Wandering through the realms of the cosmos, pondering its huge vastness

Supernumerary Rainbow over Cebu City, Philippines

Last February 5, I and some friends flew to Cebu City, the ‘Queen City of the South’ to tour around and explore some of its famous landmarks. As we were walking towards the harbor during the late afternoon, we felt a slight drizzle rain down our heads.

One of us thought that these conditions were perfect to see a rainbow. When white light from the Sun hits the raindrops at a certain angle, rainbow formation is possible. The angle is important as it effect the direct the light travels after it hits the raindrops and that determines whether or not we will see a rainbow. It is best if the sun is fairly low in the sky such as dawn or late afternoon just like during that time. To view a rainbow, your back must be to the sun as you look at an approximately 40 degree angle above the ground into a region of the atmosphere with suspended droplets of water. By knowing this, one of us found the brightly colored arc which formed above us. At first it was faint, until it become more and more visible. A secondary rainbow has also formed –  seen above and outside the primary arc, and has the order of its colors reversed (red faces inward toward the other rainbow, in both rainbows).

It was really a splendid sight!😀

As I looked more closely, I noticed something unusual yet strangely familiar phenomenon. On the inner (blue) side of the primary bow, there were also several slightly detached and pastel colored bands that do not fit the usual pattern. It was then that I realized that we were actually looking at a supernumerary rainbow!

Notice the closely spaced greenish purple arcs on the inner (blue) side of the primary bow. (see inset picture)

A supernumerary rainbow—also known as a stacker rainbow—is an infrequent phenomenon, consisting of several faint rainbows on the inner side of the primary rainbow, and very rarely also outside the secondary rainbow.

The primary rainbow results from a single internal reflection of refracted light inside a raindrop, and the secondary rainbow results from a double internal reflection. But the additional rainbows are not explainable by geometric optics, and hence had been termed “supernumerary”.

Supernumerary rainbows result from interference of light which undergoes single internal reflection but travels along different paths inside a raindrop.

Seeing this rainbow is a gift from above.  It seemed like it has served as a lucky sign that we would have a nice weather throughout our whole trip; and indeed it happened.😀

“I have set my rainbow in the clouds, and it will be the sign of a covenant between me and the earth.” (Genesis 9:13,15)

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