The visible spectrum runs from about 400 nanometres (violet) to 700 nanometres (far red). The spectrum produced by a rainbow will depend on two factors: absorption by water, mainly in the form of vapour (because light must pass through water droplets to form a rainbow), and the light source, in this case direct sunlight. Water transmits light best at 400 nanometres. That is why everything looks blue underwater - the longer wavelengths are being absorbed. Water transmits reasonably well in the ultraviolet up to 200 nanometres, so the strength of the violet end of a rainbow depends on the light source.
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