Rainbow | The provinces

The following lines are devoted to a simple, although not complete explanation, of the rainbow, a natural phenomenon that makes someone indifferent. The two necessary circumstances to see a rainbow are, with the sun behind us, there are droplets hanging in front of us (rain, a cataract, waves, etc.). If we are lucky that the other conditions arise, we can see the rainbow. Sometimes two are seen. In the lower, primarily called, the colors, of the red violet, go from the inside. Upstairs it seems to be the secondary, less intense, in which the colors are in the opposite direction. Among them there is a dark area called Alejandro’s band. With happiness, under the primary rainbow you can see multicolored arches called supernumeraria. And with great luck you will see the tertiary Rainbow, The Quaternary, etc. René Descartes published the first acceptable theory based on geometric optics. In the past there were quality descriptions of the Greeks and, in some cases, explanation. Let’s see what Descartes has explained.

When the light influences a surface that separates two media, two effects occur: a reflection and a refraction. In the refraction, the light goes to the other side of the surface and the direction changes. When the light is white, you can also experience a dispersion that manifests itself as a series of colors. Draw a drop of spherical water (a circumference) and a radius of parallel light on the ground that influences a point on the surface of the drop that looks in the sun. Part of the light penetrates by changing direction in the drop by changing directions and dissolving colors. The rays, already of colors, reach the inner surface of the drop where they are reflected with different angle, lose intensity and changing order. Those reflected rays leave the drop at the surface that looks at the sun that suffers from a refraction. The separation between the violet and the red is about 2 degrees and the angle between the beam that reaches the drop and the one that leaves is approximately 42 degrees. When this happens in thousands of (millions of) drops, we see the primary rainbow. When the rays that enter the drop -drop in two reflections, when they leave, they form a corner with the beam that arrives at about 50 degrees and with the inverted colors. It is the secondary rainbow, about 8 degrees above the primary. When losing the intensity in every reflection, it also looks weaker than the primary rainbow. With three reflections, the tertiary rainbow would occur, with four the quaternir, etc. Because of their weakness they can only be seen in exceptional circumstances. The theory shows that the rays that only suffer a reflection (primary rainbow) cannot go under a corner of more than 42 degrees and for two reflections, at an angle of less than 50 degrees. That is, between those two corners there is no light from the drop. That explains the band of Alejandro.

Supernumerary arches, the intensity of colors … must take into account the golf and corpuscular character of the light. For another occasion.