Light

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Light

• The Human Perception of Color

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Color what to learn in this lesson

• Color isnt real, it is a figment of our
  imagination
• Color only exists in the human mind
• Energy is the real characteristic of every light
  particle
• Our eyes can detect only a small range of the
  possible energies
• Goal today is to understand this . . .

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Description of LIGHT

• Called the Electromagnetic spectrum
• Based on the energy of the photons

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The EM spectrum

• LIGHT comes in two flavors
• 1. VISIBLE LIGHT
  the very small range of energies of
  light which our eyes can detect.
• 2. There is MUCH, MUCH more light which is not
  visible to human eyes.

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HIGHER ENERGY than visible light

• EXAMPLES
  cosmic rays
  gamma rays
  X rays
  ultraviolet

Energies too high for human vision
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The ELECTROMAGNETIC SPECTRUM

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LOWER ENERGY than visible light

• EXAMPLES
  infrared (heat)
  microwaves radio TV
  waves

Energies too low for human vision
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The ELECTROMAGNETIC SPECTRUM

Energy is too LOW
for human eyes, normal
vision
LOW f Long ?
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The VISIBLE LIGHT spectrum

• the portion of the electromagnetic spectrum we
  can sense with our EYES.
• Our skin can detect other portions as well . . .
  Leading to sunburn, cancer . . .
• Both UV and IR are invisible to our eyes, but are
  sensed by the skin.
• Some of the energies, colors of light, which
  are invisible to humans are visible to other
  species.

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The VISIBLE LIGHT spectrum
R O Y G
B I V
Red LOW energy LOW frequency LONG wavelength
Blue HIGH energy HIGH frequency SHORT wavelength
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SUMMARY EM spectrum
Radio TV micro IR visible UV
X rays ? rays
long WAVELENGTH
short
low FREQUENCY
high
low ENERGY
high
low DANGER to LIFE
high
Know this table and ROY G BIV See page 521
in your text
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Where does light come from?

• Photons are emitted when electrons lose energy
• All atoms are surrounded by electrons
• CHEMISTRY CONNECTION these electrons live in
  certain energy levels
• Electrons can change energy states in an atom
  if they
• Absorb light gain energy
• Loose energy emit light

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OUTCOME

• Atoms of each element have a unique,
  characteristic electron configuration.
• Each element has a unique,
  characteristic light spectrum.
• A specific element can only emit and absorb
  particular energies of light.
• We can only see some of those colors .

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Atomic emission spectra
SOURCES of light In order, from left to right

• Atomic sources
• Hydrogen
• Helium
• Neon
• Argon
• Krypton
• Mercury

• Compound sources
• H2O
• CO2

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Our eyes see a collection of different energies
. . . .

• We normally see a lot of different colors
  SIMULTANEOUSLY.
• To see what colors are really there, we have to
  use a diffraction grating.
• Notice the differences when we look at each
  light, one at a time

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Hydrogen Helium Neon Argon Krypton Mercury
H2O CO2
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How do we see colors ?

• We see when light enters the eye.
• No light DONT SEE ANYTHING.
• Our brain processes light information two
  ways 1) in the where system 2) in the
  what system
• The following examples of visual stimuli
  demonstrate some of the things we experience
  because of our color vision.

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Example 1

• The twinkling black spots do not actually exist
  in the intersections of the gray lines on the
  next slide.
• You can discover this by focusing on one
  particular intersection.
• The spots appear because of the way the cones
  and rods in your eyes respond to light.

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This is easy to read, so you can decide to ignore
it very rapidly
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Very low contrast text is read by the part of the
brain which locates objects, the where
system. As long as the words are easy to
recognize, you can read rather rapidly, but
recondite or infrequently encountered words seem
unfamiliar and have to be read letter-by-letter.
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This is also hard to read. It jumps around and
seems unstable because the where system has
trouble seeing it. Advertisers use this trick to
make you pay attention because you have to slow
down and read each individual word.
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This is also hard to read. It jumps around and
seems unstable because the where system has
trouble seeing it. Advertisers use this trick to
make you pay attention because you have to slow
down and read each individual word.
Changing the color of the letters helps a lot
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This is also hard to read. It jumps around and
seems unstable because the where system has
trouble seeing it. Advertisers use this trick to
make you pay attention because you have to slow
down and read each individual word.
Changing the background color also helps a lot
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This is hard to read even though each individual
letter is easy to see, so you have to pay a lot
of attention in order to read it.
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Why do things like this happen ?
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When light enters the eye . .

• Light from some source strikes the eye
• Passes through the CORNEA
• Amount let in controlled by the IRIS
• which adjusts the opening of the PUPIL
• The light is focused by the LENS,
• passes through some transparent stuff
• and strikes the RETINA.
• It is absorbed, creating an electrical signal
• which is sent by the optic nerve to the brain
• where it is perceived as an image.

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COLOR VISION ONLY EXISTS IN THE BRAIN

• Biology connection
• Color is not something real or special about
  the visible spectrum.
• Most species do not see color.
• We see color because of the way our eyes are
  built . . . . . and the way our brain interprets
  what we see.

