Flowing into Indigo

Celia Berrell

A rainbow of light-waves
can power right through
the top layers of water
to brighten our view
of colourful coral
and cute fishes too
in dazzling hues
of red, yellow, blue.

But these pigments get drowned
as we dive further down.
Yellows turn grey
and reds turn to brown
as part of that rainbow
abandons dive’s quest
‘til only the indigo
blues dive the best.

This Sunlight Zone goes
for two hundred metres.
By then all those colours
are losing their features.
An indigo world
of deep monochrome
then welcomes us to
the Twilight Zone.

The Science

Sunlight can generate rainbows whenever its white light is refracted or spread out into its various light wave frequencies. The electromagnetic energy we identify as red has the longest wave frequency of the rainbow colours (ROYGBIV). Blues and violets have the shortest. (Light waves which are either shorter or longer than these certainly exist, but we can’t actually see them with our own eyes). When sunlight travels down through increasingly deepening water, we can liken the longer red wavelengths to long-legged adults, powerfully striding down into the water. In contrast, the blue-violet wavelengths are more like little kids, happily running round, bumping into molecules. They’re taking lots of tiny steps as though they have energy to spare. Red wavelengths are the first to falter. When every stride needs to be enormous, they quickly run out of impetus.

Pigments and surfaces we identify as red are absorbing the other wavelengths of visible light and predominantly reflect only the red ones. As red frequency light waves diminish, this colour begins to dull, turning brown and eventually appearing black, since all other colour frequencies were already being absorbed. A bright yellow fish could still be reflecting a proportion of white light, along with specific yellow frequency light waves. Still, as the number of yellow-frequency light waves become less, and the intensity of light in general fades, that yellow colour dulls and begins to appear grey. It too would eventually darken to black, far enough down the water column. Even in bright sunshine and clear waters, we don’t expect to detect natural colours from sunlight past 200 metre depths. There is either a faint glimpse of light from those remaining short-frequency blue-violet light waves (also known as indigo) or complete darkness. We refer to this monochromatic region as the Twilight Zone.

The Poet

Celia Berrell’s Science Rhymes regularly feature in CSIRO’s Double Helix magazine & Australian Children’s Poetry. They have been published in school textbooks around the world. The Science Rhymes Book (Jabiru 2018) has 70 scientifically accurate poems relevant to the primary science curriculum. The Science Rhymes website shares student poems, promoting Australia’s National Science Week each August. Poems in anthologies include Penguin DK's A World Full of Poems for ‘Peace by Piece’ and The Emma Press's The Bee is not Afraid of Me for ‘True Bugs are Suckers’.

Next poem: Flying kaleidoscope by Vaishnavi Sridhar