Rainbow

Image credit – Sarang – public domain

Cloud of the Day – Rainbow

Photo credit – Wing-chi Poon – CC-BY-SA

There are many optical meteorological phenomena, and we’ve discussed a few, including aurora, halo and corona. Today’s example is the rainbow. Rainbow is a good name. It’s bow-shaped and it forms in rain. To be accurate, the part we usually see is bow-shaped. If the ground wasn’t in the way, the rainbow would be circular. In fact, many people have seen circular rainbows when they were flying. On the ground though, we only get the bow.

Photo credit – Brocken Inaglory – CC-BY-SA

You see a rainbow when the Sun is directly behind you and you’re looking toward falling rain. The Sun’s rays enter the raindrops, are refracted, spreading out into the colors, then are reflected off the back of the drops to your eyes. The maximum brightness of the reflected light is at a 42 degree angle, which explains the dimensions of the rainbow. You only see the colors from the droplets that are at that angle. A person standing next to you will see their rainbow from different raindrops.

Photo credit – Leonardo Weiss – CC-BY

Double rainbows form when the Sun’s rays are reflected twice on the back of the drop. The bow from the second reflection has its colors reversed, with the red on the inside rather than the outside. The area between double bows appears darker because it’s between two bands of maximum brightness.

Photo credit – Eric Rolph – CC-BY-SA

Supernumerary rainbows sometimes form on the inner edge of the primary rainbow. They aren’t caused by refraction and reflection only, but by subsequent interference, an effect we saw in corona and irisation. Click on the photo above to really see it.

Photo credit – Tomasz Sienicki – CC-BY

Rainbows don’t form only in rain. They’re also seen in waterfalls, fountains and sprinklers. We still call them rainbows, though. There are also many flags that use the rainbow.