Formation of a Rainbow and a Double Rainbow
Rainbows after a good rain are really the stuff of magic aren’t they? Something that all children truly believe is magic during their childhood and the awe they view it in truly never dissipates even if they have fully grown up. Now, what if they saw two rainbows in one go? Well that’s twice the delight! The magical beauty of this phenomenon remains even if the science behind it is known.
Now, are these two rainbows real or the stuff of wild imaginations? Well, they aren’t just wild imaginations! They are called double rainbows and the second rainbow is just as real as the first one! Now, this brings to question, how do two rainbows occur simultaneously? In order to answer this question we first need to understand how a single rainbow comes about!
Formation of Primary/Single Rainbow
Rainbows are formed when sunlight is refracted, reflected and refracted again off of water droplets. This happens because water is denser than air and when sunlight that is passing through air moves to a water droplet, it’s speed and angle tends to change due to variation of density between the two mediums. The process that induces the change in angle and direction in light when it moves from one medium to another is termed as Refraction and this phenomenon acts as the crux for rainbow formation.
In order for rainbows to be viewed, the sun has to be behind the viewer and rain or some other source of water droplets should be in front of the viewer (could even be drops from fountains if one looks carefully enough). The light also has to hit the water drop at a 40 degree — 42 degree angle, as lighting hitting water droplets at angles lower or higher will just pass through or be reflected back out of the raindrop without refraction taking place.
Water droplets are nearly spherical despite what most images suggest, and when light passes through such droplets, it splits according to its various wavelengths and moves accordingly. These various wavelengths correspond to different colors in the visible spectrum and it is known that sunlight or even white light is actually composed of many colors having different wavelengths.
Once the light is refracted into the droplet, it is internally reflected by the droplet’s inner surface towards the outside. Here, since refraction has already taken place and sunlight has split into different colors each having their own wavelength, colors with shorter wavelengths like blue and violet experience an increased change in direction when compared to colors with longer wavelengths like red or orange.
The internally reflected light now exits the water drop and due to the change in medium from water to air undergoing refraction again. This refracted sunlight from behind the viewer is observed by them as a rainbow. Now that we know how Normal Rainbows are formed, let’s take a peek and see how Double Rainbow formations take place!
Formation of Secondary/Double Rainbow
Anyone who has observed a double rainbow will have noticed that the second rainbow is always lighter in it’s hue when compared to the primary rainbow, is viewed on top of the primary rainbow and that it’s colors appear reverted when compared to the primary rainbow. This means that for the second rainbow, the top most color is violet (in VIBGYOR order) and not red (ROYGBIV). The secondary rainbow is above the primary rainbow because the droplets that refract and reflect the sunlight are nearly 9–10 degrees above the primary bow, meaning the light hits these droplets at an angle of 52–54 degrees.
The formation of double rainbows takes place when the sunlight is reflected twice off of a water droplet. Meaning after the initial internal reflection of the refracted sunlight takes place, rather than moving out, it hits the inner surface of the water droplet again and is then casted out through refraction. Due to the second reflection taking place, the colors are now inverted with red appearing at the bottom while violet appears on top.
The secondary rainbow is much more hazy and lighter in comparison to the primary rainbow owing to the fact that with every time the light is reflected, some of it escapes there by losing the intensity of the rainbow. The second rainbow also appears to be much wider than the primary rainbow.
It can also be noticed that the sky viewed between these two rainbows appears darker. This darker region between the two rainbows is termed as Alexander’s Band after Alexander of Aphrodisias who first described it in 200 AD. This band forms because the deviating angles of the primary and secondary rainbow causes the water droplets to refract the sunlight at angles that cannot be viewed by the observer.
Now that you know how single rainbows and double rainbows are formed, go check them out with a new found understanding and pass on this interesting knowledge to others too!
