COLOURS (Part 1 of 3)

DEFINITION: 1 the sensation resulting from stimulation of the retina of the eye by light waves of certain lengths 2 the property of reflecting light of a particular wavelength: the distinct colours of the spectrum are red, orange, yellow, green, blue, indigo, and violet, each of these shading into the next; the primary colours of the spectrum are red, green, and blue, the light beams of which variously combined can produce any of the colours 3 any colouring matter; dye; pigment; paint: the primary colours of paints, pigments, etc. are red, yellow, and blue, which, when mixed in various ways, produce the secondary colours (green, orange, purple, etc.): black, white, and grey are often called colours (achromatic colours ), although black is caused by the complete absorption of light rays, white by the reflection of all the rays that produce colour, and grey by an imperfect absorption of all these rays 4 any colour other than black, white, or grey; chromatic colour: colour is distinguished by the qualities of hue (as red, brown, yellow, etc.), lightness (for pigmented surfaces) or brightness (for light itself), and saturation (the degree of intensity of a hue) 5 colour of the face; esp., a healthy rosiness or a blush 6 the colour of a person's skin 7 skin pigmentation of a particular people or racial group, esp. when other than white 8 [pl. ] a coloured badge, ribbon, costume, etc. that identifies the wearer 9 [pl. ] a) a flag or banner of a country, regiment, etc. b) the armed forces of a country, symbolized by the flag [to serve with the colours ] 10 [pl. ] the side that a person is on; position or opinion [stick to your colours ] 11 outward appearance or semblance; plausibility 12 appearance of truth, likelihood, validity, or right; justification [the circumstances gave colour to his contention] 13 general nature; character [the colour of his mind] 14 vivid quality or character, as in a personality, literary work, etc. 15 Art the way of using colour, esp. to gain a total effect 16 Law an apparent or prima-facie right 17 Mining a trace of gold found in panning 18 Music a) timbre, as of a voice or instrument; tone colour b) elaborate ornamentation 19 Particle Physics a unique, hypothetical force or charge on each type of quark that controls how quarks combine to form hadrons: although called red, green, and blue, they are not related to visual colours 20 Photog., TV, etc. reproduction of images in chromatic colours rather than in black, white, and grey

One of the most striking features of the visible world is the abundance of colour The most extensive parts of the Earth and its atmosphere air, soil, and water are usually coloured The sky can be blue or black or grey and even reddish or purplish. Soils can be black or brown or grey and even red. Bodies of water look blue or green. One of the important ways people obtain information about the world is by looking at the colours of things.

When the green leaves of a plant turn brown, it may be a sign that the plant is sick. It can also be a sign of the season of year, since in the autumn the leaves of many trees turn brown.

The colour of a fruit can reveal whether it is ripe. A green banana is unripe, a yellow one is ripe, and a yellow banana with brown and black spots is overripe. A green tomato is unripe, but a red one is ripe. Colour can also indicate the flavour of foods. Brown rice has a different flavour from that of white rice.

What does it mean to say that a tomato is red? Is colour part of the tomato in the same way that shape is? A tomato examined in the dark is still perceived as round but not as being red. It has no colour at all. Moreover, if a bright blue light is shined only on the tomato, it does not look red but black. So colour, unlike shape, depends on light. In fact, it cannot exist apart from light. Yet in a sense the tomato can be described as red.

Somehow, if the right kind of light shines on it, the tomato looks red. The colour of the tomato has something to do with the way light interacts with it.

But colour also has something to do with the persons and animals who see it. For the tomato to be red, viewers able to perceive colour are needed. Many kinds of animals cannot distinguish colours They see only in black, white, and greys A guinea pig looking at a tomato sees only a grey object. Colour exists the tomato is red because something happens in the eyes and the brains of certain persons and animals that enables them to perceive colour

It is possible to study colour from many points of view. Chemists and physicists, for example, have a special interest in colour Sometimes the molecular structure of chemicals or the physical arrangement of their atoms may reveal why they reflect only certain kinds of coloured light. Physicists who study optics a branch of physics have developed theories of colour Biologists and psychologists use many interesting techniques to find out what enables people's eyes and brains to perceive colour

Light from the noon-time sun looks white. But if a ray of white light is aimed at a prism, a broad band of different colours looking like a rainbow emerges. This colour array is called the visible spectrum.

In the 17th century Isaac Newton discovered that a second prism could not add more colour to light that had already passed through a prism. Red stayed red, green stayed green, and so on. But he observed that the second prism could spread the colours of the spectrum farther apart. A narrow red beam entering the second prism would emerge as a wider band of red. Newton also found that if he turned the second prism upside down so that the entire coloured band coming from the first prism entered it, white light would emerge. From these experiments he concluded that white light is a mixture of many different colours and that a prism is somehow able to bend it in such a way that the individual colours separate.

In the late 19th century the theory that light travels in the form of electromagnetic waves won acceptance. Waves are described by their speed, their wavelength, and their frequency. In a given medium, such as air or a vacuum, all light waves travel at the same speed, but they differ in wavelength and frequency. Wavelength and frequency are inversely proportional to each other the longer the wavelength, the lower the frequency.

For the visible light spectrum, scientists commonly specify only the wavelength.

Each colour is associated with a range of wavelengths. The name green or red does not apply to just one colour A wide segment of the spectrum contains colours that are called green. These include blue-green, apple green, and chartreuse, as well as many intermediate greens. Another wide segment contains colours that are called red. Colours of nearly the same wavelength look exactly alike to the human eye.

