Science of Light

Lesson Seven

07-Science of Light

Color  Hues


    Unless you pass light through a prism and spread it out, you cannot tell exactly what colors, or wave lengths, are in it.  Something that appears red to the eye may prove to have some orange and yellow in its spectrum, as well as red, for example.


    Color wheels, or hue circles, show the relationship between the various colors.  They are nothing more than the visible spectrum bent into a circle.  The hues are arranged in the same order as they come out of a prism, that is according to wave length.  The one exception to this arrangement is purple and violet, which do not actually appear in the spectrum because they are a mixture of red and blue rays.  However, to form the completed circle, they are arranged in the proper order between the blue and red.


    According to Sir Isaac Newton, between the visible violet and the visible red are indigo, blue, green, yellow and orange.  As our study of color progresses this appears a limited concept, as the solar spectrum contains only seven of the multitude of colors that we are familiar with, to say nothing about the colors we are not familiar with.  There are infinite numbers of colors that we cannot perceive.  The infra-red and ultra-violet we know something about – rays that are invisible to the eye but can be demonstrated by various instruments.  Then there are the “odic” colors or the psycho-magnetic colors, which many with sight know exist.



     #1     In the pigment color chart normally used, the six basic colors of the rainbow are shown: Red, orange, yellow, green, blue and violet.  A seventh line could be added for indigo, between blue and violet. 





    According to the Brewster theory the primary colors are red, yellow and blue; the secondary colors orange, green and purple; and the tertiary colors russet, slate and citrine.  The secondary are called secondary because by a combination of two of the so-called primary colors, one of the secondary colors can be produced: red and yellow produce orange; yellow and blue produce green; red and blue, violet.


    The triad of tertiary colors is made by combining two of the secondary colors: green and violet produce olive or slate; green and orange produce citrine; orange and violet produce russet.

Lesson Seven, page 2


    From the polarity scheme described in the chromatic curve, red, yellow and blue are very convenient as primary colors, as red is exciting, blue is cool and soothing; while yellow is the medium color or the center of the luminosity.


    The hues red, blue and green have a very important property.  If we mix the right amount of light of these three hues, we can match closely any hue on the color circle.  For this reason, red, blue and green are often called primary colors.  There are other sets of three hues that

have the same property, but the red-blue-green set is the one that is used most often.



               Color Wheel #2




             The lines join

             complementary colors






    Yet another variation of spectral colors is shown in the following table.  We are presenting for comparison the views of several scientists, - all experts in the field of color - and views they sometimes seem, for color carries a personal as well as scientific connotation, so there appears no absolute criterion.



Complementary Spectral Colors


Red  …………..Blue-green


Orange………  Blue

Yellow………. Blue-violet



    Green light mixed with purple light also produces white or gray.  This pair has not been put into the table because purple is not considered by this source as a color of the spectrum.  Purple light is a mixture of red light and blue light.  So a mixture of green light and purple light is really a mixture of three spectral colors.

Lesson seven, page 3

Complementary Colors


    One way of defining complementary colors is that a complementary color is the color a normal eye will see when closed after staring at a given radiant color.  If the person stares at red they will see a greenish blue with the eyes closed.  If they stare at orange they will see a deep blue.  If at yellow, a color between blue and violet.  If they stare at greenish yellow, they will see a purple; and if at green, they will see a magenta.  If they reverse these colors and stare at greenish-blue, they will see red, and so on.


    The reasons for this phenomenon seems to be that certain of the rods and cones in the retina, which are in tune with the color that is stared at, become fatigued and call up a sympathetic action of the nerves not acted upon.  This sympathetic reaction will show up in from 10 to 60 seconds depending upon the radiance and the individual.


    There are many mixtures of light that look white or gray.  Some of them, like those reflected from a gray surface bathed in sunlight, include all the colors of the spectrum.  There are others that include only two colors.  Two colors which produce white or gray when light of these colors is mixed are called complimentary colors.  Such colors set each other off and serve especially well for purposes of contrast.


    Scientifically, complementary colors are based on wave length.  Any two surface colors (such as dyes, paints and inks) are said to be complimentary if, when mixed, they cancel each other out and produce a neutral sensation such as gray or black.


    Mixing light of different colors is not the same as mixing paints.  For example, every artist knows that a mixture of blue and yellow paints looks green.  Paints, dyes and other coloring materials are able to take away certain wave lengths from the white light that falls on them, leaving only some of the wave lengths to be reflected.  This taking-up of certain colors is called absorption.


    If you mix the proper amounts of paints of all possible colors, you get black paint.  Each kind of coloring matter in the mixture absorbs its own set of wave lengths and nothing is left to be reflected.  Anything that reflects very little light of any kind looks black, and anything that reflects a mixture of many different wave lengths looks white. Black is not a color, but the absence of color, or the absence of reflected light.


    Pigment soaks up certain color waves and reflects the rest of them.  When you see a red building, or a red cloth, the pigment in the paint and the dye absorb all of the colors except red.  Thus red is reflected to your eyes.  A paint pigment is named for the color that it does not absorb.  Plants look green because the material in them takes up almost every other color, and only the green is reflected and reaches your eyes.


