor:

• The aqueous humour is a water-like fluid which fills the front of the eye

between the lens and cornea and provides the cornea and lens with oxygen

and nutrients.

How Human Eyes works

• Light enters the eye by passing through the transparent cornea and aqueous

humor. Some of this light enters the eye through an opening called the pupil.

The iris controls the size of the pupil, which is the opening that allows light to

enter the lens. Light is focused by the lens and goes through the vitreous

humor to the retina.

• Photoreceptors (Rods and cones) in the retina translate the light into an

electrical signal that travels from the optic nerve to the brain. Then the brain

turns the signals into the images you see.

Accommodation of human eye

• The ciliary muscles adjust the shape of the eye lens for focusing on the nearby

and far objects. By changing the shape of the eye lens the eye can change

the focal length of the lens. This mechanism of the eye is called the

accommodation.

• When the ciliary muscles are relaxed the lens is thin and has a longer focal

length so that the distant objects are properly focused at the retina.

• For a nearby object, the eye needs a smaller focal length lens, which is

achieved by contracting the ciliary muscles and thereby thickening the lens.

• For a healthy human eyes, visible range is from 25 cm to infinity.

• The distant of the closest object that a human eye can see is called the near-

point. For a normal eye, the near point is located 25cm from the eye.

• The distant of the farthest object that a human eye can see is called the far-

point. For a normal eye, the far point is located at infinity.

Eye Defects

• Many people have problems with their vision, or ability to see. Often, the

problem is due to the shape of the eyes and how they focus light.

• Two of the most common vision problems are nearsightedness (Myopia)

and farsightedness (Hypermetropia/Hyperopia)

• Therefore; Eye defect is the phenomenon where by eye defeat to see clearly.

Myopia

 Is a condition which occurs when a person can see near objects clearly but

cannot see distant objects clearly (ie. distant objects appear blurry).

Causes

• When the eyeball is too long

• When the refractive power of the lens is too strong

Correction

• To wear suitable concave lenses to diverge the rays from distant objects before

they reach the eye.

• See the figure below

Hypermetropia/Hyperopia (Long – sightedness)

 Is a condition which occurs when a person cannot see near objects clearly (ie,.

nearby objects appear blurry) but can see distant objects clearly,

Causes

• When the eye ball is too short

• When ciliary muscles are weak such that unable to change the shape of the

eye lens in order to focus the image (occurs when the refractive power of the

eye lens is too weak)

Correction

 The defect can be corrected by wearing suitable convex lenses so that the rays

from the near object are made to converge and focus on the retina. See the fig

below

Other eye defects can be categorized into: - Astigmatism

• Astigmatism is a condition caused by a refractive error, in which the eye does

not focus light evenly on the retina. This results in distorted or blurred vision

at any distance.

• Astigmatism is a common vision problem caused by a fault in the shape of the

cornea, resulting in an irregular curve. This can change the way light passes

through the cornea and refracts onto the retina. People with this condition have

blurry, fuzzy, or distorted vision.

Presbyopia

• The eyes lose their power of accommodation with aging. As people grow old,

the gradual weakening of the ciliary muscles and diminishing flexibility of the eye

lens results in hardening of the eye lens, making it more difficult for the eye to

focus on close objects. This causes the near point to gradually recede away in

older people. These people may find it difficult to see nearby objects distinctly

without corrective eyeglasses. This defect of Farsightedness caused by the loss

of elasticity of the lens of the eye is called Presbyopia.

• Sometimes, a person may suffer from both farsightedness and short-

sightedness. 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 to assist distant vision. The lower part is a convex

lens to assist near vision. NB.

• Optometrists and ophthalmologists usually prescribe lenses measured in

‘’diopters’’

 The power, P, of a lens in diopters equals the inverse of the focal length in

metres

ퟏ

풇

푷풐풘풆풓 =

=

That is:

(

)

푭풐풄풂풍 풍풆풏품풕풉 풎

Similarities of Human Eye and Lens Camera

• Both have a convex lens system to focus the image

• Both save images

• Both form a real, reduced and inverted image

• The amount of light entering is controlled by a variable aperture

• They both have surfaces on which the image is formed

• The retina behaves like the photographic screen of a camera

Differences between Lens Camera and Human Eye

Camera

Eye

Lens is hard glass

Only the lens refracts the light

Lens is soft and elastic

Aqueous and vitreous humour refracts the

light

Cameras do not have blind spot Human eyes have blind spot

Diaphragm controls the amount Iris adjusts the amount of light entering the eye

of light reaching the film

through the pupil.

Focal length of the lens is fixed

Focal length of the lens can be changed

Focuses

by

changing

the Focuses by changing the thickness of the eye

distance between the lens and lens

film

Worked Examples

1. The far point of a myopic person is 40 cm. What should be the power of the lens

that he must use to see clearly?

Given: f = 40 cm = 0.4 m, Power =?

From:

2. A certain eye can focus only on objects closer than 50.0 cm. What word

characterizes this type of vision problem? What sort of contact lens (described

both in focal length and power) will correct this problem?

ANS; This eye is myopic or nearsighted. We need to find a lens that will take an

object at infinity

(풖 = ∞) and produce a virtual image at 50.0

cm (풗 = −ퟓퟎ풄풎 = −ퟎ. ퟓ풎).

On solving using the lens formula;

diopter

3. John has hyperopia. When is he more likely to need his glasses: when he reads

a book or when he watches TV?

ANS:

With hyperopia, John is farsighted. He can probably see the TV more clearly

than the words in a book because the TV is farther away. Therefore, he is more

likely to need his glasses when he reads than when he watches TV.