Spectrum of lightThe light entering the eye is either emitted by or reflected from the objects we see. Hot objects emit light with wavelengths occupying a broad continuous band of the electromagnetic spectrum, the position of the band depending on the temperature of the object – the hotter the object, the shorter the wavelength emitted (see blackbody radiation). We tend to think of the spectrum of visible light in terms of the band of colors revealed by Isaac Newton when he split up a beam of sunlight using a prism; in these terms, the shorter the wavelength, the bluer the light.
Other objects emitting light do so either at particular wavelengths or in narrow bands. Where these emission bands fall within the visible spectrum, such objects appear colored, otherwise black. Objects reflecting diffuse light appear colored by virtue of combined scattering and absorption effects, though the nature of the light source is also important.
Color addition and mixingThe eye can only see color when the light is relatively bright; the rods used in poor light see only in black and white. For this reason most stars in the night sky appear white. Blues, red, and yellows are only evident in the case of some of the brighter stars (and planets), or with the aid of binoculars or a telescope. The cones used in color vision are of three kinds, responding to light from the red, green, or blue portions of the visible spectrum. The brain adds together the responses of the different sets of cones and produces the sensation of color. The three colors to which the cones of the eye respond are known as the three primary colors of light. By mixing different proportions of these three colors, any other color can be simulated, equal intensities of all three producing white light. This is known as the production of color by addition, the effect being used in color television tubes where phosphors glowing red, green, and blue are employed. Color pigments, working by transmission or reflection, produce color by subtraction, abstracting light from white and displaying only the remainder. Again a suitable combination of a set of three pigments – cyan (blue-green), magenta (blue-red), and yellow (the complementary colors of the three primaries) – can simulate most other colors, a dense mixture of all three producing black. This effect is used in color photography but in color printing an additional black pigment is commonly used.
Color classification schemesMost colors are not found in the spectrum. These non-spectral colors can be regarded as intermediate between the spectral colors and black and white. Many schemes have been proposed for the classification and standardization of colors. The most widely used is that of Albert Henry Munsell which describes colors in terms of their hue (basic color), saturation or chroma (intensity or density), and value (the degree of lightness or brightness). Hue is denoted by its place on the circumference of the tree; chroma by its distance from the trunk; value by its place up the trunk.
A quantitative system is needed for scientific purposes and for this the color index is used.
Emission and absorption spectra
RainbowsA rainbow is a natural demonstration of the mixture of colors, or wavelengths that make up white light.
Drops of moisture in the atmosphere act as prisms, dispersing the light into its component colours. The observer sees the various colors of the spectrum emerging from many droplets. The completeness of the rainbow he sees depends on his position between the Sun and the drops of moisture and on his horizon; from aircraft at high altitudes a circle can be seen.
Sometimes two rainbows are visible: an inner – called a primary bow – and a larger outer one, or secondary bow. The colors are in opposite sequence in the two rainbows. In the primary (top illustration below), light is dispersed on entering a rain-drop, and the colours reflected from the back of the drop towards the observer. Some rays may be reflected twice within the drop (bottom illustration below) to produce a reversal of the colours in the secondary.
Electromagnetic spectrumThe spectrum of visible light is only a small part of the much greater spectrum of all electromagnetic radiation. Beyond the blue end lie the invisible ultra-violet rays, X-rays and gamma-rays, while infrared rays (heat rays), microwaves and radio waves lie beyond the red end. All electromagnetic radiation had the power to penetrate matter to a certain extent. High frequency radiation – that is, X-rays and gamma-rays – penetrates most.
So-called color charge, or simply color, which is a concept in particle physics, has nothing to do with the color perceived by the human eye.
Related category OPTICS AND OPTICAL PHENOMENA
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