Science of Light

Lesson Six


Color Spectrum


    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 prism.  The 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 shower.  Each 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 angles.  The paths of different wave lengths of light will diverge within the substance because each wave length is bent at a different angle.


    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.


    Sir Isaac, only 23 years old at the time, proved that all the colors were in light by reversing the experiment.  Using a second prism he recombined these same colors back into a single beam of colorless white light.

Lesson six, page 2


    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 Newton .


  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 wavelength.  The 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 called microns.


Some instruments used to measure light:


A spectrometer 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)


A spectroscope is any of various instruments designed for forming and examining optical spectra, constructed to enable one to make observations visually.

Lesson six, page 3


A spectrograph is used to photograph the spectrum.


A spectrophotometer is a photometer for measuring the relative intensities of light in different parts of a spectrum, or the relative intensity of two spectra.


A photometer 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 electromagnetic spectrum.  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 parts.  One 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.  Another  part 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 reflected.  A 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 absorbed.


    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 reflector”.

Lesson six, page 4


    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 structures”.




    The separation of colors in the spectrum is due to the fact that different colors have different wave lengths.  Practical 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:


Color                               Wavelength


Violet                    17 millionths of 1 inch

Indigo                    18 millionths of 1 inch

Blue                       19 millionths of 1 inch

Green                     20 millionths of 1 inch

Yellow                   23 millionths of 1 inch

Orange                   24 millionths of 1 inch

Red                        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 formed.  Light that shines into such a cut stone is refracted in many directions.  The “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.

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