Color I INTRODUCTION Color © Microsoft Corporation.

« The colors that absorb light of the additive primary colors are called subtractive primary colors.

They are magenta (purplish-pink), which absorbs green; yellow, whichabsorbs blue; and cyan (light greenish-blue), which absorbs red.

Thus, if a green light is thrown on a magenta pigment, the eye will perceive black.

These subtractiveprimary colors are also called the pigment primaries.

They can be mixed together in varying amounts to match almost any hue.

If all three are mixed in about equalamounts, they will produce black.

An example of the mixing of subtractive primaries is in color photography and in the printing of colored pictures in magazines, wheremagenta, yellow, black, and cyan inks are used successively to create natural color.

Edwin Herbert Land, an American physicist and inventor of the Polaroid Landcamera, demonstrated that color vision depends on a balance between the longer and shorter wavelengths of light.

He photographed the same scene on two pieces ofblack-and-white film, one under red illumination, for long wavelengths, and one under green illumination, for short wavelengths.

When both transparencies wereprojected on the same screen, with a red light in one projector and a green light in the other, a full-color reproduction appeared.

The same phenomenon occurred whenwhite light was used in one of the projectors.

Reversing the colored lights in the projectors made the scene appear in complementary colors. III ABSORPTION The mechanism of the absorption of light by substances to produce color is obscure.

It is apparently a function of the molecular structure of the substance.

In the caseof organic compounds, only unsaturated compounds show color, and their hue can be changed by altering the compounds chemically.

Inorganic compounds aregenerally colorless in solution or liquid form, except for compounds of the so-called transition elements. Color is also produced in other ways than by absorption.

The colors of mother-of-pearl and of soap bubbles are caused by interference.

Some crystals show differentcolors when light is passed through them at different angles, a phenomenon known as pleochroism.

A number of substances show different colors by transmitted andreflected light.

For example, a very thin sheet of gold appears green by transmitted light.

The “fire” of certain gems, notably the diamond, is due to the dispersion ofwhite light into its component spectral hues, as in a prism.

Some substances, when illuminated by light of one hue, absorb this light and reradiate light of a differenthue, always of longer wavelength.

This phenomenon is called fluorescence, or, if delayed, phosphorescence ( see Luminescence).

The blue of the sky is caused by the scattering of the short wavelength blue components of white sunlight by the molecules and atoms that make up the atmosphere.

A similar scattering can be observed ina darkened movie theater.

Seen from the side, the light beam from the projector appears blue, because of the smoke and dust in the air; yet the light on the screen iswhite.

See also Optics; Radiation. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

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