Eye defects 11.4 Eye 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 include - Eye defects include : 近視 short sight ( 近視 ) 散光 astigmatism ( 散光 ) 白內障 cataract ( 白內障 ) 色盲 colour blindness ( 色盲 )… 遠視 long sight ( 遠視 ) The common types of eye defects The common types of eye defects

Short sight and long sight and their corrections p.82 This experiment uses the flask for demonstration. A : normal eye B : short-sighted eye C : long-sighted eye retina inside wall of the flask represents the retina A B C green fluorescent green fluorescent dye solution dye solution lenses Convex lenses represent lenses Experiment Centre 11.2

I Normal sight, short sight and long sight a beam of light lens A 1 Shine a beam of light through lens A The beam of light - The 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/ behind) the ‘retina’. b Is the image formed on the ‘retina’ sharp? ______ (Yes/No) on Yes This is a model of the normal eye.

a beam of light lens B 2 Shine a beam of light through lens 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. This is a model of the short-sighted eye.

a beam of light lens C 3 Shine a beam of light through lens 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. This is a model of the long-sighted eye.

II Corrections of short sight and long sight p.84 The shapes of the convex lens and concave lens are different. Convex lens Concave lens - Thick in middle, thin in margin - Thin in middle, thick in margin Converge light - Converge light Diverge light - Diverge light

Convex lens converge parallel light Convex lens can converge parallel light Concave lens diverge parallel light Concave lens can diverge parallel light

lens B 4 Put a lens in front of lens B What kind of lens can bring the light to a focus on the ‘retina’ of short-sighted eye? _______________ Concave lens

lens C 5 Put a lens in front of lens C What kind of lens can bring the light to a focus on the ‘retina’ of long-sighted eye? _______________ Convex lens

- People with normal sight can see near and distant objects clearly. light fromdistantobject light fromnearobject light is focused on the retina sharp image is formed

distant objects clearly - Cannot see distant objects clearly near objects clearly - Can see near objects clearly Short sight light from distant object light is focused in front of the retina lightfrom near object

What happens to the lenses of your eyes? Look at the stars in the sky.

Cause of short sight lens is too thick eyeball is too long orp.84

Correction for short sight person with short sight short sight aftercorrection light fromdistantobject Wear glasses with concave lens

light from near object light fromdistantobject near objects clearly - Cannot see near objects clearly distant objects clearly - Can see distant objects clearly Long sight light is focused behind the retina

When you look at the book on the desk after looking at the blackboard, what happens to the lenses of your eyes?

p.84 Cause of long sight lens is too thin eyeball is too short or

Wear glasses with convex lens light fromnearobject aftercorrection person with long sight Correction for long sight

2003, the highest percentage of According to a study carried out in 2003, Hong Kong has the highest percentage of secondary school students with short sight in the world. p.82 Hong Kong Japan Sweden United States Percentage of secondary school students with short sight Discuss with your classmates: > > Why is it so common of students with short sight? > improvement methods? > What are the 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? Testing method: p.85 You should consult an eye doctor.

AstigmatismAstigmatism - Caused by uneven curvature of the cornea or lens Correction method - Correction method : > lenses with > Wearing lenses with different curvature at different points > Correction surgery

wearing glasses Apart from wearing glasses to correct short sight, long sight and astigmatism, some people try the surgical correction method —— LASIK. p.85 corrects the problem by using a laser beam to change the curvature of cornea

p.85 You may visit the following website to see how the surgery is carried out. /video_link /video_link

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 by people with cataract Image seen by normal people

CataractCataract - Lenses of people with cataract become cloudy p.85 => => Stop light from passing through => => Blurred images => => Lead to blindness

- Can be replaced by a ‘man-made lens’ - Main cause: people become old blindness - Main cause of blindness in mainland China. CataractCataract - Other causes: inherited defect, diabetes...

活動 園地 F p.86 Test for colour blindness Write down what you see (figure or picture) in the following test cards.123 Activity Corner F

456 If you are in doubt of having colour blindness, visit an eye doctor to have a more detailed eye check.

Colour blindness red-green colour-blindness is the most common - red-green colour-blindness is the most common => Having a defect in on the => Having a defect in at least one of the three types of colour-sensitive cone cells on the retina. Unable to distinguish red from green

The image seen by normal people The image seen by red-green colour-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 normal people The image seen by total colour-blind people

> > 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 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 you list five methods of taking care of eyes?