Chapter Twenty Eight Notes:
COLOR
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As discussed in the last chapter, electromagnetic waves are waves
which are capable of traveling through a vacuum. Unlike mechanical
waves which require a medium in order to transport their energy,
electromagnetic waves are capable of transporting energy through
the vacuum of outer space. Electromagnetic waves are produced by
a vibrating electric charge and as such, they consist of both an
electric and a magnetic component.
Electromagnetic waves exist with an enormous range of
frequencies. This continuous range of frequencies is known as the
electromagnetic spectrum. The diagram below depicts the
electromagnetic spectrum and its various regions. The longer
wavelength, lower frequency regions are located on the far left of
the spectrum and the shorter wavelength, higher frequency regions
are on the far right. Two very narrow regions within the spectrum
are the visible light region and the X-ray region. Within the visible
range, the different frequencies/wavelength determine what color
we see!
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The focus of chapter 28 will be upon the visible light region - the
very narrow band of wavelengths located to the right of the infrared
region and to the left of the ultraviolet region. Though
electromagnetic waves exist in a vast range of wavelengths, our eyes
are sensitive to only a very narrow band. Since this narrow band of
wavelengths is the means by which humans see, we refer to it as the
visible light spectrum. Normally when we use the term "light," we are
referring to a type of electromagnetic wave which stimulates the
retina of our eyes. In this sense, we are referring to visible light, a
small spectrum from the enormous range of frequencies of
electromagnetic radiation. This visible light region consists of a
spectrum of wavelengths which range from approximately 700
nanometers (abbreviated nm) to approximately 400 nm. Expressed in
more familiar units, the range of wavelengths extends from 7 x 10-7
meter to 4 x 10-7 meter. This narrow band of visible light is
affectionately known as ROYGBIV.
Each individual wavelength within the spectrum of visible light
wavelengths is representative of a particular color. That is, when light
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of that particular wavelength strikes the retina of our eye, we
perceive that specific color sensation. Isaac Newton showed that
light shining through a prism will be separated into its different
wavelengths and will thus show the various colors that visible light
is comprised of. The separation of visible light into its different
colors is known as dispersion.
Each color is characteristic of a distinct wavelength;
and different wavelengths of light waves will bend
varying amounts upon passage through a prism.
For these reasons, visible light is dispersed upon passage through a
prism. Dispersion of visible light produces the red (R), orange (O),
yellow (Y), green (G), blue (B), and violet (V)colors. It is because of
this that visible light is sometimes referred to as ROY G. BIV.
(Incidentally, the indigo is not actually observed in the spectrum but
is traditionally added to the list so that there is a vowel in Roy's last
name.) The red wavelengths of light are the longer wavelengths and
the violet wavelengths of light are the shorter wavelengths. Between
red and violet, there is a continuous range or spectrum of
wavelengths. The visible light spectrum is shown in the diagram
below.
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When all the wavelengths of the visible light spectrum strike your eye at the
same time, white is perceived. The sensation of white is not the result of a
single color of light. Rather, the sensation of white is the result of a mixture
of two or more colors of light. Thus, visible light - the mix of ROYGBIV - is
sometimes referred to as white light. Technically speaking, white is not a
color at all - at least not in the sense that there is a light wave with a
wavelength which is characteristic of white. Rather, white is the combination
of all the colors of the visible light spectrum. If all the wavelengths of the
visible light spectrum give the appearance of white, then none of the
wavelengths would lead to the appearance of black. Once more, black is not
actually a color. Technically speaking, black is merely the absence of the
wavelengths of the visible light spectrum. So when you are in a room with no
lights and everything around you appears black, it means that there are no
wavelengths of visible light striking your eye as you sight at the
surroundings.
How many colors are there in this swatch? How many were you taught in
elementary school?
red
orange
yellow
green
blue
violet
The simple named colors are mostly monosyllabic in English — red, green, blue, brown, black, white, gray.
(Yellow is the one exception to this rule, but it's still pretty simple.) Brevity indicates a pre-English, Anglo-Saxon
origin. Monosyllabic words are generally the oldest words in the English language — head, eye, nose, foot, cat,
dog, cow, eat, drink, man, wife, house, sleep, rain, snow, sword, sheath, God, and the "four letter words" —
words that go back a thousand years. Some of the names for colors are loan words from French — orange and
beige, since the "zh" sound doesn't exist in pure English (garage is a very french word) and violet and purple,
since they just sound too fancy to be anglo-saxon.
