11.3

Limitations of Limitations of

our eyesour eyes

- We can see many things, but we cannot see things at all distances or of all sizes.

- For people with normal eyesight, they still have limitations of their eyes.

11 object is too near too near

p.76p.76Limitations of our eyes Limitations of our eyes

22 object is too far awaytoo far away

33 object is too smalltoo small

cannot see /cannot see clearly

We can overcome some of the limitations by using suitable instruments.

p.77p.77

Extending our vision

Separate the following instruments into two groups according to their functions and arrange them in the order of how well they serve that function.

Activity Corner CActivity Corner C

AAAA BBBB CCCC

DDDD EEEE FFFF

Distant objects Small objects

1 Astronomical telescope

33

22

11

22

33

MicroscopeTelescope

Binoculars

Electron microscope

Hand lensVideoVideo

VideoVideo

We cannot see clearly when the object is too small, too near or too far away. There is another limitation of our eyes blind spot.

What is blind spot?What is blind spot?

Find it out from Activity Corner EActivity Corner E

p.80p.80

Why does the ghost disappear?1 Hold your book at your eye level. Cover

your left eye. Look at the cross at the top of the castle in the picture below.

Can you see the ghost and the cross at the same time? ______

Activity Corner EActivity Corner E

2 Repeat step 1. This time, keep looking keep looking at the crossat the cross while you slowly move the book towards you. Can you see the ghost all the way through?

We cannot see We cannot see the object when its image

falls on the ______________________ of the retina. blind spotblind spot

Is there also a blind spot in your left eye? ______________

3 Repeat the experiment. This time, cover your right eye. Can you see the ghost all the way through?

blind blind spotspotblind blind spotspot

p.81p.81blind spot (盲點 ) blind spot (盲點 )

- Where the optic nerve leaves the eye

- There are no light-sensitive cellslight-sensitive cells

=> => We are We are not able to see the not able to see the image image falling on it.falling on it.

=> => We are We are not able to see the not able to see the image image falling on it.falling on it.

blind blind spotspotblind blind spotspot

retina

(with light-

sensitive cells)

We are able to We are able to see see the image the image

falling on falling on itit

We are able to We are able to see see the image the image

falling on falling on itit

Why does the ghost disappear?Why does the ghost disappear?

retina

(no light-

sensitive cells)

We are We are not not able able to to see see the image the image

falling on falling on itit

We are We are not not able able to to see see the image the image

falling on falling on itit

We are able to We are able to see see the images the images at at the same time.the same time.

We are able to We are able to see see the images the images at at the same time.the same time.

We are able to We are able to see see the images the images at at the same time.the same time.

We are able to We are able to see see the images the images at at the same time.the same time.

The image of ghost is falling on the blind spot, so the ghost disappears. We are able to We are able to see see the castlethe castle only. only.

The image of ghost is falling on the blind spot, so the ghost disappears. We are able to We are able to see see the castlethe castle only. only.

Do you knowDo you know why why we we cannot see the cannot see the ghostghost all the all the

way through?way through?

Do you knowDo you know why why we we cannot see the cannot see the ghostghost all the all the

way through?way through?

Activity Corner DActivity Corner D

Why don’t weWhy don’t we noticenotice the the blind blind

spotspot easily easily??

Why do we have a pair of eyes but not an eye??

- Lights from objects enter both eyes. As the positions of reflected lights are different, there may be minor differences of the images seen by the two eyes.

- The brain combines the two images.

=> => Images with 3-D dimensions

=> => We can judge judge the distancethe distance

It shows that we cannot judge distances by using one eye.

It shows that we cannot judge distances by using one eye.

If you cover an eyecover an eye, hold a piece of pencil and walk towards another person holding a piece of pencil,

can your pencil touch the other one easily ??

Many animals have their eyes located at two sides of the head. This can broaden their visions.

e.g. elephant, rabbit…

But, they do not have 3-D vision.

What kinds of animals do not have 3-D vision?

What kinds of animals do not have 3-D vision?