The colours of the spectrum range, in order, from violet, through blue, green, yellow, and orange, to red. The wavelengths of violet are the shortest, ranging from 380 to about 450 nanometres (A nanometre is one billionth of a meter long.) Wavelengths of red are the longest, ranging from about 630 to 760 nanometres Wavelengths shorter than those of violet are called ultraviolet radiation; wavelengths longer than those of red are infra-red radiation. They produce no sensation of colour in humans. "Black" is the absence of colour

Additive Mixing with Coloured Light

Newton discovered that by mixing two differently coloured rays of light he could produce other colours When he projected light beams from different prisms onto a white background, he found that sometimes the new colour looked like one of the other colours of the spectrum.

Red and yellow, for example, could be mixed to look like the orange of the spectrum. But colours could also be created in this way that did not look like any of the spectral colours Thus red and violet could form purples that did not match any colour in the spectrum.

Newton also observed that as certain coloured lights were combined, a grey or white patch of light was produced. He found that he could often obtain white light by mixing the beams of three different colours

Almost all colours can be matched by three beams of differently coloured light. The greatest number of different colours can be produced when the three colours are chosen from the middle and the two ends of the spectrum. In other words, a combination of one of the reds, a green, and a blue or violet will produce the greatest range of colours For this reason red, green, and deep blue are called the primaries for additive colour mixing, or additive primaries. These three colours are used more than any other combination of colours to mix coloured light beams.

When only two of the additive primaries are mixed in a certain amount, the resulting colour is called the complementary colour, or complement, of the third additive primary.

When red and green light beams are mixed, the resulting colour is yellow, the complementary colour of blue. A mixture of red and blue makes a purplish colour called magenta, the complement of green. And green and blue mixed together form cyan, the complement of red.

When the additive primaries are mixed in other amounts, intermediate colours are formed. This fact is the basis of the science of colorimetry, or colour measurement. Once the three primary colours are agreed upon, most other colours can be defined by the amounts of the three primary colours that, mixed together, match the new colour

Subtractive Colour Mixing

When light strikes an object, it may be transmitted, absorbed, or reflected. A windowpane, for example, transmits almost all the light that strikes it. Since it does not change the light, the pane looks colourless, or clear. A blackboard free of chalk dust, on the other hand, absorbs almost all the light that strikes it and therefore since blackness is the absence of light looks dull and black. A plaster wall both reflects and absorbs light. If the wall is white, it reflects almost all the light that falls on it.

Sometimes a substance absorbs some but not all the colours that reach it. For example, a red tomato absorbs all wavelengths but those of red, which, after bouncing from molecule to molecule within the top layers of the tomato, are redirected outward. When blue light (which does not contain red wavelengths) shines on a tomato, the blue wavelengths are absorbed. The tomato then looks black because no light is reflected from it.

Transparent red objects such as red cellophane, red plastic, or red glass absorb all wavelengths but red ones, which they partly transmit and partly reflect. Such transparent objects are called colour filters because when white light strikes them they filter out all colours except their own, which can pass through them easily.

Colour filters are the basis of subtractive colour mixing, just as coloured beams of light are the basis of additive mixing. Subtractive colour mixing is a complicated procedure because the different dye molecules in two different filters may produce the same colour sensation yet absorb different wavelengths of light. The description of subtractive colour mixing that follows assumes that ideal filters are used.

When a beam of white light strikes a yellow filter, the wavelengths that make up yellow can pass through the filter while all other wavelengths are absorbed. Since yellow is a mixture of green and red light, the wavelengths of those colours pass through, but the wavelengths of blue the complement of yellow are absorbed. Yellow is sometimes called minus-blue, since it can filter out blue light. Similarly, a magenta filter allows wavelengths of red and blue to pass but absorbs wavelengths of its complement, green. For this reason, magenta is sometimes called minus-green.

If a yellow filter (minus-blue) is placed on top of a magenta filter (minus-green) and a beam of white light is passed through them, the yellow filter absorbs blue, the magenta filter absorbs green, and only red light emerges.

A cyan filter (minus-red) absorbs its complement, red. If a yellow, a cyan, and a magenta filter are aligned in front of a beam of white light, all three of the additive primaries are absorbed, and no light emerges. This is called subtractive colour mixing because the filters absorb, or subtract, colour from a beam of light.

Pointillism (or divisionism), impressionistic painting process; the chief exponents were French artists Georges Seurat and Paul Signac.

Paint mixtures usually exhibit the complex behaviour of subtractive mixing. A mixture of yellow and cyan watercolours gives one of several greens, depending on what pigments make up the original cyan and yellow paints. If magenta is then added, black or grey results. However, pigments can be combined in additive mixtures by means of special techniques. A famous method is divisionism, sometimes called pointillism, which was used by some post-impressionist painters. They painted tiny dots of pure spectrum colours next to one another so that light reflected by one dot would combine with light reflected by a second dot in an additive mixture. One of the most famous paintings of this school is Georges Seurat's 'Sunday Afternoon on the Island of the Grande Jatte'.

Colours produced by the subtraction of wavelengths, or filtering, often occur in nature. The reds and oranges of a sunset are caused by the filtering action of the sky. The sky scatters light of short wavelengths, such as blue. At midday, when the sun is overhead, the scattered blue light does not have to travel through very much air to reach a viewer.

The sky looks blue because a great deal of blue light is reflected from it. But at sunset the light must travel through much more air on its way to Earth. The blue is soon scattered, and only the colours of longer wavelengths combined to appear orange and red can be seen.