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We have previously learned that visible light waves consists of a
continuous range of wavelengths or frequencies. When a light wave
with a single frequency strikes an object, a number of things could
happen. The light wave could be absorbed by the object, in which
case its energy is converted to heat. The light wave could be
reflected by the object. And the light wave could be transmitted by
the object. Rarely however does just a single frequency of light
strike an object. While it does happen, it is more usual that visible
light of many frequencies or even all frequencies are incident
towards the surface of objects. When this occurs, objects have a
tendency to selectively absorb, reflect or transmit light certain
frequencies. That is, one object might reflect green light while
absorbing all other frequencies of visible light. Another object might
selectively transmit blue light while absorbing all other frequencies
of visible light. The manner in which visible light interacts with an
object is dependent upon the frequency of the light and the nature
of the atoms of the object. In this section we will discuss how and
why light of certain frequencies can be selectively absorbed,
reflected or transmitted.
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Atoms and molecules contain electrons. It is often useful to think of
these electrons as being attached to the atoms by springs. The
electrons and their attached springs have a tendency to vibrate at
specific frequencies. Similar to a tuning fork or even a musical
instrument, the electrons of atoms have a natural frequency at which
they tend to vibrate. When a light wave with that same natural
frequency impinges upon an atom, then the electrons of that atom will
be set into vibrational motion. (This is merely another example of the
resonance principle introduced earlier.) If a light wave of a given
frequency strikes a material with electrons having the same vibrational
frequencies, then those electrons will absorb the energy of the light
wave and transform it into vibrational motion. During its vibration, the
electrons interacts with neighboring atoms in such a manner as to
convert its vibrational energy into thermal energy. Subsequently, the
light wave with that given frequency is absorbed by the object, never
again to be released in the form of light. So the selective absorption of
light by a particular material occurs because the selected frequency of
the light wave matches the frequency at which electrons in the atoms
of that material vibrate. Since different atoms and molecules have
different natural frequencies of vibration, they will selectively absorb
different frequencies of visible light.
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Reflection of light waves occur because the frequencies of the light
waves do not match the natural frequencies of vibration of the objects.
When light waves of these frequencies strike an object, the electrons in
the atoms of the object begin vibrating. But instead of vibrating in
resonance at a large amplitude, the electrons vibrate for brief periods of
time with small amplitudes of vibration; then the energy is reemitted as
a light wave. If the object is opaque, then the vibrations of the electrons
are not passed from atom to atom through the bulk of the material.
Rather the electrons of atoms on the material's surface vibrate for short
periods of time and then reemit the energy as a reflected light wave.
Such frequencies of light are said to be reflected.
The color of the objects which we see are largely due to the way those
objects interact with light and ultimately reflect it to our eyes. The color
of an object is not actually within the object itself. Rather, the color is in
the light which shines upon it and is ultimately reflected to our eyes. We
know that the visible light spectrum consists of a range of frequencies,
each of which corresponds to a specific color. When visible light strikes an
object and a specific frequency becomes absorbed, that frequency of light
will never make it to our eyes. Any visible light which strikes the object and
becomes reflected to our eyes will contribute to the color appearance of that
object. So the color is not in the object itself, but in the light which strikes
the object and ultimately reaches our eye. The only role that the object plays
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is that it might contain atoms capable of selectively absorbing one
or more frequencies of the visible light which shine upon it. So if an
object absorbs all of the frequencies of visible light except for the
frequency associated with green light, then the object will appear
green in the presence of ROYGBIV. And if an object absorbs all of the
frequencies of visible light except for the frequency associated with
blue light, then the object will appear blue in the presence of
ROYGBIV.
Consider the two diagrams on the following page.
The diagrams depict a sheet of paper being
illuminated with white light (ROYGBIV). The papers
are impregnated with a chemical capable of absorbing
one or more of the colors of white light. Such chemicals which are
capable of selectively absorbing one or more frequency of white light are
known as pigments. In Example A, the pigment in the sheet of paper is
capable of absorbing red, orange, yellow, blue, indigo and violet. In
Example B, the pigment in the sheet of paper is capable of absorbing
orange, yellow, green, blue, indigo and violet. In each case, whatever
color is not absorbed is reflected.
