Social Studies Connection

The rainbow actually consists of two rainbows, the primary and the secondary. Between the bows, the sky is darker than otherwise. This area is called Alexander's dark band after Alexander of Aphrodisias (A.D. 200), who was the first to describe the dark area.

The first to really try to describe the rainbow was Aristotle. He presumed that the rainbow was caused by reflection of sunlight in the clouds. The light was reflected at a certain angle. That means the rainbow consists of a cone of rays. Aristotle was thus the first to explain the rainbow's circular shape and that the rainbow is not located at a definite place on the sky, but is seen in a certain direction.

Roger Bacon measured the angle of the rainbow cone as 42° in 1266 (the secondary rainbow is 8° higher on the sky). In 1304 the German monk Theodoric of Freiberg proposed the hypothesis that each raindrop in the cloud makes its own rainbow. He verified his hypothesis by observing the diffraction of sunlight in a circular bottle. Notice that Aristotle believed that the rainbow originated from the entire cloud. Theodoric's results remained unknown for three centuries, until Descartes (in 1656) rediscovers diffraction in the drop. Both Theodoric and Descartes knew that the rainbow consisted of 2 bows. In the primary rainbow, the rays are reflected once within the drop, in the secondary rainbow, the rays are reflected twice within the drop. Furthermore, they noticed that only one color is seen when looking at the drop (the bottle) in a given direction. From this they concluded that the colors of the rainbow arrive (in the eye) from different drops in the cloud. Those were the principal ideas of the rainbow.

A more thorough study of the ray's traveling through the drop requires knowledge of the laws of reflection and refraction. The law of reflection is easy to »comprehend«, whereas the law of refraction takes knowledge of the velocity of light in different materials. The law of refraction is due to the Dutchman Willibrord Snell (16

21).