    The flakes of “coloring matter” in white paint may not be white at all.  Under a microscope, these flakes may look as clear and colorless as glass, but because they reflect day-light so well, the paint looks white.  Snow, which consists of colorless crystals of ice looks white for the same reason.

Lesson seven, page 4


    In blending colored light, you add colors, the additive process.  In blending pigment color, the subtractive process applies.


    What happens when a blue filter and a yellow filter are placed one after the other in the path of a beam of white light?  The blue filter removes or subtracts from the white light all of the red and yellow light that is in it.


    When a painter mixes two pigment colors together, he is subtracting colors.  The pigment in each color is like a filter that removes some colors from white light.  Mixing the pigments is like placing one filter after the other in the path of the light that strikes the paint.


    When a blue spotlight and a yellow spotlight shine on the same white surface, this kind of mixture is called additive, because it is made by adding one kind of light to another.


    Additive primaries are said to be red, green and blue.  One color wheel uses orange, green and blue.  This is based on mixing actual light rays, adding colors together.  As white is the effect produced by all colors of light shining together, there will be a space of white at the center where the colors blend.


    Color filters transmit light, but strain out certain colors and let others pass through.  Most filters are made of glass, gelatin or plastics, and these work by taking up part of the light passing through them.  Traffic lights are a practical example of their use.


    Most of the things we see around us are opaque and do not allow light to pass through them at all, all the light that falls on them being either absorbed or reflected.  If the object absorbs some colors and reflects others, then it looks colored.  Its color is the color of the light it reflects just as the color of a filter is the color of the light it transmits.


    Colors are classified in two major categories: achromatic colors – these include black and white, and the whole series of intermediate grays, varying only in brilliance; and chromatic colors (of the spectrum) that vary also in hue and saturation.  (Greek “Chroma”: color)


    Hue is a difference in the color itself, and is what most people think of as “color”.  The colors of the spectrum are hues, and are dependent on the wave-length of light.


    Value or brightness is the difference in the brightness or luminosity of the same colors within each hue – this correlating with the energy of the stimulus, i.e. the amplitude of the wave.


    Chroma is the purity of a color.  A truly pure color is free of any gray.  Saturation or purity is the quality of brilliance or cleanness, as opposed to drab or dirty, and is dependent on the mixture of short and long wave lengths.  Monochromatic wave lengths give the purest saturation.


    Tints, shades and hues are often spoken of indiscriminately, but this is not correct.


Lesson seven, page 5


    When we combine a color with white, we produce a variation in the brightness of the color, called a tint.


    When we combine a pure color with black, we produce a variation in saturation or purity, called a shade.  By mixing a color with its complement, we can also darken it without changing its hue.


    When we combine gray (which is a mixture of black and white) with a color, we produce a tone.



In the printing trade:


    The subtractive primaries as used by printers are cyan, magenta, and yellow.


    The complement of red is blue-green called cyan.

    The complement of green is a red-purple called magenta.

    The complement of blue is yellow.


                              Color Wheel #4


    Another variation of color compliments as outlined by Paschel.


    In this color wheel, the complementary colors fall directly opposite each other.





        In surface colorants such as paints, inks, and dyes, the fundamental or primary colors (according to Herbert Paschel) are magenta, yellow and cyan.  They are called primary because every other color can be produced by mixing them together in various proportions or by adding white, gray or black to them.


    By overlapping circles of color, it can be seen that a mixture of equal parts of magenta and yellow produces red; a mixture of cyan and yellow produces green; and a mixture of cyan and magenta produces blue.  The mixture of all three colors in equal parts produce black.


    Where color dots are used side by side as colorants (in printing) a surface made up of a mixture of equal parts of magenta and yellow appears red.  When white light strikes it, the magenta absorbs green rays from the light, and the yellow absorbs blue rays.  Since both the magenta and yellow reflect all the red rays, the surface appears red.


    Cyan and yellow produce green because the cyan subtracts the red rays, and the yellow subtracts the blue rays, and both reflect the green rays.  Since magenta absorbs green light, the cyan absorbs red rays, only the blue rays remain to be reflected from this surface.


Lesson seven, page 6




    The world is colorless to some people who are called color-blind.  Their optical nerves do not react to color.  It is generally accepted that the world was colorless to prehistoric man who was color-blind.  The faculty to see color has developed slowly over the ages.


    Color blindness is the inability to distinguish between certain colors.  Genuine color-blindness, or complete inability to see colors, is quite rare, affecting only one person in 300,000.


    What is more generally inferred by the term is some form of color-deficiency, the most common of which is the red-green confusion which affects about 8 million persons in the United States.  There is no known cure, but it is not troublesome.  Due to the commoner red-green color-deficiency, most traffic lights have been changed from red and green to orange-red and blue-green, colors which are more easily distinguished.


General Visibility: 


    Many tests have been made to find out what combinations of colors are easiest to see.  This knowledge is useful for signs, posters, license plates, traffic signs, etc.  The visibility scale is as follows in order of effectiveness:


 1.  Black on yellow

 2.  Green on white

 3.  Red on white

 4.  Blue on white

 5.  White on blue

 6.  Black on white

 7.  Yellow on Black

 8.  White on Red

 9.  White on Green

10.  White on Black


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