Example A: Green will be reflected and so the paper appears green to
an observer.
Example B: Red will be reflected and so the paper appears red to an
observer.
Example A: Green will be transmitted and so the object
appears green to an observer.
Example B: Both green and blue will be transmitted
and so the object appears greenish-blue to an
observer.
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Transmission of light waves occur because the frequencies of the
light waves do not match the natural frequencies of vibration of the
objects. When light waves of these frequencies strike an object, the
electrons in the atoms of the object begin vibrating. But instead of
vibrating in resonance at a large amplitude, the electrons vibrate for
brief periods of time with small amplitudes of vibration; then the
energy is reemitted as a light wave. If the object is transparent, then
the vibrations of the electrons are passed on to neighboring atoms
through the bulk of the material and reemitted on the opposite side
of the object. Such frequencies of light waves are said to be
transmitted.
The color of the objects which we see are largely due to the way
those objects interact with light and ultimately transmit it to our
eyes. The color of an object is not actually within the object itself.
Rather, the color is in the light which shines upon it and is ultimately
transmitted to our eyes. We know that the visible light spectrum
consists of a range of frequencies, each of which corresponds to a
specific color. When visible light strikes an object and a
specific frequency becomes absorbed, that frequency of light
will never make it to our eyes. Any visible light which strikes
the object and becomes transmitted to our eyes will
contribute to the color appearance of that object. So the color
is not in the object itself, but in the light which strikes the
object and ultimately reaches our eye. The only role that the
object plays is that it might contain atoms capable of
selectively absorbing one or more frequencies of the visible
light which shine upon it. So if an object absorbs all of the
frequencies of visible light except for the frequency
associated with green light, then the object will appear green
in the presence of ROYGBIV. And if an object absorbs all of
the frequencies of visible light except for the frequency
associated with blue light, then the object will appear blue in
the presence of ROYGBIV.
e.g.
R
O
Y
G
B
I
V
blue filter
We See Mostly Blue
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The sun emits light waves with a range of frequencies. Some of
these frequencies fall within the visible light spectrum and thus are
detectable by the human eye. Since sunlight consists of light with
the range of visible light frequencies, it appears white. This white
light is incident towards Earth and illuminates both our outdoor
world and the atmosphere which surrounds our planet. As discussed
earlier, the interaction of visible light with matter will often result in
the absorption of specific frequencies of light. The frequencies of
visible light which are not absorbed are either transmitted (by
transparent materials) or reflected (by opaque materials). As we
sight at various objects in our surroundings, the color which we
perceive is dependent upon the color(s) of light which are reflected
or transmitted by those objects to our eyes. So if we consider a
green leaf on a tree, the atoms of the chlorophyll molecules in the
leaf are absorbing most of the frequencies of visible light (except for
green) and reflecting the green light to our eyes. The leaf thus
appears green. And as we view the black asphalt street, the atoms of
the asphalt are absorbing all the frequencies of visible light and no
light is reflected to our eyes. The asphalt street thus appears black
(the absence of color). In this manner, the interaction of sunlight with matter
contributes to the color appearance of our surrounding world. In later
sections, we will focus on the interaction of sunlight with atmospheric
particles to produce blue skies and red sunsets. While sunlight consists of
the entire range of frequencies of visible light, not all frequencies are
equally intense. In fact, sunlight tends to be most rich with yellow light
frequencies. For these reasons, the sun appears yellow during midday due
to the direct passage of dominant amounts of yellow frequencies through
our atmosphere and to our eyes.
The appearance of the sun changes with the time of
day. While it may be yellow during midday, it is often
found to gradually turn color as it approaches sunset.
This can be explained by light scattering. As the sun
approaches the horizon line, sunlight must traverse a
greater distance through our atmosphere; this is
demonstrated in the diagram below.
