Chapter 6 Human Vision can be corrected and extended using optical systems.

Chapter 6

Human Vision can be corrected and extended using optical systems

In chapter 6 you will learn:

• How human vision works• The parts of the eye• Ways to fix poor eyesight• Dissect a sheep’s eye and compare it to a

human eye• Explain how optical devices such as

microscopes and telescopes use lenses and mirrors to magnify objects

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Parts of the eye

• Pupil- this is the hole the light passes through• Iris- is the colored circle of muscle surrounding the pupil

(your eye color)- this allows the pupil to get smaller in bright light and larger in dim light.

• Dilated pupil- dim light- larger- more light can get in



• Contracted pupil- bright light- smaller-prevents too much light from getting in

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• Cornea- is a transparent tissue that covers the pupil and iris

• Sclera- is a strong tissue that surrounds the eye and holds it together



• Lens- is behind the pupil and is convex. As the light passes through it, it is bent and the image falls on the back of the eye on the retina.

• Retina- has special light sensitive cells called rods and cones.The cones see colored things and the rods work in dim light and can only pick up black and white

• Optic nerve- signals picked up by cells in the retina are changed to electrical signals and these are passed along a thick nerve fiber called the optic nerve which goes to the brain

• Brain- this interprets the electrical signals and gives them meaning (eye sight) . This is the main organ of hearing.



• Aqueous humor- this is liquid located in front part of the eye. It provides nutrients

• Vitreous humor- this is thick jelly behind the lens- this gives the eye shape and strength




Cornea-lens-retina system

1. Light rays begin to be focused as soon as they pass into the cornea. (most focusing is here)

2. Rays next pass through the lens which focuses a bit more.the lens can change shape-when it is fat it focuses close up and when stretched and thin focuses far away

3. The light rays hit the retina where they form an upside down (inverted) image

4. The brain now interprets the image as the right side up

Blind spotBlind spot

• Area where the optic nerve enters the retina does not have any light sensing cells. This area is known as the blind spot

• Try the exercise on page 205.





Black and white and color vision

• We have 2 kinds of cells in the retina

• Long skinny cells called rod cells make up our black and white vision system and in low light conditions they help us to see shapes and movement



• Cone cells- allow us to detect color. We have three kinds of cone cells (red, blue and green). We can see all colors of the rainbow using these 3 color cells.





Activity:

• Do Activity 6.2 on page 207. Do all questions

Correcting Focus Problems

• Near sightedness- means you can see nearby objects clearly but not faraway objects (called myopia)

• This happens because the image falls in front of the retina

• Concave lenses fix this condition



• Far-sightedness-this is when you can see far way objects but close up objects are fuzzy.

• The object image falls behind the retina• You need convex lenses to fix this







• Astigmatism- this happens when the cornea is uneven in shape.It is corrected with laser surgery or with contact lenses or glasses



• Blindness-most blind people can perceive some light or have very limited vision. Lots of different kinds of blindness. Can be caused by disease, birth,or malnutrition. Can be curable sometimes




• Snow blindness- painful condition of temporary or complete blindness caused by overexposure to the glare of sunlight





• Night blindness- condition where you cannot see in dim light. Not curable



• Color blindness- ability to see only in shades of grey. Some forms involve not seeing one of the primary colors

• Most common type is inability to tell red and green apart



• Assignment: read the letter on page 211 and answer the questions. Be prepared to discuss these questions

Optical instruments• Mirrors and lenses are used alone or in

combination in instruments which magnify such as microscopes, telescopes and binoculars






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• Compound microscopes use two convex lenses with short focal lengths to magnify small, close objects

Refracting telescopes

• These enlarge distant things using 2 convex lenses . They work like microscopes but have a longer focal length.



Limitations

• Refracting telescopes are limited in their use. To see really far away they need large lenses which are expensive and heavy.

Reflecting telescopes

• Most large telescopes are reflecting telescopes.• These use a concave mirror, a lens and a plane

mirror to work

Hubble Space Telescope

• The Hubble Space Telescope gets better views of space objects because the atmosphere is not present to distort the images





Binoculars

• These are really two telescopes mounted side by side

• The telescopes are shortened by placing prisms inside that serve as plane mirrors.

Cameras

• These work like our eyes.• Our pupil is like the camera aperture• Both are adjustable depending on light conditions





• Wide angle lenses produce pictures with a wider field of view but smaller objects. Good for scenery





• Telephoto lenses take great close up pictures• They have a longer focal length than wide-angle

lenses



Eye camera comparison



• Eye/camera comparisons: use page 223 to compare the parts of the eye to a camera:

• Pupil=__________________

• Eyelid=_________________

• Iris=____________________

• Retina=_________________

• Rods & cones=____________________

Chapter 6 Human Vision can be corrected and extended using optical systems.

Documents

light sensing cells

color cells

special light sensitive

kinds of cells

awaythe light rays

eye color

low light conditions

rod cells