What causes a double rainbow?

After experiencing a small rain storm yesterday outside of the apartment window, I anticipated a rainbow to appear. However, I was thrilled to see not only one rainbow, but two. I then began to wonder, why that happens?







With a basic understanding, I know that rainbows only appear when light is present along with rain. And that a rainbow appears when the path of a ray of sunlight enters and is reflected inside the raindrops.

However after some research, I read that not all of the energy of the ray escapes the raindrop after it is reflected once. A part of the ray is reflected again and travels along inside the drop to emerge from the drop. The rainbow we normally see is called the primary rainbow and is produced by one internal reflection; the secondary rainbow arises from two internal reflections and the rays exit the drop at an angle of 50 degrees° rather than the 42°degrees for the red primary bow. Blue light emerges at an even larger angle of 53 degrees°. This effect produces a secondary rainbow that has its colors reversed compared to the primary.

Furthermore, the area of unlit sky between the two bows lies is referred to as "Alexander's band". Light which is reflected by raindrops in this region of the sky cannot reach the observer, though it may contribute to a rainbow seen by another observer elsewhere.

*Lastly, here are some short videos of the storm that happened just before it...



(Lighting Strike at the 1 second mark)



(Lighting Strike at the 23 second mark)



(Lighting Strike at the 8 second mark)