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- 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.
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- 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
22 object is too far awaytoo far away
33 object is too smalltoo small
cannot see /cannot see clearly
p.77p.77
Extending our visionSeparate 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
AA BB CC
DD EE FF
Distant objects Small objects1 Astronomical te
lescope
33
22
11
22
33
MicroscopeTelescope
Binoculars
Electron microscope
Hand lens
Galileo, an Italian physicist physicist and and astronomer, astronomer, was the was the first person to point afirst person to point a telescope to the sky.
Using his small telescope, Galileo observed the mountains on the Moon.
p.78p.78
Hubble Space Telescope is the world’s first space-based optical telescope.
HowHow can it can it ‘take’ ‘take’ the the imageimage??
WhoWho orders orders its jobs?its jobs?
WhenWhen do do the orders send?the orders send?
http://www.phy.cuhk.edu.hk/phyworld/
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Visit the following website to know more about the work of Hubble Space Telescope.
Spitzer Space Telescope was named after the great scientist, Lyman Spitzer.
How great is he?
http://www.spitzer.caltech.edu/
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Visit the following website to know more about the work of Spitzer and to see more images taken by the Spitzer Space Telescope.
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
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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 spotspot
p.81p.81blind 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.
blind 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
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 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.
Do you knowDo you know why why we we cannot see the cannot see the ghostghost all the all the
way through?way through?
Why don’t weWhy don’t we noticenotice the the blind blind
spotspot easily easily??
- 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.
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?
