How Many Rainbows?

Another advantage to living in the Far North is the high frequency of seeing rainbows, a consequence of the low sun angle. Rainbows are caused by internal reflection and refraction of sunlight or moonlight inside raindrops.

Light entering a raindrop is bent by an amount that depends upon the wavelength (color) of the light. Each raindrop acts as a tiny spherical prism. A rainbow is seen whenever there are enough raindrops distributed properly with respect to the viewer and the sun and there is not too much absorption of the light in the rain. But rainbows are seen only in specific directions relative to the direction of the sun, directions that are determined by the number of reflections within individual raindrops.

A single reflection in the raindrop produces the brightest rainbow seen. This primary bow has a red outer boundary which is located at an angle of 42 degrees away from a line from the sun to the viewer. To see the primary bow the viewer must face away from the sun.

A secondary bow with colors reversed so that the red edge is toward the inner border is frequently seen about 8 degrees outside the primary bow. It is weaker than the primary bow because the light undergoes two reflections, and consequently more absorption, inside the raindrops.

Three reflections inside the drops cast the light back toward the sun to create a tertiary rainbow only 52 degrees away from the sun. This bow is rarely seen because it is weaker than the primary and secondary bows, and the brightness of the sky this close to the sun overpowers the rainbow light. Furthermore, a person watching the primary and secondary bows would have the tertiary bow at his back.

An extra partial copy of the primary bow is sometimes seen parallel to and just inside the primary bow. Similarly, a partial copy of the secondary bow may be seen outside it. These usually narrow bands of red or red and green are called supernumerary rainbows. They are caused by light leaving the raindrops along different paths in such a way that the colored rays destructively and constructively interfere with each other so that colors are seen to be bright in certain places and absent in others. Hence, the supernumerary bows are "interference fringes" of the primary and secondary rainbows.