Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Color, physical phenomenon of light or visual perception associated with the various wavelengths in the visible portion of the electromagnetic spectrum (see Electromagnetic Radiation; Spectrum). As a sensation experienced by humans and some animals, perception of color is a complex neurophysiological process. The methods used for color specification today belong to a technique known as colorimetry and consist of accurate scientific measurements based on the wavelengths of three primary colors.


The human eye does not function like a machine for spectral analysis, and the same color sensation can be produced by different physical stimuli. Thus a mixture of red and green light of the proper intensities appears exactly the same as spectral yellow, although it does not contain light of the wavelengths corresponding to yellow. Any color sensation can be duplicated by mixing varying quantities of red, blue, and green. These colors, therefore, are known as the additive primary colors. If light of these primary colors is added together in equal intensities, the sensation of white light is produced. A number of pairs of pure spectral colors called complementary colors also exist; if mixed additively, these will produce the same sensation as white light. Among these pairs are certain yellows and blues, greens and blues, reds and greens, and greens and violets.

Most colors seen in ordinary experience are caused by the partial absorption of white light. The pigments that give color to most objects absorb certain wavelengths of white light and reflect or transmit others, producing the color sensation of the unabsorbed light.

The colors that absorb light of the additive primary colors are called subtractive primary colors. They are magenta (purplish-pink), which absorbs green; yellow, which absorbs 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 subtractive primary 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 equal amounts, 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, where magenta, yellow, black, and cyan inks are used successively to create natural color. Edwin Herbert Land, an American physicist and inventor of the Polaroid Land camera, demonstrated that color vision depends on a balance between the longer and shorter wavelengths of light. He photographed the same scene on two pieces of black-and-white film, one under red illumination, for long wavelengths, and one under green illumination, for short wavelengths. When both transparencies were projected 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 when white light was used in one of the projectors. Reversing the colored lights in the projectors made the scene appear in complementary colors.