The Tribune, Chandigarh, India

Fascinating world of colours
D.S. Chhabra

WE all know that colour photographs or movies are more enjoyable as compared to their black and white counterparts since they present to us a realistic and lifelike picture. The vivid colours of the peacock and butterfly wings, rainbow, neon signboard etc are very fascinating to watch. Colours play a very important rule in our daily life — be it food, clothes, living room etc. Our life would be dull without colours. It is, therefore, natural that we have a curiosity to know as to how the colours arise and are seen by us.

The visible light which our eyes can detect is only a small portion of the natural radiations emitted by the sun. The extent of visible light is defined in terms of the wavelength or frequency of the light waves involved. The colours is the physiological perception of the eye-brain combination to light waves of different wavelengths falling upon the light sensitive retina — the inner layer of our eyeball. The shortest wavelength of light that we can perceive corresponds to the colour violet, with a wavelength near to 400nm (1nm= 10-7 cm). The longest wavelength of light perceived corresponds to the colour red with a wavelength close to 700nm. Between these two limits colours of the light occur in the sequence from violet to indigo, blue, green, yellow, orange and finally to red (VIBGYOR). The wavelengths of light corresponding to various colours are given in the table below.

In general, colours arise by the various optical phenomenon like refraction, reflection, interference, diffraction, scattering, absorption and polarisation.

Rainbows provide a familiar example which are formed when sunlight coming from behind the observer, enters a raindrop and is totally internally reflected once and then refracted to reach the eye. The white light is spread out into a spectrum of colour due to the lesser refractive index of water for red as compared to violet wavelength of light. Water drop will cause red light to deviate least and violet to deviate most with intermediate wavelengths falling between these limits. Newton was the first scientist to show that light can be dispersed into its various colour components by means of a glass prism.

Structural colours are widely distributed throughout the biological world, notably among animals with such non-living investitures as feathers, scales, or insect cuticle. The scales (the fine dust that covers the wings) on the butterflies and moths are arrays of repeating structures that interact with light to produce the colour — and interference phenomenon. These can be called as biological thin film interference colour filters.

The colours observed from soap bubbles and from a film of oil on a rain drenched road are also due to interference effect. The light waves reflected from both sides of the film interfere to produce constructive and destructive interference. The colour seen are a result of this process summed over all the visible wavelengths. The magenta, blue and greenish colours reflected rom the camera/binocular lenses and ophthalmic lenses are due to optical interference phenomenon occurring in the anti-reflection coating — a layer of a substance with a refractive index lower than the glass deposited onto the lens surface to increase transmission and thereby enhance contrast through multilens system and also to improve the scratch resistance of glass surface. The colour on these lenses varies when seen from different angles. This variation of the colour is due to the increase in the path length of the light refracting and reflecting through the layer at an increasing angle.

We all know that the light is generated by heating a substance. The heating of the substance results in supplying energy to its atoms. This causes shifting of the position of the electrons spinning in their orbits around the nucleus resulting in the change of energy of the atom. Depending upon the atom’s structure, the shifting of the electrons (either from higher to lower or lower to higher orbit) and the energy requirement for electrons in their new locations, excess energy is released in the form of light. The colours (band of wavelengths) of the generated light depends upon the amount of energy released. As the temperature of the substance is increased, the colour of the light changes from red to orange, yellow and blue.

Colour vision

The retina where the picture of the object is formed by the eyelense is a multi-layered (about 10 layers) structure of cells. It is a thin sheath and can be compared with a film of the camera on which the image is formed. There are two types of cells in the retinal photo-receptor layer known as “Rods” and “Cones”. Colour vision in humans is based on the presence of three types of cones; red, green and blue. The blue cones are rather few in numbers and absorb light primarily of short wavelength. The green and red cones absorb medium and longer wavelength of light. Therefore, the colour of any object perceived by us is determined only by the amount of effective absorption of light by three types of cones. These cones in the second layer are connected to the ganglion cells in the eighth and ninth layers through bipolar cells in the sixth and seventh layers of the retina as shown in the figure. Each ganglion cell is linked with one nerve-fibre of the optic nerve which in turn is connected to the brain.

The cones operate under high radiant flux than rods. That is why cones are active during daytime and rods during night or when the illumination conditions are poor. In other words we can say that human being may be assumed to have two retinas — a day retina consisting of only cones and a night retina having rods only. Since the cones have low response during night, the colour vision becomes poor under low light level.

It appears that an average person can distinguish a large number of different colour hues. The majority of colour hues can be produced by mixing just three primary colours, red, green and blue.

Colour vision deficiency

Certain people who confuse colours, which a normal person would distinguish easily, are said to have anomalous colour vision due to deficiency of cones. Deficiencies of red or green photo receptor cones can be complete or only partial. Complete deficiency of either red or green cone results in colour vision based on only two remaining cone types, and is thus known as dichromatic colour vision abnormalities. Incomplete deficiencies of red and green cones are referred to as anomalous trichromatic colour vision. Extremely poor colour vision is associated with the absence of all three cones — a rare deficiency known as monochromatic vision.

The most common cases are abnormalities of the red cone which cause confusion of greys with reds; and abnormalities of the green cone, which cause confusion of greys with purples. These typical red-green colour vision abnormalities are commonly congenital and incurable. It is believed that colour vision deficiencies are relatively common in men (8%) and they inherit this defect from their parents whereas the inheritance is rare in women.

