11.4 Eye defects Eye defects
11.4
Eye defectsEye defects
How many people in the class are wearing glasses?
Do you know the reasons?
- Ageing or inherited defect may cause eye defects.
- Eye defects Eye defects includeinclude :
short sight ( 近視近視 )short sight ( 近視近視 )
astigmatism( 散光散光 )astigmatism( 散光散光 )
cataract ( 白內障白內障 )cataract ( 白內障白內障 )
colour blindness( 色盲色盲 )…colour blindness( 色盲色盲 )…
long sight( 遠視遠視 )long sight( 遠視遠視 )
The common
types
of eye defects
The common
types
of eye defects
Short sight and long sight and their corrections
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This experiment uses the flask for demonstration.
A : normal eyeA : normal eye
B : short-sighted eye
B : short-sighted eye
C : long-sighted eye
C : long-sighted eye
inside wall of the flask represents the retinaretina
AB
C
green fluorescent green fluorescent dye solutiondye solution
Convex lenses represent lenseslenses
Experiment Centre 11.2Experiment Centre 11.2
I Normal sight, short sight and long sight I Normal sight, short sight and long sight
1 Shine a beam of light a beam of light through lens Alens A
- The beam of lightThe beam of light gets narrower and meets at one point, focus.
- A sharp image is formed
a Where does the beam of light come to a focus?
It comes to a focus ____
____ (in front of/on/ behin
d) the ‘retina’.
b Is the image formed on the ‘retina’ sharp?
______ (Yes/No)
onon
YesYes
This is a model of the This is a model of the normal eye.normal eye.This is a model of the This is a model of the normal eye.normal eye.
2 Shine a beam of light a beam of light through lens Blens B
a Where does the beam of light come to a focus?
__________________
b Is the image formed on the ‘retina’ sharp?
_________________________
In front of the ‘retina’
No, the image is not sharp.No, the image is not sharp.
This is a model of the This is a model of the short-sighted eye.short-sighted eye.This is a model of the This is a model of the short-sighted eye.short-sighted eye.
3 Shine a beam of light a beam of light through lens Clens C
a Where does the beam of light come to a focus?
__________________
b Is the image formed on the ‘retina’ sharp?
_________________________
Behind the ‘retina’
No, the image is not sharp.No, the image is not sharp.
This is a model of the This is a model of the long-sighted eye.long-sighted eye.This is a model of the This is a model of the long-sighted eye.long-sighted eye.
II Corrections of short sight and long sight
II Corrections of short sight and long sight
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The shapes of the convex lens and concave lens are different.
Convex lens
Convex lens
Concave lens
Concave lens
- Thick in middle, thin in margin
- Thin in middle, thick in margin
- Converge lightConverge light - Diverge lightDiverge light
Convex lens Convex lens can converge converge parallel lightparallel light
Concave lensConcave lens can diverge diverge parallel lightparallel light
4 Put a lens in front of lens Blens B What kind of lens can bring the light to
a focus on the ‘retina’ of short-sighted eye?
_______________Concave lensConcave lens
5 Put a lens in front of lens Clens C What kind of lens can bring the light to
a focus on the ‘retina’ of long-sighted eye?
_______________Convex lensConvex lens
- People with normal sight can see near and distant objects clearly.
lightfrom
distant distant objectobject
lightfromnear near
objectobject
light light is is focused focused onon the the retinaretina
light light is is focused focused onon the the retinaretina
sharp sharp image image is is formedformed
sharp sharp image image is is formedformed
- Cannot see distant objects clea distant objects clearlyrly
- Can see near objects near objects clearlyclearlyShort Short sightsightShort Short sightsight
lightfrom
distant objectdistant object
light light is is focused focused in front in front ofof the the retinaretina
light light is is focused focused in front in front ofof the the retinaretina
light from from
near objectnear object
What happens to the What happens to the lenses lenses of your eyesof your eyes??
Look at theLook at the stars stars in the sky. in the sky.
Cause Cause of of short sightshort sightCause Cause of of short sightshort sight
lens is too thick
lens is too thick
eyeball is too long
eyeball is too long
or
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Correction forCorrection for short sight short sightCorrection forCorrection for short sight short sight
person withperson with short sightshort sight
after after correctioncorrection
lightfrom
distant distant objectobject
WearWear glasses glasses withwith concave lensconcave lens
WearWear glasses glasses withwith concave lensconcave lens
lightfrom
near objectnear object
lightfrom
distant distant objectobject
- Cannot see near objects clearl near objects clearlyy
- Can see distant objects distant objects clearlyclearlyLong Long sightsightLong Long sightsight
light light is is focused focused behind behind
the the retinaretina
light light is is focused focused behind behind
the the retinaretina
When you look at the When you look at the book on the book on the desk desk after looking at the blackboardafter looking at the blackboard,,
what happens to the what happens to the lenses lenses of your eyesof your eyes??
