Archive for May 2012

COLOURS (Part 1 of 3)   Leave a comment

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.

Posted 2012/05/20 by Stelios in Education

Tagged with ,

COLOURS (Part 2 of 3)   Leave a comment

Colour Classification Systems

People who make, sell, or use nail polish, lipstick, paint, ink, and many other products deal with very small variations in colour Colour classification systems have been developed that enable them to specify and obtain the precise colours they want. Some of these systems show how colours differ in ordinary daylight. Others calculate the wavelengths of light that pass through filters of different colours when a special light source, such as a tungsten lamp, is used.

The Munsell system arranges colour samples according to three qualities hue, value, and chroma. Hue is what is usually meant by the word colour Red, blue, green, and yellow are hues. The Munsell system divides all hues into ten categories: yellows, green-yellows, greens, blue-greens, blues, purple-blues, purples, red-purples, reds, and yellow-reds. Hues are often arranged in a circle. Value is the Munsell term for the lightness of a coloured sample. A yellow material may be light while a blue material may be dark. A series of greys, from black to white, best define value. Chroma defines the amount of hue in a given sample. The word chroma is related to the word chromatic and describes colours ranging from grey to vivid hues. A brick and a ripe tomato, for example, may have the same red hue and the same value. Their difference in colour is a difference in chroma. Thus colours can vary in hue, value, and chroma.

In addition to the Munsell system, there are many colour classification systems that relate to colour perception. Two examples are the Optical Society of America Uniform Colour Scales (OSA-UCS) and the Swedish Natural Colour System (NCS). The NCS system arranges colours based on the perceptions of white, black, red, green, yellow, and blue, with only four perceptions for a given colour A purple, for example, may consist of white, black, red, and blue. With experience, one can assign percentages to each perception.

There are also classification systems based on colourant mixtures. These systems are very useful to help visualize how colours mix together. Examples include the Ostwald System and the Pantone Matching System.

Finally, numerical systems have been developed based on the knowledge of the eye’s physiology and extensive experimental studies. These numerical systems have been standardized by the Commission Internationale de l’Eclairage (International Commission on Illumination), or CIE. A colour is specified by three numbers relating to the eye’s three colour receptors. To specify a colour, one first performs physical measurements on how much a coloured material will reflect or transmit light across the visible spectrum. Computations follow using this wavelength information to arrive at the three numbers. This system is used to specify the colour of most man-made products and forms the basis for standardizing present and future colour television signals, including high-definition television (HDTV).

Colour Chemistry

Subtractive mixing is based on the way matter affects light. Somehow a beam of white light is changed when it meets certain kinds of matter. Some of the light stays in the matter is absorbed. As a result, the light that emerges is reflected or transmitted has a different colour This happens in part because of the way matter is constructed.

All matter consists of atoms. Each atom contains a dense, heavy centre called a nucleus and one or more electrons that are in continuous motion around the nucleus. According to atomic theory, distinct quantities of energy are available to each of these electrons. An electron can have the quantity of energy dictated by one or another of the atom’s energy levels, but it cannot have an intermediate quantity.

Transition element, any of various chemical elements that have valence electrons in two shells instead of one.

Sometimes an atom has two electronic energy levels whose difference is equal to the amount of energy possessed by a light quantum associated with a certain wavelength. This is a characteristic of a series of chemical elements called the transition elements. In chemical combination the atoms of these elements can absorb visible light. The light energy boosts the electrons into higher energy levels. The electrons then dissipate this energy in the form of heat and return to their normal energy levels. The compounds of the transition element cobalt, for example, are known for the brilliant blue that is left after they absorb and dissipate red light.

Many kinds of molecules, or combinations of atoms that form a chemical substance, have electronic energy levels that lie close together, a situation similar to that of the transition elements. The molecules that can absorb and dissipate visible light usually contain many double bonds. These light-sensitive coloured molecules make up a very important group of chemicals. The green pigment chlorophyll, found in the leaves of plants, absorbs light energy that is then converted to food energy. Four types of similar light-sensitive molecules are involved in human vision, each sensitive to a different range of wavelengths.

Dyes are another group of chemicals that often contain many double bonds. The exact structure of a given dye determines the energy levels available to the electrons and, therefore, the wavelengths that the dye will be able to absorb. For example, when the molecules of a substance can be linked chemically with a textile fibre, the substance can be used as a dye.

Colour Perception

In a psychological sense colour can exist without light. People in a completely dark room can “see” colour by shutting their eyes tightly. When they do this, coloured spots called phosphenes seem to appear in front of their eyes. Phosphenes have also been produced by direct stimulation of the brain and by stimulation of the eye with pressure or electricity.

