Presentation Fizik
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Transcript of 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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