p.84p.84Cause Cause of of long sightlong sightCause Cause of of long sightlong sight
lens is too thin
lens is too thin
eyeball is too short
eyeball is too short
or
WearWear glasses glasses withwith convex lensconvex lensWearWear glasses glasses withwith convex lensconvex lens
lightfromnear near objectobject
after after correctioncorrection
person withperson withlong sightlong sight
Correction forCorrection for long sight long sightCorrection forCorrection for long sight long sight
According to a study carried out in 2003, 2003, Hong Kong has the highest percentage the highest percentage of of secondary school students with short sight in the world.
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0 10 20 30 40 50 60 70
美國
日本
HongKong
Japan
Sweden
UnitedStates
Percentage of secondary school students with short sight
Discuss with your classmates:> > Why is it so common of students with short si
ght?> > What are the improvement methods?improvement methods?
Look at the centre with one eye. Note whether some lines look more blurry. If they do, you may have astigmatism.
Do you have astigmatism?
Do you have astigmatism?
Testing method:Testing method:
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You should consult an eye doctor.
You should consult an eye doctor.
AstigmatismAstigmatismAstigmatismAstigmatism
- Caused by uneven curvature of the cornea or lens
- Correction methodCorrection method :
> > Wearing lenses with lenses with different curvature a
t different points
> > Correction surgeryCorrection surgery
Apart from wearing glasseswearing glasses to correct short sight, long sight and astigmatism, some people try the surgical correction method —— LASIK.
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corrects the corrects the problem by usingproblem by using a a
laser beamlaser beam to to change the change the
curvature of cornea curvature of cornea
corrects the corrects the problem by usingproblem by using a a
laser beamlaser beam to to change the change the
curvature of cornea curvature of cornea
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You may visit the following website to see how the surgery is carried out.
http://www.lzr.com/am_i_a_candidate /video_link
http://www.lzr.com/am_i_a_candidate /video_link
Have you heard of the ‘Lifeline Express train’ ( 健康快車 )?Have you heard of the ‘Lifeline Express train’ ( 健康快車 )?
These trains go to the faraway, poor rural areas of China and provide free
operations to the people having cataract.
Image seen byImage seen bypeople with people with cataractcataract
Image seen byImage seen bynormal peoplenormal people
CataractCataractCataractCataract
- Lenses of people with cataract become cloudy
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=> => Stop light from passing through => => Blurred images
=> => Lead to blindness
- Can be replaced by a ‘man-made lens’
- Main cause: people become old
- Main cause of blindnessblindness in mainland China.
CataractCataractCataractCataract
- Other causes: inherited defect, diabetes...
活動園地 F p.86p.86
Test for colour blindnessWrite down what you see (figure or picture) in the following test cards.
11 22 33
Activity Corner FActivity Corner F
44 55 66
If you are If you are in doubt of havingin doubt of having colour colour blindnessblindness, visit, visit an eye doctor an eye doctor to havto have a more detailede a more detailed eye check. eye check.
If you are If you are in doubt of havingin doubt of having colour colour blindnessblindness, visit, visit an eye doctor an eye doctor to havto have a more detailede a more detailed eye check. eye check.
Colour bliColour blindnessndnessColour bliColour blindnessndness
- red-red-green green colour-blindnesscolour-blindness is the most common is the most common
=>=> Having a defect in Having a defect in at least one of the three types of colour-sensitive cone cells on the on the retina.
Unable to Unable to distinguishdistinguish red red
fromfrom green green
Unable to Unable to distinguishdistinguish red red
fromfrom green green
The image seen The image seen by by normal normal
peoplepeople
The image seen The image seen byby red-green col red-green colour-blind peopleour-blind people
-- People with total colour blindness are unable to distinguish all colours.
-- Colour blindness is an inherited defect.
-- More commonly found in males.
The image seen by The image seen by normal peoplenormal people
The image seen by The image seen by total colour-blind ptotal colour-blind p
eopleeople
> > can also live normally just as other people do.
-- Colour-blind people:
> > are not able to work in areas like colour matching.
-- Cannot be Cannot be corrected
Colour blindnessColour blindnessColour blindnessColour blindness
80% of our received messages comes from eyes. If our eyes have any defect, it seriously affects our sensations towards the environment. So we should take care of our eyes.
Can youCan you list list five five methods methods of of taking care of eyes?taking care of eyes?
Can youCan you list list five five methods methods of of taking care of eyes?taking care of eyes?