The First Rainbow

Category: File - :Double-alaskan-rainbow.jpg|thumb|right|300px|Semicircular double rainbow. Supernumerary rainbows on the inside of the primary arc. Shadow of the photographer marks the centre of the rainbow circle (antisolar point).

A rainbow is an optical and meteorological phenomenon that causes a spectrum of light to appear in the sky when the Sun shines onto droplets of moisture in the Earth's atmosphere. They take the form of a multicoloured arc, with red on the outer part of the arch and violet on the inner section of the arch.

Category: File - :Rainbow-diagram-ROYGBIV.svg|left|90px

A rainbow spans a continuous spectrum of colours; the discrete bands are an artefact of human colour vision. The most commonly cited and remembered sequence, in English, is Newton's sevenfold red, orange, yellow, green, blue, indigo and violet (popularly memorized by mnemonics like Roy G. Biv).

Rainbows can be caused by other forms of water than rain, including mist, spray, and dew.

Category: File - :TakakkawFalls2 edit.jpg|right|300px|thumb|Rainbows may also form in mist, such as that of a waterfall

Category: File - :rainbow 02.jpg|thumb|300px|Rainbow with a faint reflected rainbow in the lake

A spectrum (plural spectra or spectrumsDictionary.com. The American Heritage Dictionary of the English Language, Fourth Edition. Houghton Mifflin Company, 2004. (accessed: January 25, 2008).) is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by analogy to many fields other than optics. Thus, one might talk about the spectrum of political opinion, or the spectrum of activity of a drug, or the autism spectrum. In these uses, values within a spectrum may not be associated with precisely quantifiable numbers or definitions. Such uses imply a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion.

In most modern usages of spectrum there is a unifying theme between extremes at either end. Some older usages of the word did not have a unifying theme, but they led to modern ones through a sequence of events set out below. Modern usages in mathematics did evolve from a unifying theme, but this may be difficult to recognize.