The visual system appears to combine the output of the red and green cones into an internal yellow signal, which is then compared with the response of the blue cones. Abnormalities in this blue/yellow axis occur only rarely on an inherited basis, but are more frequently encountered as aquired disorders in patients with cataract, glaucoma or optic nerve diseases.

Diagnosis

Most patients with colour vision deficiency are readily able to correctly identify the colours of everyday objects. This is because very few individuals are truly colour blind who have no colour discrimination. Most people learn soon enough that the grass is green and the sky is blue. Effective tests are available which provide the patients the opportunity to confuse either very similar colours (tests of colour discrimination), or to confuse quite dissimilar colours.

Nagel anomaloscope is an instrument which measures trichromatic deficiency. The person having trichromatism looks into the instrument where he sees a light patch divided into two halves — one of which is illuminated by a yellow radiation while the other by red and green radiation. There are two graduated knobs, one of which regulates the intensity of yellow colour and the other, the ratio of red to green intensities. The colour match of the two halves made by the person (with trichromatic deficiency) by manoeuvring the knobs enables him to detect the amount of trichromatic deficiency in terms of too much of red or too much of green colour.

The writer is Ex-Senior Scientist and Head, Thin film coating lab, Central Scientific Instruments Organisation, Chandigarh.

NEW PRODUCTS & DISCOVERIES
Astronomy next big thing

THE OWL (Overwhelmingly Large Telescope) is an awesome project which requires international effort to make it happen. It would open an enormous new window on the universe

This huge telescope — its main mirror would be more than 100 metres across - would have a predicted resolution 40 times better than the Hubble Space Telescope and a sensitivity several thousand times greater.

It would be sited at an altitude of 5,000 metres and would be operated almost as a space observatory, with a base camp for the human operators nearby at a lower height of no more than 3,000 metres.

The European Southern Observatory (ESO)organisation, the prime mover behind the project, is currently negotiating rights to a site with Chile, the would-be host country. Eso members have been discussing the OWL project at their Council Meeting in London.

The preferred site would be in the Atacama Desert, which is said to be the driest in the world, making it a perfect location for astronomy. Already, telescopes are located there from the US, Europe and the ESO’s newest member, the UK.

OWL is currently in the design phase, with the cost and timescale still to be fixed. But the aim is to take advantage of the latest developments in telescope technology to make the next giant leap forward in observing.

The mirror, much like the US 10-metre Keck telescopes in Hawaii, would be made of 1,500 hexagonal segments and would use some of the clever computer techniques — active and adaptive optics — that further improve resolution.

Those involved believe the Owl could revolutionise ground-based astronomy.

Roberto Gilmozzi, director of Eso’s Paranal Observatory, the site of the Very Large Telescope (VLT), in Chile, says: “At first sight it seems half-crazy. This is a telescope with a primary mirror the size of a football field.

“But it is something within reach of current technology and it has freed astronomers from the old straitjacket which is the belief that every new generation of telescopes will be twice as big as the previous one.

“Now, we can go beyond this. Owl would have the same resolution as the VLT, but in the VLT Interferometer the maximum light you can see is the sum of the light coming from the four telescopes.

“With a telescope that has 10 times the collecting area of every telescope ever built, you would be able to go down several thousand times fainter than the faintest thing you see today with those telescopes.” BBC

UNDERSTANDING THE UNIVERSE
WITH PROF YASH PAL

I am not a scientist. My grandson asked me why the full moon looks bigger at moonrise than when it is up in the sky. I explained it by saying that the thick layer of the atmosphere at the time of moonrise acts as a lens. The child was satisfied when I showed him that the lines on its hands looked thicker when seen through a lens. Was my explanation correct?

I am sorry to tell you that your explanation was not correct. One can measure the size of the moon by using a simple survey instrument like a theodolite or, more simply by using a transparent scale at arm’s length - you might have to use torchlight to read the scale. You will find that there is no difference in the size of the full moon while it goes from moonrise to a high position in the sky! The difference we all observe is entirely psychological! The reason for this has been discussed extensively. Two of the explanations given are the following:

1) When we look at the moon near the horizon we also happen to see trees, people and buildings at the same time. Many of these are far away and look very small. But our brain knows that in actual fact the are much bigger. Therefore it automatically makes a correction in the observed size of the moon and makes us perceive it as bigger. I am not entirely happy with this explanation. Why only so much bigger?

Incidentally, the same observation is made in respect of the rising or setting sun and the very same explanation is given. Some day I would like to see the sun high up with a number of airplanes in the same field of view. I wonder if the sun would start looking bigger. If some of you have chance share this observations with me.

2) There is yet another explanation has been offered. We all have sensation that our sky is like dome. One could argue about the way this feeling came about but there is no question that we have that impression. In a dome the ceiling is never as far as its diameter. That is our experience of all the domes we have ever built. Therefore our brain place the moon or the sun near the horizon at a greater distance than when it is high up in the sky. It makes a correction for this difference and makes the moon or the sun look bigger near the horizon.

I have given you the explanations I am aware of. One thing one can definitely say - that there is no physical difference in size. It is all psychological. There is a long way to go before our brain really understands itself.