Presentation Fizik

8/3/2019 Presentation Fizik

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The eye

The normal eye focuses light and produces a sharpimage Essential parts of the eye

Cornea light passes through this transparentstructure

Aqueous Humor clear liquid behind the cornea The pupil

A variable aperture An opening in the iris

The crystalline lens Most of the refraction takes place at the outer surface ofthe eye Where the cornea is covered with a film of tears


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The iris is the colored portion of the eye : It is a muscular diaphragm that controls pupil size

The iris regulates the amount of light entering the eyeby dilating the pupil in low light conditions and

contracting the pupil in high-light conditions. The f-number of the eye is from about 2.8 to 16


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operation

The cornea-lens system focuses light onto the backsurface of the eye

This back surface is called the retina

The retina contains receptors called rods and cones These structures send impulses via the optic nerve to

the brain

The brain converts these impulses into our

conscious view of the world


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Rods and Cones

Chemically adjust their sensitivity according to the

prevailing light conditions The adjustment takes about 15 minutes

This phenomena is getting used to the dark

Accommodation

The eye focuses on an object by varying the shape ofthe crystalline lens through this process

An important component is the ciliary musclewhich issituated in a circle around the rim of the lens

Thin filaments, called zonules, run from this muscle to

the edge of the lens


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focusing

The eye can focus on a distant object

The ciliary muscle is relaxed

The zonules tighten

This causes the lens to flatten, increasing its focallength

For an object at infinity, the focal length of the eye isequal to the fixed distance between lens and retina

This is about 1.7 cm


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The eye can focus on near objects

The ciliary muscles tenses

This relaxes the zonules

The lens bulges a bit and the focal length decreases

The image is focused on the retina


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The Eye Near and Far Points

The near pointis the closest distance for which the lenscan accommodate to focus light on the retina

Typically at age 10, this is about 18 cm

It increases with age

The far pointof the eye represents the largest distancefor which the lens of the relaxed eye can focus light onthe retina

Normal vision has a far point of infinity


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Conditions of the Eye

Eyes may suffer a mismatch between the focusingpower of the lens-cornea system and the length of theeye

Eyes may be

Farsighted

Light rays reach the retina before they converge toform an image

Nearsighted Person can focus on nearby objects but not those

far away


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Farsightedness

Also called hyperopia

The image focuses behind the retina

Can usually see far away objects clearly, but notnearby objects


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Correcting Farsightedness

A converging lens placed in front of the eye cancorrect the condition

The lens refracts the incoming rays more toward theprinciple axis before entering the eye

This allows the rays to converge and focus on the

retina


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Nearsightedness

Also called myopia

In axial myopiathe nearsightedness is caused by thelens being too far from the retina

In refractive myopia, the lens-cornea system is toopowerful for the normal length of the eye


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Correcting Nearsightedness

A diverging lens can be used to correct the condition

The lens refracts the rays away from the principleaxis before they enter the eye

This allows the rays to focus on the retina


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Presbyopia and Astigmatism

Presbyopia is due to a reduction in accommodationability

The cornea and lens do not have sufficient focusingpower to bring nearby objects into focus on the retina

Condition can be corrected with converging lenses

In astigmatism, there's a problem with the curve of thecornea. This causes part of the eye's image to be blurry.

Corrective lenses such as contact lenses or glasses canusually correct vision in people with astigmatism.


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Other common vision problems :

Blindness. This is the loss of useful sight. Blindness

can be temporary or permanent and it has manycauses. Birth defects or damage to any portion of theeye, the optic nerve, or the area of the brainresponsible for vision can lead to blindness.

Cataracts. Some children are born with cataracts, acloudiness of the eye's lenses that prevent imagesfrom being seen clearly or at all. But cataracts aremuch more prevalent in the elderly among whom

surgery to remove cataracts and correct vision iscommon.


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Color blindness. Color blindness is caused byproblems in the pigments of the cones in the retina.Most people who are color-blind can see some colors.In most cases, someone who is color-blind confusessome colors with others, usually red and green.

Eye injuries. Injuries to the eye are one of the mostcommon preventable causes of blindness. Eyeinjuries may be caused by irritants such as sand, dirt,or other foreign bodies on the eye's surface.Chemicals or foreign bodies that become embedded

in the eye can also cause pain and loss of vision.


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