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Color Addition
Color perception, like sound perception, is a complex subject
involving the disciplines of psychology, physiology, biology, chemistry
and physics. When you look at an object and perceive a distinct color,
you are not necessarily seeing a single frequency of light. Consider
for instance that you are looking at a shirt and it appears purple to
your eye. In such an instance, there my be several frequencies of light
striking your eye with varying degrees of intensity. Yet your eye-brain
system interprets the frequencies which strike your eye and the shirt
is decoded by your brain as being purple.
The subject of color perception can be simplified if we think in terms
of primary colors of light. We have already learned that white is not a
color at all, but rather the presence of all the frequencies of visible
light. When we speak of white light, we are referring to ROYGBIV - the
presence of the entire spectrum of visible light. But combining the
range of frequencies in the visible light spectrum is not the only
means of producing white light. White light can also be produced by
combining only three distinct frequencies of light, provided that they
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are widely separated on the visible light spectrum. Any three colors
(or frequencies) of light which produce white light when combined
with the correct intensity are called primary colors of light. There are
a variety of sets of primary colors. The most common set of primary
colors is red (R), green (G) and blue (B). When red, green and blue
light are mixed or added together with the proper intensity, white
(W) light is obtained. This is often represented by the equation
below:
R+G+B=W
In fact, the mixing together (or addition) of two or three of these
three primary colors of light with varying degrees of intensity can
produce a wide range of other colors. For this reason, many
television sets and computer monitors produce the range of colors
on the monitor by the use of of red, green and blue light-emitting
phosphors.
The addition of the primary colors of light can be demonstrated
using a light box. The light box illuminates a screen with the three
primary colors - red (R), green (G) and blue (B). The lights are often
the shape of circles. The result of adding two primary colors of light
is easily seen by viewing the overlap of the two or more circles of
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primary light. The different combinations of colors produced by red,
green and blue are shown in the graphic below. (CAUTION: Because
of the way that different monitors and different web browsers render
the colors on the computer monitor, there may be slight variations
from the intended colors.)
These demonstrations with the color box illustrate that red light and
green light add together to produce yellow (Y) light. Red light and
blue light add together to produce magenta (M) light. Green light
and blue light add together to produce cyan (C) light. And finally,
red light and green light and blue light add together to produce
white light. This is sometimes demonstrated by the following color
equations and graphic:
R+G=Y
R+B=M
G+B=C
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Yellow (Y), magenta (M) and cyan (C) are sometimes referred to as
secondary colors of light since they are produced by the addition of
equal intensities of two primary colors of light. The addition of these
three primary colors of light with varying degrees of intensity will
result in the countless other colors which we are familiar (or
unfamiliar) with.
Newton’s Color Wheel
Prism spectrum is a straight line, so
why did Isaac Newton describe color
using a circular wheel?
Stream of
red & green photons
looks same as
yellow photons
(metamerism)
or
Theatrical lighting
Eye to
Brain
Notice overlap of red, green, & blue is
seen as white light
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Any two colors of light which when mixed together in equal
intensities produce white are said to be complementary colors of each
other. The complementary color of red light is cyan light. This is
reasonable since cyan light is the combination of blue and green light;
and blue and green light when added to red light will produce white
light. Thus, red light and cyan light (blue + green) represent a pair of
complementary colors; they add together to produce white light. This
is illustrated in the equation below:
R + C = R + (B + G) = White
Each primary color of light has a secondary color of light as its
complement. The three pairs of complementary colors are listed
below. The graphic at the right is extremely helpful in identifying
complementary colors. Complementary colors are always located
directly across from each other on the graphic. Note that cyan is
located across from red, magenta across from green, and yellow
across from blue.
Complementary Colors of Light
Red and Cyan
Green and Magenta
Blue and Yellow
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The production of various colors of light by
the mixing of the three primary colors of light
is known as color addition. The color addition
principles discussed on this page can be used
to make predictions of the colors which would
result when different colored lights are
mixed.
LARRY
R
MOE
C
CURLY
After-image of
red is cyan
because Larry
gets tired so
when white light
excites all three
Stooges, Moe &
Curly stronger
than Larry.
Cyan = White - Red
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Color Subtraction
The previous lesson focused on the principles of color addition.
