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VISION

VIEWS: 4 PAGES: 38

									VISION
  Physics 6B
  Fall 2004
  Elizabeth Ko, Rana
  Hojatmehr, Francis
  Baking, Steven
Vision: An Introduction

                    The visual field is
                     mapped as an image on
                     a surface made up of
                     light-sensitive cells
                    Absorption of light by
                     pigments
                    Electrical effect ensues—
                     leads to an impulse in a
                     fiber of the optic nerve,
                     which travels to the
                     brain
The Human Eye: Basic Anatomy
Rods and Cones

Rods:
 Vision in low light
 Rhodopsin


Cones:
 Color vision and
  detail
 Color pigments
Perceiving Light

   Rhodopsin: a mixture of
    scotopsin and 11-cis-
    retinal
   Eventually forms
    activated rhodopsin
   Activated rhodopsin
    causes electrical impulses
    that are transmitted to
    the brain and interpreted
    as light
   Rhodopsin reformed
Activated Rhodopsin

   The cell membrane of a rod
    cell has an electrical
    charge—activated rhodopsin
    causes this electrical charge
    to increase. This produces
    an electrical current along
    the cell.

   Path of electric impulse:
    ganglion cell → optic nerve
    → optic chiasm → optic tract
    → occipital lobe → primary
    visual cortex
The Vision Process

   Monochrome vision
    (occurs in the rod
    cells)

   Color vision ( occurs
    in the cone cells)
Monochrome Vision (Rod Cells)

   Isomerization of
    retinal
   Protein
    conformational
    changes
   Signal
    transduction
    cascade to
    generate a nerve
    impulse
Isomerization of Retinal

   Chromophore
    11-cis-retinal
    isomerizes to all-
    trans-retinal.
    This event is
    best understood
    in terms of
    molecular
    orbitals, orbital
    energy, and
    electron
    excitation.
Isomerization of Retinal

   Absorption of a photon by 11-cis-retina
    promotes a p electron to a higher-energy
    orbital (a p-p* excitation).
   This excitation "breaks" the p component of
    the double bond, thus allowing free rotation
    about the bond between carbon atom 11 and
    carbon atom 12.
   The double bond then reforms and locks the
    molecule back into position in a trans
    configuration of the all-trans-retinal.
   11-cis-retinal has a maximum absorbance in
    the ultraviolet part of the spectrum, but the
    maximum absorbance for rhodopsin is 500
    nm (in the visible green part of the
    spectrum). The observed color of a substance
    is actually the complementary color to the
    color that is absorbed. Thus, the name "visual
    purple" describes the complementary color for
    rhodopsin.
Conformational Changes

   The all-trans chromophore
    adopts a twisted conformation,
    which is energetically
    unfavorable. Therefore, a
    series of changes occurs to
    expel the chromophore from
    the protein.
Phototransduction Pathway
Signal Transduction Cascade to
Generate a Nerve Impulse

   Activation of the enzymes transducin
    and phosphodiesterase
   Hydrolysis of cyclic GMP
   Closing of Na+ channels
    (hyperpolarization)
   Propagation of an electrical impulse to
    the brain
The Threshold of Sensitivity

   Rods are most sensitive near 500 nm

   Threshold: 90 photons/s entering the
    eye in .1 sec !
     E=90 hf =100 hc/lamda =4x10^-17
    watts
                 (10% efficiency)
Refraction

   Light rays bend
    when they pass from
    one transparent
    medium to another
   When a ray strikes
    the cornea, because
    the speed of light
    differs in the two
    media, it will bend.
How does the brain interpret images?
   Rays of light reflected off of an image are focused
    through the lens onto the back of the eye, forming an
    upside-down image on the retina.
   We can think of the image as a pixellate map of
    activated and non-activated photocells on the retina.
   A nerve from each photocell connects to a particular
    location in the visual cortex of the brain. The photocells
    that are activated send a nerve impulse to the brain,
    while the photocells that are not activated do not send
    any impulse to the brain.
   The brain, when it receives a collection of nerve signals
    from the eye, interprets where each signal comes from,
    and reconstructs the pixellate map. The brain judges
    the image location to be the location where light rays
    appear to originate from.
Why we see a world full of color
Electromagnetic Spectrum




~400nm                     ~750nm
The major importance of light intensity

   Colored vision depends heavily on intensity of
    light
    –   Cones will only work when there is a high intensity
        of light
    –   Low intensity light triggers vision by the rods


   Purkinje effect
    –   Because rods can see more toward the blue
        spectrum colors which appear brightest in the dark,
        will not appear so in the light
    –   Triggered by differences in rods and cones
Color Sensation

   Any color can be defined by a basic equation:
    –   C=aA+bB+cC
            A, B, and C represent the primary colors Red, Blue,
             and Green
            a, b, and c represent relative amounts of each primary
             color
            Two different colors can be added together and be
             plotted as vectors (e.g. C=aA+bB+cC and
             C’=a’A+b’B+c’C)
    –   To simplify the drawing of vectors, we assume that
        everything will be set to the same light intensity
            a+b+c=n, where N is a constant number
            We can then colors along the XY plane using XY
             coordinates
Mechanism of colored vision

   The three primary
    colors are detected by
    the cones located in
    the retina
   The highest
    concentration of
    cones is in the fovea
Three types of cones
General Test of dependence on
stimulated cones
Some Problems that occur with
colored vision

   Colorblindness
    –   8 percent of males
    –   0.5 percent of females
   Types of Colorblindness
    –   Red/Green Colorblindness
    –   Dichromats
            Missing one of the cone pigments
    –   Deuteranopes
            See colors not as points but as lines
    –   Protanopes
            Focus is near the red end of the visible spectrum
Deuteranope   Protanope
WHY IS SKY BLUE?
Speculations

   Is it because air molecules are by
    themselves blue?
   Is it because there is only blue light in
    the sky?
   Or is there any other reason?
Light in the sky

   From the sun
   Visible white light that contains all
    wavelengths = all colors
   Thus, there is more than blue light in
    the sky
Sunlight

   Electromagnetic wave
   Oscillate charged particles (protons and
    electrons) in air molecules
   Oscillation produces electromagnetic
    wave of same frequencies.
   Result, scattering of sun light
   Also, air molecules absorb all
    frequencies.
Physics of Blue

   Higher frequency than other colors (except
    violet)
   Molecules oscillate more -> blue light
    scattered more strongly
   Acceleration œ square of frequency
   Intensity of light œ square of frequency
   Thus, light intensity œ frequency ^4
   And blue light is scattered ten times more
    than red
Sky is Blue

   The color we see in the sky is the
    redirected sun light
   We see blue because it has higher
    frequency and is scattered much more.
   Why don’t we see violet sky?
   Somehow, our eyes are more sensitive
    to the frequency of blue light.
Works Cited

Casiday, Rachel and Frey
  Regina. Vision and Light-Induced Molecular
  Changes. 2000.

Clayton, Roderick K. Light and Living Matter,
  Vol. 1: The Physical Part. McGraw Hill, Inc.
  1970.

Clayton, Roderick K. Light and Living Matter,
  Vol. 1: The Biological Part. McGraw Hill, Inc.
  1970.

								
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