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Bright rainbows are rarer than many people imagine, and are seen less than a dozen times each year in most locations in England. Early mornings and late afternoons are the most likely times to see them because they can form only when it is raining and the sun is less than 42 degrees above the horizon. The centre of the rainbow circle is directly opposite the sun, so the lower the sun the higher in the sky the rainbow forms. Red is always outermost in a primary rainbow with orange, yellow, green, blue, indigo and violet within. So what is a rainbow?

The white light that we see illuminating our world results from the collective effect of rays having a range of visible wavelengths. Each wavelength behaves slightly differently when it passes through a falling spherical raindrop. The longer wavelengths at the red end of the visible spectrum are refracted, or bent, less than the shorter-wavelengths at the blue end of the spectrum. The consequent effect is that the various wavelengths that exist within the sun's white light are deflected to different locations and seen in isolation as distinct colours.

Rays reflected once inside rain drops generate the primary rainbow. Since red light is refracted less than blue, and its minimum angle of deviation is less than for other wavelengths, it appears on the outer edge of the primary bow. Colours are produced by the two refractions as light enters and leaves each raindrop. Rays are ultimately deviated back towards the sun to form a bow appearing in the sky opposite the sun.

The colours of the visible spectrum, as seen in a rainbow, do not really exist. They are the merely the reactions of the various types of cones in the human eye to the different wavelengths of electromagnetic radiation. The eyes of animals, birds and insects are not only variously sensitive to significantly different ranges of wavelengths, but also respond differently to the radiation visible to human eyes. Some birds see the world bathed in ultraviolet, many mammals have bichromatic vision, and various insects may see only brightness. See also human perception of colour.


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