The Electromagnetic and Visible Spectra

Unlike mechanical waves that 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 entire range of the spectrum is often broken into specific regions. The subdividing of the
entire spectrum into smaller spectra is done mostly on the basis of how each region of
electromagnetic waves interacts with matter. 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. You are undoubtedly familiar with some of the other regions of the electromagnetic
spectrum.

Visible Light Spectrum

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 that 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 that
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 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 colors red (R), orange (O), yellow (Y), green (G), blue (B), and violet (V). 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.

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 that 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.

Visible Light and the Eye's Response

The graphic below depicts the approximate range of wavelengths that are associated with the
various perceived colors within the spectrum.
Color Cones

Light that enters the eye through the pupil ultimately strikes the inside surface of the eye known
as the retina. The retina is lined with a variety of light sensing cells known as rods and cones.
While the rods on the retina are sensitive to the intensity of light, they cannot distinguish
between lights of different wavelengths. On the other hand, the cones are the color-sensing cells
of the retina. When light of a given wavelength enters the eye and strikes the cones of the retina,
a chemical reaction is activated that results in an electrical impulse being sent along nerves to the
brain. It is believed that there are three kinds of cones, each sensitive to its own range of
wavelengths within the visible light spectrum. These three kinds of cones are referred to as red
cones, green cones, and blue cones because of their respective sensitivity to the wavelengths of
light that are associated with red, green and blue. Since the red cone is sensitive to a range of
wavelengths, it is not only activated by wavelengths of red light, but also (to a lesser extent) by
wavelengths of orange light, yellow light and even green light. In the same manner, the green
cone is most sensitive to wavelengths of light associated with the color green. Yet the green cone
can also be activated by wavelengths of light associated with the colors yellow and blue. The
graphic below is a sensitivity curve that depicts the range of wavelengths and the sensitivity level
for the three kinds of cones.

The cone sensitivity curve shown above helps us to better understand our response to the light
that is incident upon the retina. While the response is activated by the physics of light waves, the
response itself is both physiological and psychological.

Now suppose that light in the yellow range of wavelengths (approximately 577 nm to 597 nm)
enters the eye and strikes the retina. Light with these wavelengths would activate both the green
and the red cones of the retina. Upon striking the retina, the physiological occurs: electrical
messages are sent by both the red and the green cones to the brain. Once received by the brain,
the psychological occurs: the brain recognizes that the light has activated both the red and the
green cones and somehow interprets this to mean that the object is yellow. In this sense, the
yellow appearance of objects is simply the result of yellow light from the object entering our eye
and stimulating the red and the green cones simultaneously.

If the appearance of yellow is perceived of an object when it activates the red and the green
cones simultaneously, then what appearance would result if two overlapping red and green
spotlights entered our eye? Using the same three-cone theory, we could make some predictions
of the result. Red light entering our eye would mostly activate the red color cone; and green light
entering our eye would mostly activate the green color cone. Each cone would send their usual
electrical messages to the brain. If the brain has been psychologically trained to interpret these
two signals to mean "yellow", then the brain would perceive the overlapping red and green
spotlights to appear as yellow. To the eye-brain system, there is no difference in the
physiological and psychological response to yellow light and a mixing of red and green light.
The brain has no means of distinguishing between the two physical situations.

In a technical sense, it is really not appropriate to refer to light as being colored. Light is simply a
wave with a specific wavelength or a mixture of wavelengths; it has no color in and of itself. An
object that is emitting or reflecting light to our eye appears to have a specific color as the result
of the eye-brain response to the wavelength. So technically, there is really no such thing as
yellow light. Rather, there is light with a wavelength of about 590 nm that appears yellow. And
there is also light with a mixture of wavelengths of about 700 nm and 530 nm that together
appears yellow.

Light Absorption, Reflection, and Transmission

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 is 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.

Atoms and molecules contain electrons. The electrons and their attached springs have a tendency
to vibrate at specific frequencies. 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. 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 interact 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.

Reflection and 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. 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 that we see are largely due to the way
those objects interact with light and ultimately reflect or transmit it
to our eyes. The color of an object is not actually within the object
itself. Rather, the color is in the light that shines upon it and is
ultimately reflected or 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 that strikes
the object and becomes reflected or 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 that 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 that 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 below. 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 that 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.

Color Addition
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 are widely separated on the visible
light spectrum. Any three colors (or frequencies) of light that 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
red, green and blue light-emitting phosphors.
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

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 that we are familiar (or unfamiliar) with.

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 involves 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. Materials that have been
permeated by specific pigments will selectively absorb specific frequencies of light in order to
produce a desired appearance.

Materials contain atoms that are capable of selectively absorbing one or more frequencies of
light. Consider a shirt made of a material that is capable of absorbing blue light. Such a material
will absorb blue light (if blue light shines 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 is 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 gives 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

Complementary Colors and Color Subtraction

In the above example, the paper absorbed blue light. Paper that absorbs blue light is permeated
by a pigment known as a yellow pigment. While most pigments absorb more than a single
frequency (and are known as compound pigments), it becomes convenient for our discussion to
keep it simple by assuming that a yellow pigment absorbs a single frequency. A pigment that
absorbs a single frequency is known as a pure pigment. The following rule will assist in
understanding what colors of light are absorbed by which pigments.

Pigments absorb light. Pure pigments absorb a single frequency or color of light. The color of
light absorbed by a pigment is merely the complementary color of that pigment.

Thus, pure blue pigments absorb yellow light (which can be thought of as a combination of red
and green light). Pure yellow pigments absorb blue light. Pure green pigments absorb magenta
light (which can be thought of as a combination of red and blue light). Pure magenta pigments
absorb green light. Pure red pigments absorb cyan light (which can be thought of as a
combination of blue and green light). And finally, pure cyan pigments absorb red light.

Now lets combine the process of color subtraction with an understanding of complementary
colors to determine the color appearance of various sheets of paper when illuminated by various
lights.

Example 1
 Magenta light shines on a sheet of paper containing a yellow pigment.
Determine the appearance of the paper.

Magenta light can be thought of as consisting of red light and blue light. A yellow pigment is
capable of absorbing blue light. Thus, blue is subtracted from the light that shines on the paper.
This leaves red light. If the paper reflects the red light, then the paper will look red.

M - B = (R + B) - B = R
Primary Colors of Paint

The three primary colors of paint used by an artist, color printer or film developer are cyan (C),
magenta (M), and yellow (Y). Artists, printers, and film developers do not deal directly with
light; rather, they must apply paints or dyes to a white sheet of paper. These paints and dyes must
be capable of absorbing the appropriate components of white light in order to produce the
desired effect. Most artists start with a white canvas and apply paints. These paints have to
subtract colors so that you might see the desired image. An artist can create any color by using
varying amounts of these three primary colors of paint.

Each primary color of paint absorbs one primary color of light. The color absorbed by a primary
color of paint is the complementary color of that paint. The three colors that are primary to an
artist (magenta, cyan, and yellow) subtract red, green, and blue individually from an otherwise
white sheet of paper. Thus,

Magenta paints absorb green light.

Cyan paints absorb red light.

Yellow paints absorb blue light.