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 :
short sight (近視)
long sight(遠視)
astigmatism(散光 )
cataract (白內障)
colour blindness(色盲)…
Experiment Centre 11.2
p.82
This
uses
the lenses
flask
demonstration.
Shortexperiment
sight and long
sight
andfor
their
corrections
Convex
represent
lenses
A : normal eye
B : short-sighted
eye
green fluorescent
dye solution
C : long-sighted
eye
C
A
B
inside wall
of the flask
represents
the retina
I Normal sight, short sight and long sight
1 Shine a beam of light through lens A
- 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 ____
on (in front of/on/
____
behind) the ‘retina’.
b Is the image formed on the ‘retina’ sharp?
Yes (Yes/No)
______
This is a model of the normal eye.
2 Shine a beam of light through lens B
a Where does the beam of
light come to a focus?
In front of the ‘retina’
__________________
This is a model of the
short-sighted eye.
b Is the image formed on the ‘retina’ sharp?
No, the image is not sharp.
_________________________
3 Shine a beam of light through lens C
a Where does the beam of
light come to a focus?
Behind the ‘retina’
__________________
This is a model of the
long-sighted eye.
b Is the image formed on the ‘retina’ sharp?
No, the image is not sharp.
_________________________
II Corrections of short sight and
long sight
The shapes of the convex lens and
concave lens are different.
Convex
lens
- Thick in middle,
thin in margin
- Converge light
p.84
Concave
lens
- Thin in middle,
thick in margin
- Diverge light
Convex lens can converge parallel light
Concave lens
can diverge
parallel light
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
_______________
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
from
distant
object
light
from
near
object
light is
focused
on the
retina
sharp
image is
formed
Short sight
- Can see near objects clearly
light
from
near object
- Cannot see distant objects
clearly
light
from
distant object
light is
focused
in front
of the
retina
Look at the stars in the sky.
What happens to the lenses
of your eyes?
Cause of short sight
lens is too
thick
or
eyeball
is too
long
p.84
Correction for short sight
light
from
distant
object
Wear glasses with
concave lens
person with
short sight
after
correction
Long sight
- Can see distant objects clearly
light
from
distant
object
- Cannot see near objects
clearly
light
from
near object
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?
Cause of long sight
lens is too
thin
or
eyeball
is too
short
p.84
Correction for long sight
light
from
near
object
Wear glasses with
convex lens
person with
long sight
after
correction
p.82
According
to aclassmates:
study carried out in 2003,
Discuss
with your
Hong
the highest
percentage
of
> Why Kong
is it sohas
common
of students
with short
secondary
school students with short sight
sight?
in
the world.
> What
are the improvement methods?
Hong
Kong
Japan
日本
Percentage of
secondary
school
students with
short sight
Sweden
United
美國
States
0
10 20 30 40 50 60 70
Do you have
astigmatism?
p.85
Testing method:
Look at the centre with one eye. Note
whether some lines look more blurry. If
they do, you may have astigmatism.
You should consult an eye
doctor.
Astigmatism
- Caused by uneven curvature of the cornea
or lens
- Correction method :
> Wearing lenses with different curvature
at different points
> Correction surgery
p.85
Apart from wearing glasses to correct
short sight, long sight and astigmatism,
some people try the surgical correction
method —— LASIK.
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.
http://www.lzr.com/am_i_a_candidate
/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 Image seen by
normal people people with
cataract
Cataract
p.85
- Lenses of people with cataract
become cloudy
=> Stop light from passing through
=> Blurred images
=> Lead to blindness
Cataract
- Main cause: people become old
- Other causes: inherited defect, diabetes...
- Can be replaced by a ‘man-made lens’
- Main cause of blindness in
mainland China.
活動園地
F F
Activity
Corner
p.86
Test for
colour
Write
down
whatblindness
you see (figure or
picture) in the following test cards.
1
2
3
4
5
6
If you are in doubt of having colour
blindness, visit an eye doctor to
have a more detailed eye check.
Colour
blindness
=> Having a defect in at
least one of the three
types of colour-sensitive
cone cells on the retina.
- red-green colour-blindness is the most common
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.
The image seen by
normal people
The image seen by
total colour-blind
people
- Colour blindness is an inherited defect.
- More commonly found
in males.
Colour blindness
- Cannot be corrected
- Colour-blind people:
> can also live normally just as
other people do.
> are not able to work in areas like
colour matching.
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?