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?