Light which comes directly from the sun, from
electricity, candles or fire, though not alike, is called
white light because it does not appear to have any
particular color. Actually
it appears colorless because it is an almost perfectly
balanced mixture of all the wave lengths of visible light.
The composite nature of white light was first
demonstrated by Sir Isaac Newton in the 1660’s when he
passed a beam of sunlight through a glass prism, where it
broke up into the familiar rainbow colors when projected
onto a screen.
A spectrum is formed whenever light goes through a
light may come from the hot wire in an electric bulb, a
glowing coal in the fireplace or melted iron in a foundry.
All of these are hot, glowing objects.
The spectrum is a band of colors, going smoothly
from red at one side to violet at the other, with no
spaces between them. Scientists
call this a continuous spectrum.
Spectrum colors are the same as those of the
rainbow, and there is a good reason for this.
A rainbow is a spectrum.
It is formed, not by a glass prism, but by very
tiny drops of water that float in the air after a rain
drop acts like a prism and spreads out the colors.
The result is the familiar curved band of color
that is called the rainbow.
A ray of light is refracted or bent as it passes
obliquely from one medium to another of different density
through a triangular prism.
It is always refracted whenever it crosses the
boundary between air (or a vacuum), and a transparent
substance, except when it strikes the substance at right
paths of different wave lengths of light will diverge
within the substance because each wave length is bent at a
Red is bent least; then, in order, orange; yellow;
green; blue; indigo; and violet, the latter being slowed
and bent the most. The
different wave lengths are dispersed in sequence; in this
way the colors are spread out and can be seen separately,
but when all are mixed together the eye sees no color at
all, only white light.
Thus the sensation of “color” is due to certain
wave lengths of light.
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In 1800 W. Herschel studied heat distribution with
the aid of thermometers and found the maximum temperature
beyond the red end of the spectrum, thus discovering the
infrared spectrum. Then
in 1801 J.W. Ritter, in studying the effect of spectral
light on silver salts, found this action extending beyond
the violet and thus discovered the ultraviolet spectrum.
The next year Thomas Young established the first
connection between the wave-length theory and the
spectrum; and he calculated the approximate wave lengths
of the colors recognized by
Throughout the 19th century further
discoveries were made by famous scientists in measuring
the solar spectrum, and comparing this with the spectrum
of flames, or the sparks of pure elements.
In 1868 A.J. Angstrom established and loaned his
name to a new unit of wave-length measurement, the
angstrom, which in spectroscopy is one ten-millionth part
of a millimeter, or one ten-billionth of a meter.
Another method of studying the spectrum is by use
of a grating; ruling fine parallel lines with a diamond
point on glass or copper, then studying the rays, which
are diffracted in this case.
The advantage of this method lies in the
possibility of studying not only the visible spectrum, but
also the ultraviolet and infrared spectra as well.
There are generally four ways of observing spectra: that is, visually, photoelectrically, radiometrically, and photographically. Each is useful in a different way. Although the average human eye is most sensitive to green light (5500 angstroms), its sensitivity declines rapidly to zero for infrared (7700 A), and ultraviolet (3800 A).
Visual methods of observation are of little value
in the study of spectroscopy.
Eyesight is too selective, variable and restricted
to one octave. Therefore
extensive instruments have been devised to eliminate error
in judgment. These
have become so refined that the presence of helium was
detected on the sun in 1868, a generation before it was
shown to be present on earth.
Spectroscopic measurements are concerned
essentially with energy distribution as a function of
visible spectrum ranges from about 3800A (violet limit) to
7700A (red limit). Extremely
short waves, such as those in the x-ray range are only
about 0.1A to about 100A, and therefore a different, more
practical unit of measurement is used for x-ray, called
the x unit, and still another for the extremely long
infrared heat waves, which are measured in larger units
Some instruments used to measure
is an instrument used to determine the index of refraction
by measuring the external angle of a prism, and its angle
of minimum deviation.
(Also classified among spectroscopes)
is any of various instruments designed for forming and
examining optical spectra, constructed to enable one to
make observations visually.
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is used to photograph the spectrum.
is a photometer for measuring the relative intensities of
light in different parts of a spectrum, or the relative
intensity of two spectra.
is any instrument for measuring the intensity of light, or
comparing the relative intensity of several lights.
Color is a response of the human observer to
visible light energy, which is a small part of the total
An object is visible because it is able to reflect
light, for color is not in an object but in the reactions
of the eye to the vibration of the object.
There are at least six things that work together to
produce the color we see.
They are the light source, the light itself, the
material medium through which the light travels, the
object on which the light falls, the eye, and the brain.
When we see the color of an object, it is the last
step in a chain of events that begins at a light source.
The light source generally sends out a mixture of
light rays of many wave lengths.
As the rays pass through the air to the object, the
air attenuates some of the light of short wave-length by
scattering it. When
the light falls on the object, the object removes some
more light by absorbing it.
What is left of the light is then reflected or
transmitted from the object through the air to our eyes.
Once again the air removes some light.
The color we see depends on the kind of light
mixture that finally reaches our eyes.
But it depends, too, on the nature of our eyes, and
the message that our eyes send to the brain.
When the light of one color and wave length falls
on an object, the object divides the light into three
part of the light is allowed to pass right through, as if
the object were full of holes and the light went through
the holes. We
say that this part of the light has been transmitted.
of the light bounces back from the object, the way a ball
bounces back from a wall.
We say that this part of the light has been
third part of the light is trapped in the object and is
not allowed to escape.
Usually the trapped light is turned into heat.
We say that this part of the light has been
While white objects reflect back all colors, and
black reflect none, there are some things that reflect
only part of the light that shines on them, and treat all
colors impartially. These
objects appear gray, because gray is an “impartial
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A gray object may look white when it is the
brightest thing of all that can be seen at the same time.
It looks black when it is the least bright of all
things visible. But
it looks gray when there are both brighter and less bright
objects within view. So
whether an object looks white or black or gray depends not
only on the light that it sends to your eyes, but also on
a comparison between this light and the light sent by
other objects. This
comparison is made by your brain, unconsciously.
The dictionary defines color as “a visual
attribute of bodies or substances, distinct from their
characteristics of size, form or texture.
The appearance of color depends upon the spectral
composition of wavelengths of radiant energy capable of
stimulating the retina and its associated neural
The separation of colors in the spectrum is due to
the fact that different colors have different wave
limits of the visible wave lengths are about 16 millionths
and 28 millionths of an inch.
Each different wave length produces in the eye the
sensation of a different color.
The given wave length associated with each color of
the visible spectrum is as follows:
17 millionths of 1 inch
18 millionths of 1 inch
19 millionths of 1 inch
20 millionths of 1 inch
23 millionths of 1 inch
24 millionths of 1 inch
27 millionths of 1 inch
Precious stones such as diamonds flash colored
light that is very beautiful. These transparent stones are
formed of materials that slow down light waves very
markedly. In a
pure diamond, made only of carbon atoms, light travels
scarcely half as fast as it does in air.
The gems are cut and polished to have dozens of small
surfaces, called facets, so that many little prisms are
that shines into such a cut stone is refracted in many
“fire” of a good diamond comes from the breaking up of
white light into many colors by its tiny prisms.
The greater the index of reflection, the greater the
extent to which a light beam is deflected upon entering or
leaving that medium. Diamonds
owe their brilliance to that very high index of refraction.