Limitations of our eyes 11.3 Limitations of our 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.

1 too near 1 object is too near p.76 Limitations of our eyes 2too far away 2 object is too far away 3too small 3 object is too small cannot see / cannot see clearly

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

p.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 C

AABBCC DD EEFF Distant objects Small objects 1 Astronomical telescope Microscope Telescope Binoculars Electron microscope Hand lens

physicist and astronomer, was the first person to point a Galileo, an Italian physicist and astronomer, was the first person to point a telescope to the sky. Using his small telescope, Galileo observed the mountains on the Moon. p.78

Hubble Space Telescope is the world’s first space-based optical telescope. How can it ‘take’ the image? Who orders its jobs? When do the orders send?

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?

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? Find it out from Activity Corner E

p.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 E

keep looking at the cross 2 Repeat step 1. This time, keep looking at 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 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 spot p.81 blind spot ( 盲點 ) - Where the optic nerve leaves the eye light-sensitive cells - There are no light-sensitive cells => We are not able to see the image falling on it. blind spot

retina (with light- sensitive cells) We are able to see the image falling on it Why does the ghost disappear?

retina (no light- sensitive cells) We are not able to see the image falling on it

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

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

Do you know why we cannot see the ghost all the way through?

Why don’t we notice the blind spot 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 => judge the distance => We can judge the distance

It shows that we cannot judge distances by using one eye. cover an eye If you cover 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?