Transcript Light - Southern Methodist University

Light
The Human Perception
of Color
Color: what to learn in this lesson
• Color isn’t “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 . . .
Description of LIGHT
• Called the Electromagnetic spectrum
• Based on the energy of the photons
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.
HIGHER ENERGY
than visible light:
• EXAMPLES:
HIGHER
ENERGY
cosmic rays
gamma rays
X rays
ultraviolet
Energies too high for human vision
LOWER
ENERGY
The ELECTROMAGNETIC
SPECTRUM
•
Energy is too HIGH
human eyes, normal vision
HIGH f
for
Short 
LOWER ENERGY than
visible light:
• EXAMPLES:
HIGHER
ENERGY
infrared (heat)
microwaves
radio & TV waves
Energies too low for human vision
LOWER
ENERGY
The ELECTROMAGNETIC
SPECTRUM
•
Energy is too LOW
for human eyes, normal vision
LOW f
Long 
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.
The VISIBLE LIGHT spectrum
R
O Y
Red
LOW energy
LOW frequency
LONG wavelength
G
B
I
V
Blue
HIGH energy
HIGH frequency
SHORT wavelength
SUMMARY:
Radio & TV
rays
long
low
low
low
micro
IR
EM spectrum
visible
UV
X rays

WAVELENGTH
FREQUENCY
ENERGY
short
high
high
DANGER to LIFE
high
Know this table and “ROY G BIV”
See page 521 in your text
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
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 .
Atomic emission spectra
SOURCES of light: In order, from left to right
Atomic sources
• Hydrogen
• Helium
• Neon
• Argon
• Krypton
• Mercury
Compound sources
• H2 O
• CO2
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:
Hydrogen
H2 O
Helium
CO2
Neon
Argon
Krypton
Mercury
How do we see colors ?
• We see when light enters the eye.
• No light: DON’T 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.
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.
This is easy to read, so you can
decide to ignore it very rapidly
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.
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.
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
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
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.
Why do things like this happen ?
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.
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.
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”.
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.
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.
W
Testing rods . . .
D
F
N
K
Z
G
O
A
Q
V
W
G
M
A
C
L
Y
M
S
B
B
H
R
U
T
Q
T
J
P
Y
P
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.
How does color vision work?
• Three kinds of cones:
3 million in the fovea.
about
• “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”)
The VISIBLE LIGHT spectrum
BLUE cones
GREEN cones
RED cones
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 . . . .
At night, we see the higher energy photons
much better that during the daytime
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
Primary colors
GREEN
of light
RED
to “see” all of the colors known to humans.
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.
Primary colors of light
BLUE
RED
GREEN
Secondary colors are formed
by addition of primary colors
BLUE
MAGENTA
CYAN
GREEN
RED
YELLOW
White formed by addition of
all three primary colors
BLUE
WHITE
RED
GREEN
White also formed by addition
of all three secondary colors
MAGENTA
CYAN
WHITE
YELLOW
Color of light relationships
BLUE
MAGENTA
CYAN
WHITE
GREEN
RED
YELLOW
PRIMARY and SECONDARY
colors for pigments
Works for all materials which
color by absorption
Seeing REFLECTED light.
Seeing TRANSMITTED light.
contro
Pigments (paint, ink, filters)
• DEFINE: Primary colors of pigment
• Absorb only one primary color of light.
• YELLOW
absorbs only
• CYAN
absorbs only
• MAGENTA absorbs only
BLUE
RED
GREEN
Pigments (paint, ink, filters)
• DEFINE: Secondary colors of pigment
• Absorb two primary colors of light.
• BLUE absorbs both
• RED
absorbs both
• GREEN absorbs both
RED and GREEN
BLUE and GREEN
RED and BLUE
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.
Comparison of
PRIMARY colors
• EMISSION
BLUE
RED
GREEN
• REFLECTION or
TRANSMISSION
YELLOW
MAGENTA
CYAN
Comparison of
SECONDARY colors
• EMISSION
YELLOW
MAGENTA
CYAN
• REFLECTION
OR
TRANSMISSION
BLUE
RED
GREEN
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.
EXAMPLES of
color by subtraction
• COLOR SLIDES
• Photographic printing,
magazines,
computer printers,
newspapers.
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
IMPORTANT CONCLUSION
In order to observe
all “human” colors,
you must have a
“white light source”.
What you THINK you see
vs.
what is REALLY THERE
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.
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 isn’t
present in the light SOURCE.
One More Reason Why Physics is Better Than Drugs
Sunset on Maui