Why is the sea blue, and why can’t dogs see all the colours that we can?

The Irish Times - Weekend Review - - ENVIRONMENT - Michael Viney

‘Green, how I love you green . . .” Lorca’s first line sounds even bet­ter in what’s left of my school­boy Span­ish (“Verde, que te quiero, verde”). It leads his ro­man­tic poem into sur­real di­rec­tions – green wind, green hair – but his words ring out at this time, when green fills so much of my hill­side world.

Not 40 shades of it but hun­dreds, of every hue and gra­da­tion, one plant to an­other, one tree to the next, one leaf to the next, from vivid light to deep shadow. Some greens are quite thrilling, if that’s al­lowed, es­pe­cially the new, golden-green shoots that May fills with sun­light.

What’s green to peo­ple is chloro­phyll, mol­e­cules that help a plant to use sun­light to make sug­ars for en­ergy out of water and car­bon diox­ide. But green is also a colour we make in our minds, when in­for­ma­tion from our eyes is pro­cessed in a cor­ner of our brains.

A long so­journ with Google has left me won­der­ing if any liv­ing thing sees Earth’s “true” colours at all.

The Nat­u­ral His­tory Mu­seum in Lon­don staged a big ex­hi­bi­tion, Colour and Vi­sion, on this last year. Some of it can still be en­joyed on its web­site, at nhm.ac.uk. It of­fers ways of com­par­ing our vi­sion with how other crea­tures see na­ture.

The vivid, Tech­ni­color sight of hu­mans de­pends on mil­lions of cells in the retina, which lines the back of the eye. Along with rod cells around the edges that help us see in poor light, they in­clude three kinds of colour-sen­si­tive cones that ab­sorb par­tic­u­lar wave­lengths of light – red, green and blue.

Our brain blends them into al­most any colour. (We seem to “see” yel­low just by sub­tract­ing the blue.)

Some wave­lengths in the spec­trum of light are ab­sorbed by leaves, flow­ers and feath­ers, or what­ever, and oth­ers are re­flected and picked up by our eyes. The chloro­phyll of leaves re­flects the green wave­lengths.

When they die back in au­tumn, other pig­ment mol­e­cules show up: carote­nenoids, re­flect­ing yel­lowy-orange, and an­tho­cyanins, re­flect­ing reds and pur­ples: hence the richer ta­pes­try of the fall.

A dog has only two sets of cones, spe­cialised for pick­ing up yel­low and blue-to-ul­tra­vi­o­let light, and misses much of the colour we can see. As for snails, right at the heart of green­ness, their eyes can’t fo­cus or see colour: their world is a mono­chrome blur.

Many other crea­tures, how­ever, see colours when we can’t – at night, for ex­am­ple, or at the ul­tra­vi­o­let wave­lengths of light. In the 1970s it was shown that hov­er­ing kestrels can spot their tiny ro­dent pray by ul­tra­vi­o­let re­flected from their trails of urine.

Ul­tra­vi­o­let (UV) vi­sion had long been known from the in­sect world, guid­ing bees to par­tic­u­lar flow­ers, but­ter­flies and moths to wing pat­terns on their mates, and spi­ders to their prey. Some in­sect vi­sion can be al­most as com­plex as that of hu­mans.

Many, if not most, birds have UV vi­sion that leads them to berries or choice of a mate. Among blue tits, for ex­am­ple, the crest feath­ers of males re­flect UV in the spring, and fe­males will pick the bright­est. Adding UV per­cep­tion to red, green and blue, birds may dis­tin­guish even more colours than we do.

Some of their own colours are the most bril­liant in na­ture, like those of the most gor­geous flow­ers. Both are usu­ally aimed at re­pro­duc­tion – the birds by at­tract­ing mates, the flow­ers by at­tract­ing pol­li­nat­ing in­sects (or birds).

Colour in the ocean, too, has ev­ery­thing to do with light. “Why is the sea blue?” must be one of the most googled ques­tions, and the an­swer – that short-wave­length blues and vi­o­lets are the colours the ocean re­flects – in­tro­duces many peo­ple to the physics of per­cep­tion.

‘‘ A dog has only two sets of cones, for pick­ing up yel­low and blue-to-ul­tra­vi­o­let light, and misses much of the colour we can see

Be­neath the sea’s sur­face, sun­light or its slow thin­ning to the dark is just as vi­tal to vis­i­ble colour, de­pend­ing on whose vi­sion it is. Fish also have cone cells for colour and UV vi­sion, and big­ger eyes, to gather more light, the deeper and darker they live.

Sub­sea, the magazine of Ire­land’s scuba divers, of­ten en­chants me with its pho­tographs of un­der­wa­ter life. But along with the shots of corals, sponges and psychedelic fish from the clear­est and bright­est of trop­i­cal wa­ters come those, nearer home, whose deeper beauty must de­pend on ar­ti­fi­cial light, to feed the dig­its of hu­man vi­sion.

When ocean trawlers be­gan haul­ing up the schools of orange roughy from the shad­ows of the seamounts, the bril­liance of their colour sur­prised me. But that was to ig­nore what hap­pens to colour at the deep­est reach of light.

Blue light strikes fur­thest, and at 100m down red light starts to dis­ap­pear: it has the long­est wave­length and the least en­ergy in the spec­trum. So a red or orange fish, ab­sorb­ing all other wave­lengths of colour, fades into a shad­owy ob­scu­rity.

Michael Viney’s Re­flec­tions on An­other Life, a se­lec­tion of col­umns from the past four decades, is avail­able from irish­times.com/irish­times­books

IL­LUS­TRA­TION: MICHAEL VINEY

Depth per­cep­tion: orange roughy, and sim­i­larly coloured fish, fade into the shad­ows more than 100m down, as red light be­gins to dis­ap­pear.

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