Download Ch.6 Vision

Chapter 6
Human Vision can be corrected and
extended using optical systems
In chapter 6 you will learn:
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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
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• 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
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• 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.
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• Aqueous humor- this is liquid located in front part
of the eye. It provides nutrients
• Vitreous humor- this is thick jelly behind the lensthis gives the eye shape and strength
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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 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.
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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
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• 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.
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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
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• 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
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• Astigmatism- this happens when the cornea is
uneven in shape.It is corrected with laser surgery
or with contact lenses or glasses
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• 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
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• Snow blindness- painful condition of temporary or
complete blindness caused by overexposure to the
glare of sunlight
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• Night blindness- condition where you cannot see
in dim light. Not curable
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• 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
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• 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.
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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
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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
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• Wide angle lenses produce pictures with a wider
field of view but smaller objects. Good for scenery
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• Telephoto lenses take great close up pictures
• They have a longer focal length than wide-angle
lenses
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Eye camera comparison
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• Eye/camera comparisons: use page 223 to
compare the parts of the eye to a camera:
• Pupil=__________________
• Eyelid=_________________
• Iris=____________________
• Retina=_________________
• Rods & cones=____________________