Science ABC: how rainbows form

Photo by wikipedia

Rainbows are probably the most spectacular manifestations of light; they’ve fascinated people and stimulated the imagination of millions, and continue to do so even ’til this day. There are all sorts of legends that suround them, from the treasure at the end of the rainbow to the house of gods, for which the rainbow is a bridge too. Still, unfortunately not so many people know the real facts about rainbows.

Rainbows are optical and meteorological phenomena that cause a spectrum of light to appear in the sky (mostly) when the Sun shines upon drops of moisture in our planet’s atmosphere. These rays take the form of a multicoloured bow, with 7 colours: red, orange, yellow, green, blue, indigo and violet (there are some interesting mnemonics to remember those, such as “Roy G. Biv” and “Richard Of York Gave Battle In Vain”). The thing is that these colours are not separated, as is traditionally believed. Actually the rainbow spans a continuous spectrum of colours.

The rainbow is formed by dispersion of sunlight as it goes through raindrops. The light is first refracted as it enters the surface of the raindrop, reflected off the back of the drop, and again refracted as it leaves the drop. The angle is independent of the size of the drop, but does depend on its refractive index. This is visible to the naked eye by a misalignment of these bows.

So actually, a rainbow doesn’t exist at a particular location in the sky. It’s just an optical illusion. The position of a rainbow in the sky is always in the opposite direction of the Sun with respect to the observer, and the interior is always slightly brighter than the exterior. These effects are due to the fact that despite the fact that all raindrops reflect sunlight the same, only some of them can be seen by the viewer’s eye. There are many things that could be still added, but these are the really important things. Also, did you know that there’s such a thing as a moonbow? As the name states, it’s created by the moon instead of the sun. It is difficult to discern colours in a moonbow because the light is usually too faint to excite the cone colour receptors in our eyes. As a result, they often appear to be white. Catch a glimpse below:

Photo by wikipedia