F2 Worksheet 11.3, 11.4 answers

F.2 Integrated Science                                11.3    Limitations of our eyes

                                                                    11.4    Eye defects

i      Words to know:

	English term	Write the words again	Chinese term
1	Limitation	Limitation	限制
2	Short / Long sight	Short / Long sight	近 / 遠視
3	Cataracts	Cataracts	白內障
4	Colour-blindness	Colour-blindness	色盲
5	Concave / Convex lens	Concave / Convex lens	凹 / 凸透鏡

i      Fill in the blanks:

1       Our eyes have limitations. We cannot see objects clearly when they are too close or too far         away. We cannot see objects clearly when they are too small such as the micro-organisms in the sea.

2     We can use suitable instruments to overcome some of the limitations of our eyes, e.g. hand lens, telescope and microscope.

3      Besides limitations of our eyes, if we have an eye defects, we cannot see things clearly.

4        The followings are some examples of eye defects:

           a        short sight

           b        long sight

           c         cataracts

           d        colour-blindness

5       Two common eye defects are short sight and long sight which can be corrected by wearing spectacles with suitable lenses.

6        There are two types of lens, concave lens and convex lens.

Eye defects:

Short sight

-         A short-sighted person can see ­near objects but not distant objects clearly.

-         It occurs mainly because the lens is thicker than normal or the eyeball is too long.

-         The light from a distant object is focused in front of the retina.

-         It can be corrected by wearing spectacles with concave lenses.

Long sight

-         A long-sighted person can see distant objects but not near objects clearly.

-         It occurs mainly because the lens is thinner than normal or the eyeball is too short.

-         The light from a near object is focused behind the retina.

-         It can be corrected by wearing spectacles with convex lenses.

Cataracts

-         The lens of an eye is normally clear / colourless / transparent.

-         If the lens becomes cloudy, vision becomes unclear. This eye defect is called a cataract.

-         Cataracts can be treated by replacing the cloudy lens with an artificial lens.

Colour-blindness

-        Colour-blind people cannot distinguish certain colours.
The most common one is red-green colour-blindness. The patient cannot distinguish         between red and green.

-         It is a hereditary disease and cannot be cured.