prism

Prism, in optics, a piece of translucent glass or crystal used to form a spectrum of light separated according to colors. Its cross section is usually triangular. The light becomes separated because different wavelengths or frequencies are refracted (bent) by different amounts as they enter the prism obliquely and again as they leave it (see refraction). The shorter wavelengths, toward the blue or violet end of the spectrum, are refracted by the greatest amount; the longer wavelengths, toward the red end, are refracted the least. The Nicol prism is a special type of prism made of calcite; it is used for polarization of light.

Antioxidant

Antioxidant, type of molecule that neutralizes harmful compounds called free radicals that damage living cells, spoil food, and degrade materials such as rubber, gasoline, and lubricating oils. Antioxidants can take the form of enzymes in the body, vitamin supplements, or industrial additives. They are routinely added to metals, oils, foodstuffs, and other materials to prevent free radical damage.

Free radicals are produced under certain environmental conditions and during normal cellular function in the body. These molecules are missing an electron, giving them an electric charge. To neutralize this charge, free radicals try to steal an electron from, or donate an electron to, a neighboring molecule. This process, called oxidation, creates a new free radical from the neighboring molecule. The newly created free radical, in turn, searches out another molecule and steals or donates an electron, setting off a chain reaction that can damage hundreds of molecules.

Antioxidants halt this chain reaction. Some antioxidants are themselves free radicals, donating electrons to stabilize and neutralize the dangerous free radicals. Other antioxidants work against the molecules that form free radicals, destroying them before they can begin the domino effect that leads to oxidative damage.

Learn more: Antioxidants in the Human Body; Dietary Sources of Antioxidants; Antioxidants in Industry

Toxin

Toxin, poisonous substance produced by the metabolic activities of certain living organisms, including bacteria, insects, plants, and reptiles.

Some bacteria secrete toxins in tissues that they colonize; these are true toxins. Other bacteria retain most of the poisonous material within themselves, and the toxins are liberated only when the bacteria become disintegrated by chemical, physical, or mechanical means. In addition to bacterial toxins, the characteristic poisons and venoms produced by various plants are called phytotoxins, and those produced by animals are called zootoxins. The more important true toxins causing infection in humans are those of botulism, dysentery, tetanus, and diphtheria. Because of their extreme susceptibility to various chemical and physical influences, such as light, heat, and age, toxins are difficult to isolate, and knowledge of toxins has been gained through the lesions and symptoms that they produce when injected into animals.

Although all toxins are poisonous, in order to become effective they must chemically combine with the animal cells. With the exception of botulin, they are destroyed by the gastrointestinal juices. Although the exact chemical nature of toxins is unknown, they are generally thought to be toxalbumins, substances closely allied to proteins. It has also been abundantly demonstrated that toxins are colloid in nature and bear a close resemblance to enzymes. Toxins are absolutely specific synthetic products, unlike ptomaines, which are cleavage products from the medium on which the bacteria grow. In certain forms, toxins can give rise to antibodies, natural defensive substances produced in the body. Toxoids are toxins that are treated to destroy their toxicity but that remain potent enough to create antibodies when injected into the body.