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Rainbow formation. Historical artwork explaining why primary (lower) and secondary rainbows form. Rainbows occur when sunlight is refracted (bent) as it moves through water drops (depicted as four circles) in the sky and reflected back by the drops' rear surface to an observer. The colours making up white sunlight are bent by different amounts to create the rainbow effect. In secondary rainbows the light is reflected twice so they have reversed colours and are fainter than primary rainbows. This explanation by Rene Descartes shows that the angle of the sunlight to the drops and back to the observer is between 40-42 degrees. From Phisices Elementa by W. J. Gravesande (1722).
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