What is the true shape of a rainbow?

Following a passing rain shower, it's not uncommon to see a rainbow.

Maida Follini emailed me saying once on an airplane crossing the Atlantic to England there was a perfectly circular rainbow.

Maida made note about how, on ground, rainbows are typically half-circle arcs

and asked: “What causes the shape of these different rainbows?”

The truth is a full rainbow is circular, not arc shaped.

You may now be asking: why do we only see an arc if a full rainbow is circular? Let's first look at the science

of a rainbow. Light from the sun enters water droplets. The light entering the droplet slows down, bending it and changing its direction. This is known as refraction. The colours in the visible light spectrum are separated and reflected out of the droplet, making up the colours we see in a rainbow.

As to why we only see an arc if a full rainbow is circular, it's because we can only see the part visible above the horizon.

If you were like Maida and flying in an airplane, you may just see a full circular rainbow.

Sadly, since this means there is no end to a rainbow, you might not want to hold your breath for a pot of gold.