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How do we see the different colors?

• Occurs because of the construction of the retina.
• The retina is composed of millions of
  photoreceptor cells
• called rods and cones.

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rods

• Can not distinguish energies (see color)
• Can only sense brightness.
• Extremely sensitive to dim light.
• Are use to identify object location using the
  where portion of the brain perceptual
  processing centers.
• The way rods work are behind the checker board
  pattern illusion.

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rods

• Do not detect edges well so are not very
  useful in reading. ( Large pixels)
• Used to see at night. Helpful in looking at
  stars.
• Quite sensitive to peripheral motion.

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Testing rods . . .
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cones

• Function only under bright light conditions
  (takes a lot of photons).
• Cones can distinguish different photon energies.
• Very sensitive to edges good focus. VERY Small
  pixels
• About 3 million packed in fovea, center part of
  the retina.
• Fovea is where image is formed when we look at
  something.

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How does color vision work?

• Three kinds of cones about 3
  million in the fovea.
• Blue cones about 1
• The rest are Green and Red
• Each type of cone is sensitive to a range of
  energies (what we call colors)

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The VISIBLE LIGHT spectrum
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Basic understanding

• All of the colors of the rainbow actually
  exist in an energy sense.
• But because of the way our eyes work
• 1. we can see yellow when we see yellow or
• 2. we can see yellow when only the colors red
  and green are both present at the same time . .
  . .

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At night, we see the higher energy photons much
better that during the daytime
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What are the PRIMARY COLORS ?

• Based on the way the human eye works
• we synthesize colors in our brain.
• The human mind creates our color palate.
• In practice, it only takes three colors
• BLUE
• GREEN
• RED
• to see all of the colors known to humans.

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Examples of 3 color process

• Called color by addition adding colors of
  light.
• Television sets and computer monitors all are
  RGB
• Works for all SOURCES OF LIGHT.
• Remember that each type of cone SEES more than
  a single shade of color.

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Primary colors of light
BLUE
GREEN
RED
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Secondary colors are formed by addition of
primary colors
BLUE
GREEN
RED
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White formed by addition of all three primary
colors
BLUE
GREEN
RED
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White also formed by addition of all three
secondary colors
MAGENTA
CYAN
YELLOW
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Color of light relationships
BLUE
GREEN
RED
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PRIMARY and SECONDARY colors for
pigments
Works for all materials which control color by
absorption
Seeing REFLECTED light. Seeing TRANSMITTED light.
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Pigments (paint, ink, filters)

• DEFINE Primary colors of pigment
• Absorb only one primary color of light.
• YELLOW absorbs only BLUE
• CYAN absorbs only RED
• MAGENTA absorbs only GREEN

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Pigments (paint, ink, filters)

• DEFINE Secondary colors of pigment
• Absorb two primary colors of light.
• BLUE absorbs both RED and GREEN
• RED absorbs both BLUE and GREEN
• GREEN absorbs both RED and BLUE

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What is IMPORTANT

• ASK What colors are eliminated from
  the light source ?
• When any light hits a page, some of the
  light is absorbed.
• The rest is reflected into your eye.
• The color you see is based on the reflected
  colors that enter your eye.

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Comparison of PRIMARY colors

• EMISSION
• BLUE
• RED
• GREEN

• REFLECTION or TRANSMISSION
• YELLOW
• MAGENTA
• CYAN

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Comparison of SECONDARY colors

• REFLECTION OR TRANSMISSION
• BLUE
• RED
• GREEN

• EMISSION
• YELLOW
• MAGENTA
• CYAN

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Filters and slides

• Theater filters are used to eliminate colors
  from white.
• Slide projectors and movie film is designed to
  eliminate colors from white.
• Some glasses are designed to eliminate colors
  from white.
• Colored glass also
  eliminates colors from white.

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EXAMPLES of color by subtraction

• COLOR SLIDES
• Photographic printing, magazines,
  computer printers,
  newspapers.

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Three basic ways to see color

• Color by emission
  the object emits colors of light.
• Color by transmission
  a filter absorbs some colors while
  letting others pass through
• Color by reflection
  pigment absorbs some colors
  while letting others reflect off.

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IMPORTANT CONCLUSION
In order to observe
all human colors, you
must have a
white light source.
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What you THINK you see vs.

what is REALLY THERE
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• EXAMPLE
  You can see yellow
  two different ways
• 1. When only YELLOW LIGHT is present it
  stimulates both red and green cones.
• ONLY YELLOW LIGHT is really present.
• 2. When the RIGHT AMOUNT of both RED and GREEN
  are present
• It APPEARS to be YELLOW but only RED and GREEN
  are actually being seen.

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Key question

• What color(s) of light are actually entering your
  eye?
• To see a particular color, the right colors of
  light have to (1) be present and (2) entering the
  eye to stimulate the cones by the right amount.
• You CAN NOT see a color that isnt present in the
  light SOURCE.

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One More Reason Why Physics is Better Than Drugs
Sunset on Maui