Light and Vision How do we see?

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Light and VisionHow do we see?
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The Electromagnetic Spectrum
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Things to Think About Today

• Light waves travel in straight lines until they
  meet matter
• Then, light is reflected, absorbed, refracted
• Why do animals have eyes? How did eyes evolve?
• What are the parts of the eye and how do they
  work to bend light

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Light Energy Meets Matter
Refracted (Bent)
Reflected
Absorbed
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• Light waves travel in straight lines until they
  meet matter
• Some matter ( lenses) focus rays and bend them in
  defined ways

• Pin activityBoxes with solutions/laser pointer,
  ray box
• Ray boxes with lenses, gelatin lenses

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Convex Lenses
Magnifying Lenses are Convex lenses
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Concave Lenses(divergent lenses)
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Eyes Detect and Focus Light

• Eyes are organs that detect light, and convert it
  to electro-chemical impulses in neurons
• In higher organisms the eye is a complex optical
  system which
• collects light from the surrounding environment
• regulates its intensity through a diaphragm
• focuses it through an adjustable assembly of
  lenses to form an image
• converts the image into a set of electrical
  signals
• transmits these signals to the brain, through
  complex neural pathways that connect the eye, via
  the optic nerve, to the visual cortex and other
  areas of the brain.

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3 types of eyes have evolved

• Single centered lens
• Advanced mollusks (octopus), spiders,vertebrates
• Many tiny lenses
• Arthropods (trilobites)
• Ex. Ants (50 images) Horsefly (7,000 images)
  Dragonfly (30,000 images)
• Hole without lens
• snakes

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Parts of the Eye that you can See!
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Functions of Parts of the Eye

• Eyelid- protection of eye from injury
• Pupil- hole in the center of the iris that lets
  light in
• Appears black because the eye tissue absorbs most
  of the light
• Sclera-tough outer layer
• collagen and elastin
• Iris-colored area surrounding pupil
• Composed of smooth muscle

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Interior Parts of the Eye
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Path of an Image

• Goes through the Cornea and Lens
• Hits the back of the retina
• Activates rod and cone cells
• Transmit impulse through optic nerve to the brain

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Rod and Cone Cells

• Rod Cells- numerous ( 130 million in each eye
• Responsible for Twilight vision, Gray shades
• Very sensitive to light
• Found on outside of
• retina
• Cone Cells-
• less numerous
• ( 7 million)
• Responsible for
• Ability to see Color
• Found in center of the retina
• (macula)

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Now, lets think about what might go wrong
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Myopia- Near-sighted

• Genetic Basis
• Eye is longer than normal, Cornea steeper
• Correct with glasses, contacts, surgery
• Very common!
• Can make it better by squinting!- Changes the
  eyeball shape!

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Myopia Near Sightedness
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Hyperopia- Far-sighted

• Image is focused behind the eyeball
• Cornea is flatter, eye is shorter, focusing power
  is weaker
• Young people can accommodate by changing shape
  of eye As people get older, lens get harder and
  cant be bent.
• Lasix surgery changes shape of cornea
• http//www.ncbi.nlm.nih.gov/pubmedhealth/PMH000485
  3/

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Hyperopia Farsightedness
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Cataracts

• Clouding of the lens
• Caused by aging or damage to the proteins that
  make up the lens
• Chemicals, Diabetes, Injury, Smoking are
  causative
• Surgery to remove lens

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Normal/Cataract
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Retinitis Pigmentosa

• Rare genetic disease (1/4000 people in US)
• Rods die and leave dark deposits in retina
• Lose peripheral
• and night vision

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Normal/Retinitis
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What about compound vision?
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A Grid- Seen by a Human and an Insect
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A spider web- seen by a human and an insect
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Bee as seen by a mammal and an insect
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Insect images
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Insects See Colors Differently
http//www.naturfotograf.com/UV_flowers_list.html
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What about other animals?
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Bird Vision

• How do nocturnal birds see?
• Rod cells are very sensitive in low light
• Humans have around 200,000/mm2, some owls have
  over a million/mm2
• Birds also have larger pupils which let more
  light in
• What about day foragers
• Have more cones than humans
• May have more than one fovea area of sharpness)

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Ruminant Pupils

• Slit-like in bright light, circular in dim light
• Gives permanent wide-angle vision whether in dim
  or bright light -makes watching for predators
  more efficient.

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Making the connection between physical science
and biology

• Light travels in straight lines
• Light bends when it meets matter
• Lenses are matter that bend light
• How animals see
• Eye diseases
• Do you need light to see?

• Ray box
• Bending Experiment
• Play with Lenses
• Eye Dissection
• Annenberg video

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Video
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Wrap up Questions

• What is seeing?
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• Can a human being see in the dark? Why?
• How would a scientist test if you could see in
  the dark?

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Reading Resources

• A Natural History of the Senses-Diane Ackerman
• Video- Annenberg Foundation
• GEMS- http//lawrencehallofscience.org/gems/gemsgu
  idestopic.html

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Exploring Light and Lenses

• Concave Lenses and light
• Pinhole Viewer (Camera)
• Camera Obscura

Camera Box
Camera Obscura (CA)