These principles govern the perceived color resulting from the
mixing of different colors of light. Principles of color addition have
important applications to color television, color computer monitors
and on-stage lighting at the theaters. Each of these applications
involve the mixing or addition of colors of light to produce a
desired appearance. Our understanding of color perception would
not be complete without an understanding of the principles of color
subtraction. In this part of the chapter, we will learn how materials
which have been permeated by specific pigments will selectively
absorb specific frequencies of light in order to produce a desired
appearance.
We have already learned that materials contain atoms which are
capable of selectively absorbing one or more frequencies of light.
Consider a shirt made of a material which is capable of absorbing
blue light. Such a material will absorb blue light (if blue light shines
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upon it) and reflect the other frequencies of the visible spectrum.
What appearance will such a shirt have if illuminated with white
light and how can we account for its appearance? To answer this
question (and any other similar question), we will rely on our
understanding of the three primary colors of light (red, green and
blue) and the three secondary colors of light (magenta, yellow and
cyan)
To begin, consider white light to consist of the three
primary colors of light - red, green and blue. If
white light is shining on a shirt, then red, green and
blue light are shining on the shirt. If the shirt absorbs blue light,
then only red and green light will be reflected from the shirt. So
while red, green and blue light shine upon the shirt, only red and
green light will reflect from it. Red and green light striking your eye
always give the appearance of yellow; for this reason, the shirt will
appear yellow. This discussion illustrates the process of color
subtraction. In this process, the ultimate color appearance of an
object is determined by beginning with a single color or mixture of
colors and identifying which color or colors of light are subtracted
from the original set. The process is depicted visually by diagram at
the right. Furthermore, the process is depicted in terms of an
equation in the space below.
W - B = (R + G + B) - B = R + G = Y
 Now suppose that cyan light is shining on the same shirt - a shirt
made of a material which is capable of absorbing blue light. What
appearance will such a shirt have if illuminated with cyan light and
how can we account for its appearance? To answer this question, the
process of color subtraction will be applied once more. In this
situation, we begin with only blue and green primary colors of light
(recall that cyan light consists of blue and green light). From this
mixture, we must subtract blue light. After the subtractive process,
only green light remains. Thus, the shirt will appear green in the
presence of cyan light. Observe the representation of this by the
diagram at the right and the equation below.
C - B = (G + B) - B = G
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From these two examples, we can conclude that a shirt which looks
yellow when white light shines upon it will look green when cyan
light shines upon it. This confuses many students of physics,
especially those who still believe that the color of a shirt is in the
shirt itself. This is the misconception which was targeted earlier in
the chapter as we discussed how visible light interacts with matter
to produce color. In that part of the chapter, it was emphasized that
the color of an object does not reside in the object itself. Rather, the
color is in the light which shines upon the object and which
ultimately becomes reflected or transmitted to our eyes. Extending
this conception of color to the above two scenarios, we would
reason that the shirt appears yellow if there is some red and green
light shining upon it. Yellow light is a combination of red and green
light. A shirt appears yellow if it reflects red and green light to our
eyes. In order to reflect red and green light, these two primary
colors of light must be present in the incident light.
A clear cloudless day-time sky is blue because molecules in the air
scatter blue light from the sun more than they scatter red
light. When we look towards the sun at sunset, we see red and
orange colors because the blue light has been scattered out and
away from the line of sight.
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The white light from the sun is a mixture of all colors of the
rainbow. This was demonstrated by Isaac Newton, who used a
prism to separate the different colors and so form a
spectrum. The colors of light are distinguished by their different
wavelengths. The visible part of the spectrum ranges from red
light with a wavelength of about 720 nm, to violet with a
wavelength of about 380 nm, with orange, yellow, green, blue and
indigo between. The three different types of color receptors in the
retina of the human eye respond most strongly to red, green and
blue wavelengths, giving us our color vision.
Tyndall Effect
The first steps towards correctly explaining the color of the sky
were taken by John Tyndall in 1859. He discovered that when
light passes through a clear fluid holding small particles in
suspension, the shorter blue wavelengths are scattered more
strongly than the red. This can be demonstrated by shining a
beam of white light through a tank of water with a little milk or
soap mixed in. From the side, the beam can be seen by the blue
light it scatters; but the light seen directly from the end is
reddened after it has passed through the tank. The scattered light
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can also be shown to be polarised using a filter of polarised light,
just as the sky appears a deeper blue through polaroid sun glasses.
