rainbow fish: see killifish.

rainbow, arc showing the colors of the spectrum, violet inside and red outside, which appears when the sun shines through water droplets. It often appears while the sun is shining after a brief thundershower in the late afternoon or on fog layers. The sun, the observer's eye, and the center of the arc must be aligned—the rainbow appears in the part of the sky opposite the sun. The rainbow is an arc of 180° if the sun is at the horizon, and it cannot appear if the sun is high in the sky. It is caused by the refraction and reflection of rays from the sun on a "sheet" of water droplets. The light is refracted as it enters the sphere of the individual water drop, then is reflected from the drop's opposite side, and is again refracted as it leaves the drop and passes to the observer's eye. When conditions are suitable, a double rainbow may be seen; a larger, paler, secondary rainbow with colors reversed (red inside) outside the primary arc is caused by two refractions and two reflections of the ray while it is inside a drop. The "rainbows" of mist, lawn spray, and spray from a waterfall are similarly caused. The lunary rainbow, seen much less often, is usually observable soon after dark following a brief summer storm or shower when the moon is nearly full. Aristotle was first to devote serious attention to the rainbow, but his mistaken explanation of it misled thinkers for centuries. Descartes in the 17th cent. also attempted to account for the phenomenon but the correct explanation of it could not be furnished until the physics of light and its reflection and refraction were understood and the spectrum explained. In religion and art the rainbow symbolizes God's promise of mercy to mankind after the Deluge (Gen. 9.13). The Greeks and Romans called the rainbow the sign of Iris, messenger of the gods. The Inca and other Native Americans regarded the rainbow as a gift from the sun-god. There are fairy tales of searches for the pot of gold at the foot of the rainbow.

Rainbow Division, nickname of the 42d Division of the U.S. army. The first U.S. combat division to arrive in France in World War I, it participated in the second battle of the Marne, fought in the counteroffensive at Château-Thierry, and spearheaded attacks at Saint-Mihiel and in the Meuse-Argonne sector. Throughout the war the division suffered heavy losses. It later became a part of the National Guard.

Rainbow Bridge National Monument, 160 acres (65 hectares), S Utah; est. 1910. Rainbow Bridge, the largest natural bridge in the world, is a symmetrical, pink, sandstone arch, 309 ft (94 m) high, 33 ft (10 m) wide, with a 278-ft (85-m) span. Located in one of the most rugged and remote regions of the United States, it was discovered in 1909 by an expedition that set out to find the great stone arch rumored by the Native Americans. It is now accessible by boat from Lake Powell. See National Parks and Monuments (table).

Species (Oncorhynchus mykiss) of fish in the salmon family (Salmonidae) noted for spectacular leaps and hard fighting when hooked. It has been introduced from western North America to many other countries. A brightly coloured fish of lakes and swift streams, it is covered with small black spots and has a reddish band along either side. The steelhead, a large, bluish, oceangoing form, is also a prized game fish. Rainbow trout may weigh 6 lb (2.8 kg); steelheads (and rainbows in large lakes) may weigh 10–20 lb (4.5–9 kg) or more. Another form of rainbow, the Kamloops, or Kootenay, trout of Idaho, may exceed 30 lb (14 kg).

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Series of concentric, coloured arcs that may be seen when light from a distant source—usually the Sun—falls on a collection of water drops such as in rain, spray, or fog. The coloured rays of the rainbow are caused by the refraction and internal reflection of light rays that enter the drop, each colour being bent through a slightly different angle. Hence, the combined colours are separated upon emerging from the drop. The most brilliant and most common rainbow is the so-called primary bow, which results from light that emerges from the drop after one internal reflection. The colours of the arc (from outside to inside) are red, orange, yellow, green, blue, indigo, and violet. Occasionally a less-intense secondary bow may be observed; it has its colour sequence reversed.

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Natural area, southern Utah, U.S. Located on the Navajo Indian Reservation near the Utah-Arizona border, the monument was established in 1910 and occupies 160 acres (65 hectares). It centres on a rainbow-shaped bridge of pink sandstone 290 ft (88 m) above a creek that winds toward the Colorado River. The bridge is 275 ft (84 m) long and is one of the world's largest natural bridges. Embedded among canyons, the area is accessible only on foot, by horseback, or by boat on Lake Powell.

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