Rainbow

Rainbow. A rainbow is a meteorological and optical phenomenon caused by the refraction, dispersion, and reflection of sunlight in water droplets, resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc, most commonly observed after a rain shower when the Sun is low in the sky. Rainbows have been a source of fascination and inspiration across human cultures, science, and art for millennia.

Formation and physics

The scientific explanation for the rainbow was systematically advanced by scholars like Persian astronomer Ibn al-Haytham in his Book of Optics and later refined during the Scientific Revolution. The foundational physics was established by René Descartes, who used geometrical optics in his work Les Météores, and fully explained by Isaac Newton through his experiments with prisms. The phenomenon requires a light source, such as the Sun, and a multitude of spherical water droplets, typically from recent precipitation. Within each droplet, sunlight undergoes Snell's Law refraction, internal reflection off the droplet's inner surface, and a final refraction as it exits. This process separates white light into its constituent wavelengths. The primary bow is created by a single internal reflection, while a secondary, fainter bow results from two internal reflections, which reverses the color sequence. The precise angle of observation, approximately 42 degrees from the antisolar point for the primary bow, is critical for visibility, making the phenomenon highly dependent on the positions of the observer, the light source, and the water.

Color spectrum and order

The sequence of colours in a rainbow is a continuous spectrum, traditionally described in cultures influenced by Western science as comprising seven bands: red, orange, yellow, green, blue, indigo, and violet. This seven-colour delineation was popularized by Isaac Newton, who drew parallels with the musical notes of a diatonic scale. In reality, the human eye perceives a smooth gradient where colours blend seamlessly. The order is constant due to the wavelength-dependent nature of refraction; longer wavelengths, such as red, are bent less than shorter wavelengths like violet. This results in red always forming the outer edge of the primary arc and violet the inner edge, a pattern reversed in the secondary bow. The apparent brightness and width of colour bands can be influenced by atmospheric conditions, such as the size of the raindrops, with larger droplets producing more vivid and defined spectra.

Cultural significance and mythology

Rainbows hold profound symbolic meaning in the mythologies and belief systems of numerous cultures worldwide. In the Hebrew Bible, the rainbow is presented as a sign of God's covenant with Noah after the Great Flood, a narrative central to Judaism, Christianity, and Islam. Norse mythology depicted the Bifröst as a burning rainbow bridge connecting Midgard to Asgard, home of the Æsir. In Irish mythology, the leprechaun is said to hide a pot of gold at the end of a rainbow. Various indigenous traditions, such as those of the Aboriginal Australians, often view the rainbow as a serpent deity or a creator spirit. The rainbow flag, designed by Gilbert Baker, was adopted as a symbol of LGBT pride and social movements following the Stonewall riots. These diverse interpretations highlight the rainbow's universal role as an emblem of hope, promise, and bridge between realms.

Variations and related phenomena

Beyond the common primary and secondary bows, several related optical effects exist under specific conditions. A supernumerary rainbow appears as faint, pastel-coloured bands inside the primary arc, caused by wave interference and best seen with small, uniform droplets. Twinned rainbows, where two arcs split from a single base, are rare and result from non-spherical raindrops. Moonbows are lunar rainbows, fainter and often appearing colourless to the eye, observed at night near waterfalls like Victoria Falls or Yosemite Falls. Fogbows occur in fog or cloud droplets, appearing as broad, whitish arcs. Other atmospheric optics involving ice crystals include halos around the Sun or Moon, and sun dogs, often seen in cold climates like those of Antarctica or Alaska.

Observation and photography

Optimal rainbow observation occurs when the sun is low, typically in the early morning or late afternoon, with the observer's back to the sun and a region of falling rain ahead. Locations like Hawaii, the Scottish Highlands, and Iguazu Falls are renowned for frequent and vivid displays. For photography, using a polarizing filter can enhance saturation by reducing glare from the sky and wet surfaces, while a wide-angle lens is often necessary to capture the full arc. Notable photographers like Ansel Adams have captured iconic images of rainbows in landscapes, such as in Yosemite National Park. Pilots, particularly in aviation over polar regions, sometimes observe complete circular rainbows from aircraft, a perspective impossible from the ground. Scientific study of rainbows also informs fields like meteorology and climate science, aiding in the analysis of precipitation particle size and distribution.