This is most correctly called the Tyndall effect, but it is more
commonly known to physicists as Rayleigh scattering--after Lord
Rayleigh, who studied it in more detail a few years later. He showed
that the amount of light scattered is inversely proportional to the
fourth power of wavelength for sufficiently small particles. It follows
that blue light is scattered more than red light by a factor of
(700/400)4 ~= 10.
Dust or Molecules?
Tyndall and Rayleigh thought that the blue color of the sky must be
due to small particles of dust and droplets of water vapor in the
atmosphere. Even today, people sometimes incorrectly say that this
is the case. Later scientists realized that if this were true, there
would be more variation of sky color with humidity or haze
conditions than was actually observed, so they supposed correctly
that the molecules of oxygen and nitrogen in the air are sufficient to
account for the scattering. The case was finally settled by Einstein in
1911, who calculated the detailed formula for the scattering of light
from molecules; and this was found to be in agreement with
experiment. He was even able to use the calculation as a further
verification of Avogadro's number when compared with
observation. The molecules are able to scatter light because the
electromagnetic field of the light waves induces electric dipole
moments in the molecules.
Note that the blue of the sky is
more saturated when you look further
from the sun. The almost white
scattering near the sun can be
attributed to Mie scattering, which is
not very wavelength dependent.
Clouds in contrast to the blue sky
appear white to achromatic gray.
The water droplets that make up
the cloud are much larger than the
molecules of the air and the scattering
from them is almost independent of
wavelength in the visible range.
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As the path which sunlight takes through our atmosphere increases
in length, ROYGBIV encounters more and more atmospheric
particles. This results in the scattering of greater and greater
amounts of yellow light. During sunset hours, the light passing
through our atmosphere to our eyes tends to be most concentrated
with red and orange frequencies of light. For this reason, the
sunsets have a reddish-orange hue. The affect of a red sunset
becomes more pronounced if the atmosphere contains more and
more particles. The presence of sulfur aerosols (emitted as an
industrial pollutant and by volcanic activity) in our atmosphere
contributes to some magnificent sunsets (and some very serious
One More Reason Why Physics is Better Than Drugs
environmental problems).
Photograph of Maui sunset by Becky Henderson
 Red, orange, yellow are transmitted more readily through the
atmosphere
Light of lower frequencies is transmitted while light of higher
frequencies are scattered
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Why Water is Greenish Blue: Red light subtracted from white
light produces blue-green light (cyan).
When viewed from space, one of earth’s most commanding
features is the blueness of its vast oceans and lakes. Small
amounts of water do not portend the color of these large
bodies of water; when pure drinking water is examined in a
glass, it appears clear and colorless. A larger volume of water
is required to reveal the azure color.
Water’s hue depends on a number of factors. On bright, clear
days, the blue color associated with a lake or ocean may be
attributed, in large part, to the reflection of the sky by the
water. But even the water at the deep end of indoor swimming
pools seems to have a blue-green color. Why?
When light penetrates water, it experiences both absorption
and Rayleigh scattering. Water molecules are small enough to
scatter shorter wavelengths, giving water its blue-green color.
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The amount of long-wavelength absorption is a function of
depth; the deeper the water, the more red light is absorbed.
At a depth of 15 m, the intensity of red light drops to 25
percent of its incident value and fall to zero beyond a depth of
30 m. Any object viewed at this depth is seen in a blue-green
ambient light. For this reason, red denizens of the sea, such
as lobsters and crabs, appear black to divers not carrying a
lamp.
The City of San
Diego Web site
A lost heron hitching a
ride at sea.
Green water in the
Potomac River
caused by an algae
bloom
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In physics, emission is the process by which energy in the form of
a photon is released by a particle, for example, by an atom whose
electrons make a transition between two electronic energy levels.
The emitted energy is in the form of a photon with a specific
frequency. The emittance of an object quantifies how much light
is emitted by it. This may be related to other properties of the
object through the Stefan–Boltzmann law. For most substances,
the amount of emission varies with the temperature and the
spectroscopic composition of the object, leading to the
appearance of color temperature and emission lines. Precise
measurements at many wavelengths allow the identification of a
substance via emission spectroscopy.
