Life sciences of how we see so many colours

Dr Irengbam Mohendra Singh
Roses are red, violets are blue./ Sugar’s sweet, so are you… It’s an old nursery rhyme that has been quoted for many love songs, such as Roses are red, Violets [low growing plants usually with purplish-blue flowers] are blue! Love never crossed my mind until the day I met you. Roses are not always red and violets are not always blue. It’s only how the viewer sees it. The one I remember from my B Sc botany, is called Viola tricolour (violet, white and yellow) or wild pansy or Johnny Jump Up.
We live in a colourful world. Women always look in colourful dresses. Rajasthani women in India dress in bright colourful costumes and men in multicoloured turbans. Animals use colour to attract opposite numbers. But how do we and animals see these colours?
Aristotle hypothesised the first known theory of colour. He believed colours were sent by God from heaven through celestial rays of light, and that all colours came from white and black (lightness and darkness). He related them to the four elements: water, air, earth, and fire. It sounds silly now, but his beliefs on colour were held widely for over 2,000 years, until Newton replaced them by his visible spectrum of seven colours: red, orange, yellow, green, blue, indigo and violet (ROY-G-BIV or VIBGYOR) that we see in rainbows. It’s known by the popular mnemonic, The Richard Of York Gave battle In Vain.
17th century Newton wrote in his Optiks: “…if the sun’s light consisted of but one sort of rays, there would be but one colour in the world…” 18th century Goethe challenged Newton’s views on colour. He argued that colour was not simply a scientific measurement, but a subjective experience perceived differently by each viewer. His contribution was the first systematic study on the physiological effects of colour. His views were widely accepted by artists.
Newton’s division of seven colours of the light (electromagnetic) spectrum was criticised by a few scientists in later years. He chose the colours, believing in ancient Greek sophists’ (professional philosophers) belief that there was a connection between the colours, the musical notes, the known objects in the solar system, and the days of the week. Isaac Asimov had suggested that indigo should not be regarded as a colour in its own right, as some people cannot distinguish it from blue and violet.
Images of modern visible light spectrum shows that ‘indigo’ corresponds to what is today called ‘blue’, whereas ‘blue’ corresponds to ‘cyan’. Cyan colour is new to me. Browsing the net, it says: cyan is located midway between red and blue. It’s a greenish-blue colour, one of the three primary pigments along with yellow and magenta. It’s one of the four colours of ink used in colour printing and by the inkjet printer.
White light or simply light, is a combination of all colours of the rainbow in the portion of the electromagnetic spectrum that is visible to human eyes. There are other colours that are not visible to us but visible to other animals. For example, a rattle snake sees all colours that we see, as well as infra red. That is, it can see heat.
Human beings and other mammalians contain only three kinds of colour-sensing cells called cones, in the inner surfaces of our eyes called retina. In fact, human retina is “inside-out” or “front to back”. It has a forest of connecting wires in front while the photosensor cons and rods with their sensor ends facing backwards. It was an evolutionary mistake, unless it was created by God, as creationists believe. If so, it’s badly designed.
Each photoreceptor cone cell can sense only one colour ie red, green or blue. These three types of cones have different pigments namely, S-cones, M-cones and L-cones. They are sensitive to different wavelengths of light that correspond to shortwave, mediumwave and longwave light. Cones are less sensitive than rods, which work better in dim light.
There are 6-7 million cones in a human eye. These cones though present sparingly, in the rest of the retina, are densely packed in a small, central rod-free pit (fovea, 0.3 mm) located in the centre of macula (5.5 mm). Destruction of the cone cells from disease would result in colour blindness. Colour-blindness caused by abnormal photopigments located in cone cells can be inherited. The most common type is called red and green colour blindness. They have difficulty in distinguishing between reds, greens, browns and oranges.
But how can we see so many colours? It’s because a range of wavelengths of light stimulates each of these three receptor cones to varying degrees. The cone are not specific. They respond rather broadly to the light that comes into our eyes, so that red light eg, will also stimulate the green cones, and to a lesser extent the blue cones. Some blue light will stimulate the green cones, and to some extent the red cones, while some green light will stimulate the blue and red cones.
As a result, red, green and blue can be combined in different proportions to make all other colours. That’s how we see colours of light that lie between the colours of the cones. They are like mixing of paints to give different colours. Different wavelengths of light are perceived as different colours.
Physics, rather particle physics, explains how we see so many colours. There’s is a vast array of pigments in nature, like the three primary pigments in the cones of our eyes. Pigments are colourful chemical molecules that absorb selective wavelengths of light and reflect the rest. The Earth is inundated with endless showers of photons (smallest bundles of light) that come all the way from the sun for 150 million km. When a photon hits eg a red rose, it hits an electron around the atom or molecule. If the structure of the molecule is the right one, the photon will transfer all its energy to that electron. If it isn’t, the photon will not be absorbed.
In some cases, the animals and plants produce the pigments themselves, but in many cases they are absorbed into the organism through its diet. I knew the flamingos are pink-coloured for the first time in school, from the poem, The Slave’s Dream by the American poet Longfellow. In the dream, the slave recalls his regal past and describes what a flock of flamingos looks like. The flamingos absorb beta-carotene from the blue-green algae in their diet (among other things) giving them pink colour. In the zoo, to keep their colour, they are fed on a commercially prepared diet, high in carotenoids. Otherwise they will become white.
Pigments perform a large variety of functions in living things. Some, like melanin in the skin evolved to absorb light for protection. Chlorophyll in plants helped humans and other animals to exist on Earth, by absorbing photons and integrating them into a particular molecular machine, photosynthesis and giving us oxygen. Some pigments simply make organisms beautiful like colourful flowers that attract insects, or warn off a predator. The beautiful plumes of male peacocks help to attract their mates.
Because different pigments absorb only certain wavelengths of photons, plants for example, are green because green photons do not interact with molecules in the leaf and are deflected. As each pigment reacts only with a narrow range of the spectrum, there is a need for several kinds of pigments. That’s how we live in a colourful world.
Pigments however, are not always responsible for all the colours we see in the universe. Scientists have figured out why the sky on a clear day, looks blue. It’s not because the sky contains blue pigments, but the blue light in the spectrum, has shorter wavelength and is thus easily scattered in all directions by the tiny molecules of air in Earth’s atmosphere. Out of interest, no one could describe the colour “blue” until modern times (1880s). Homer is reputed for describing ‘blue’ as “wine-dark” sea in his “The Odyssey”, which the British prime minister William Gladstone first noticed.
Nor is the colour of an eye blue because of blue pigments. Eye colour is the result of variations in the amount if melanin pigment found in the iris. The lack of this pigment results in blue eyes. A certain quantity of this pigment gives green, and lots of this pigment gives brown eyes.
The sun looks red, yellow or orange at sunrise and sunset? It’s because the sun is at the horizon and its photons have to travel far through the atmosphere, where colours with long wavelengths: red, orange, and yellow sail through easily, while those of short wavelengths like blue and violet are scattered away. The real colour of the sun is white, as seen from space. All of us do not perceive these colours in the same way. A kindergarten child in America will paint the sun as yellow, while in Japan as red.
Food for thought. What puzzles me. We see colours because they are reflected. An object is black as no light is reflected eg, the black hole. How then, do we see black as no light will enter our eyes. It may be something like, we see black when our eyes are closed. Perhaps, that’s how our brain interprets.

The writer is based in the UK

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