Vision

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Vision
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Eye anatomy
Flexible, Converging lens, Whose focal length
can be changed by pulling on it!
Retina
common visual problems (or dont show)

• http//www.1800contacts.com/staticcontent/vision10
  1/frames.html

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• Accommodation the lens in our eyes can change
  shape (variable focal length) to enable us to
  focus on both near and far objects
• Ciliary muscles strained
• shorter f
• (To focus on close objects)
• Ciliary muscles relaxed
• longer f
• (To focus on far objects)

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Normal 20-20 Vision (see your handout!)
Ideally ?
Ideally 10-15 cm
Range of Clear Vision
Far Point
Near Point
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FARSIGHTED
Can only see FAR objects CLEARLY (write
this!!!) Rays from near objects dont converge
enough (eyeball is too short hyperopia) Thus
use converging lens to correct for farsightedness
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Image, at or beyond their near point
For a Farsighted Person
Object
Near Point
Far Point
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Far-sighted eye lens too weak eye too short
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NEARSIGHTED
Can only see NEAR objects CLEARLY (write
this!!!) Rays from far objects converge too much
(eyeball is too longmyopia) Thus use
diverging lens to correct for nearsightedness
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For a Nearsighted Person
Object
Near Point
Far Point
Image, at or before their far point
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Near-sighted eye lens too strong eye too long
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The unit Diopters (D)

• Eye doctors refer to the refractive Power of a
  lens in Diopters, not focal length
• P1/f
• A lens of P4 diopters
• means f1/4 m
• (greater converging power than a lens of P3
  diopters)

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Chromatic Aberration

• blue refracts more than the red,
• blue has a shorter focus. (demo this with lens
  as magnifying glass and light from OHP lens
  close to OHP, move side to side to see rainbow)

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Correcting Chromatic Aberration

• use a diverging lens with different n alongside
  the converging lens
• Achromatic doublet (achormat) is often used to
  compensate for the chromatic aberration
• the focuses for red and blue is then the same!

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Spherical Aberration

• Spherical aberration comes from the spherical
  surface of a lens
• The further away the rays from the lens center,
  the bigger the error is.
• The image is improved if we cut out (stop) the
  rays from top and bottom

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Color Blindness (1 in 12 males are color blind!
1 in 100 females!)
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• The colors affected are usually limited to green
  and red, which often appear as shades of tan or
  brown. Less often, the color blue may also be
  affected
• WHAT CAUSES IT?
• A genetic defect in your cones

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Rods and Cones

• Cones (red, green, blue) for more precise
  vision, need strong light (AKA photopic vision).
  help to see colors (thus we dont see color in
  dim light). Mostly distributed in the center of
  the retina (fovea).
• Rods for peripheral and night vision (AKA
  scotopic vision). Sensitive to light. Mostly
  distributed on sides of retina.

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Color Mixing of light

• Primary colors red, green, blue
• When mixed together
• WHITE!
• See
• http//pdukes.phys.utb.edu/PhysApplets/Colors/Tabb
  edcolorBox.html

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Eye Fun Facts

• Most people blink every 2-10 seconds.Each time
  you blink, you shut your eyes for 0.3 seconds,
  which means your eyes are closed at least 30
  minutes a day just from blinking.
• The reason cat's and dog's eyes glow at night is
  because of silver mirrors in the back of their
  eyes called the tapetum. This makes it easier for
  them to see at night.
• An ostrich has eyes that are two inches across.
  Each eye weighs more than their brain.
• A chameleon's eyes can look in opposite
  directions at the same time.
• A newborn baby sees the world upside down because
  it takes some time for the baby's brain to learn
  to turn the picture right-side up.

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A person who sees more clearly under water than
in air without eyeglasses is 1) Nearsighted 2)
farsighted 3) neither
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Nearsighted! The speed of light in water is less
than in air, so the change in speed is less as
light goes from water to your eye. Less
refraction occurs. This makes all people more
farsighted under water, which is advantageous if
youre nearsighted. If youre very nearsighted,
the image may fall on your retina and youll see
as clearly under water as a person with normal
vision who wears an air-enclosed mask.
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FunOptical Illusions
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What do you see?
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What do you see?
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Square A and B are the same shade of grey