Emission spectrum of Hydrogen
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Every element has its own specific glow
The light from the elements can be analyzed by a spectroscope
It is composed of thin slits, lenses, and a prism
It displays the spectrum of light
Line spectrum- images of the slit through which the light passes
Emission spectrum of Iron
Emission spectroscopy is a spectroscopic technique which examines
the wavelengths of photons emitted by atoms or molecules during
their transition from an excited state to a lower energy state. Each
element emits a characteristic set of discrete wavelengths according
to its electronic structure, by observing these wavelengths the
elemental composition of the sample can be determined. Emission
spectroscopy developed in the late 19th century and efforts in
theoretical explanation of atomic emission spectra eventually led to
quantum mechanics.
28.11 The Atomic Color Code—Atomic Spectra
A fairly pure spectrum is produced by passing white light through a thin
slit, two lenses, and a prism.
28.11 The Atomic Color Code—Atomic Spectra
A spectroscope separates light into its constituent frequencies. Light illuminates
the thin slit at the left, and then it is focused by lenses onto either a diffraction
grating (shown) or a prism on the rotating table in the middle.
28.11 The Atomic Color Code—Atomic Spectra
Light is emitted by excited atoms.
28.11 The Atomic Color Code—Atomic Spectra
a.
The different electron orbits in an atom are like steps in energy levels.
28.11 The Atomic Color Code—Atomic Spectra
a.
b.
The different electron orbits in an atom are like steps in energy levels.
When an electron is raised to a higher level, the atom is excited.
28.11 The Atomic Color Code—Atomic Spectra
a.
b.
c.
The different electron orbits in an atom are like steps in energy levels.
When an electron is raised to a higher level, the atom is excited.
When the electron returns to its original level, it releases energy in the
form of light.
28.11 The Atomic Color Code—Atomic Spectra
Relating Frequency and Energy
The frequency of the emitted photon, or its color, is directly proportional to the
energy transition of the electron.
f~E
A photon carries an amount of energy that corresponds to its frequency. Red light
from neon gas, for example, carries a certain amount of energy. A photon of twice
the frequency has twice as much energy and is found in the ultraviolet part of the
spectrum.
Solar Spectrum
Mixing Colored Pigments
Only four colors of ink are used to print color illustrations and photographs—
magenta, yellow, cyan, and black.
Disk painted half red, half blue looks magenta
when rapidly spinning.
Weakness or absence of one of the
three types of cones is the cause of
color blindness, leading to a reduced
ability to distinguish colors.
29 or 70?
21 or 74?
Incidence (%)
Classification
Males
Females
Anomalous
Trichromacy
6.3
0.37
Protanomaly
(Red-cone weak)
1.3
0.02
Deuteranomaly
(Green-cone weak)
5.0
0.35
Tritanomaly
(Blue-cone weak)
0.0001
0.0001
Dichromacy
2.4
0.03
Protanopia
(Red-cone absent)
1.3
0.02
Deuteranopia
(Green-cone absent)
1.2
0.01
Tritanopia
(Blue-cone absent)
0.001
0.03
Rod Monochromacy
(no cones)
0.00001
0.00001
What numbers do you see revealed in the patterns of dots below?
I am color blind, as is about 12 - 20 percent (depending on whose figures you
want to believe) of the white, male population and a tiny fraction of the female
population. Most of these circles are nothing but spots to me. Below are the
correct answers to what a person with normal color vision would see - and
what I see (and most people with Red-Green color blindness). When you see
what we can't see, you may understand why it's so tough to find the right sox
and why we like bright colors, which are often identifiable.
Normal Color Vision
Red-Green Color Blind
Left
Right
Left
Right
Top
25
29
Top
25
Spots
Middle
45
56
Middle
Spots
56
Bottom
6
8
Bottom
Spots
Spots
Another interesting color blindness test is below
The test to the right is simpler. The individual with normal color vision will
see a 5 revealed in the dot pattern. An individual with Red/Green (the
most common) color blindness will see a 2 revealed in the dots.
Stare, unfocused, at the red cross for 10 seconds then look at white wall
Cyan
Stare, unfocused, at the flag for 10 seconds then look at white wall
Cyan
Magenta
Yellow