Persistence of vision in another example of colour perception in the absence of a physical stimulus. When people watch a motion picture, they are actually observing a series of rapidly projected still pictures. During the very short interval between pictures, a person retains an image of the preceding picture. This image blends into that of the following picture, giving an impression of continuous motion. The retained image is called a positive after-image Similarly, if people look at a patch of one colour for about 30 seconds and then look at a blank sheet of white or grey paper, they will probably see a patch of colour that is the complement of the original colour This is called a negative after-image

When light reflected from an object enters a human eye, it passes through the cornea, the pupil, and the lens and lands on the retina. The retina contains two kinds of light-detecting cells. These cells are called rods and cones. The cones are colour sensors. The rods make night vision possible.

Young, Thomas (1773-1829), English physicist and physician, born in Milverton, Somerset; discovered interference of light; offered red-green-violet theory of light perception; professor of natural philosophy at Royal Institution and foreign secretary of Royal Society; helped decipher text of Rosetta stone.

In the early 1800s Thomas Young advanced a theory of human vision that was later elaborated by Hermann von Helmholtz. A modern version of the Young-Helmholtz theory states that the eye contains three kinds of colour receptors, or cones. One kind has greatest sensitivity to green light, another to red light, and the third to blue light. According to this theory, any other colour stimulates more than one kind of cone in varying amounts, depending on the mixture of wavelengths in the colour

Four different light-sensitive chemicals are found in the human eye. Rhodopsin, located in the rods, seems to be limited to black-and-white vision. The other three, called iodopsins, are involved in colour vision. However, eye chemistry alone does not account for man’s ability to identify colours A part is also played by the brain, which may contain separate colour-detection centres

Hering, Ewald (1834-1918), German physiologist and psychologist, born in Alt-Gersdorf, Saxony; advanced theory of four colours occurring in pairs as opposed to three-colour theory of Helmholtz.

In the 1870s Ewald Hering suggested that there were four primary colours blue, green, yellow, and red. He arranged these on a circle, with red opposite green and blue opposite yellow. The circle could be filled in with intermediate colours Hering considered colours opposite each other to be opponents. He regarded white and black as a special pair of opponents. Hering’s theory agrees with the common notion that red and yellow are perceived psychologically as warm colours, while blue and green, their opponents, are regarded as cool colours The three cone receptors hypothesized by Young and Helmholtz form the first stage of colour perception. Electrical signals generated by the cones combine in the retina to yield opponent signals as suggested by Hering. There are three signals: white-black, red-green, and yellow-blue. Thus colour vision begins with a two stage-process. Signals sent to the brain along the optic nerve are coded into these opponent signals.

A series of experiments performed by Edwin H. Land in the 1950s called both theories into question. Land demonstrated that a wide range of colours could be produced from a mixture of only two colours He photographed the same scene twice once with light having long wavelengths, and once with light having medium wavelengths. He called the two types of photographs that resulted the long record (for long wavelengths) and the short record. When Land projected the two records onto the same screen using red light to project the long record and white light to project the short record the image on the screen seemed to have a full range of colours Land suggested that some of the colours seen whose wavelengths were absent were perceived through a process involving the comparison of surrounding colours He surmised that the total combination of colours in a scene played an important part in colour vision. His experiments can also be interpreted as proof for the two-stage theory of colour vision.

Abnormal Colour Vision

Some people suffer from abnormal colour vision, or colour blindness. When asked to pick out chips that have the same colour a process called colour matching such people may pair chips that do not look at all alike to most observers. Abnormal colour vision is usually inherited, but it may also result from a chemical imbalance in the body or from eye injuries.

Scientists classify colour blindness into three major types. The most severe, as well as the most unusual, is mono chromatism, or total colour blindness. A person who is completely colour-blind cannot distinguish individual hues. To him all colours match with greys of the same lightness.

Dichromatism, or partial colour blindness, is a more widespread abnormality. Some dichromatic people confuse red, green, and grey but are able to distinguish blue and yellow. Others cannot see the longest wavelengths of light the red end of the visible spectrum. Rare forms of dichromatism include the inability to distinguish among blue, yellow, and grey and the inability to see light of very short wavelengths the violet end of the spectrum.

Normal colour vision the vision that most people possess is trichromatic. The most common type of colour blindness is a variation of normal colour vision called anomalous trichromatism. The whole range of colours visible to people with normal colour vision is also visible to people with this condition, but they match colours that do not appear the same to people with normal colour vision. For example, in a test in which mixtures of red and green light are varied to match a yellow sample, a person with green-weak vision adds more green light to match the yellow than does a person of normal colour vision. Green-weak and red-weak vision are the most prevalent forms of anomalous trichromatism; the blue-weak form is extremely rare.

Posted 2012/05/20 by Stelios in Education

Tagged with , , , ,

COLOURS (Part 3 of 3)   Leave a comment

Colour Vision in Animals

Since animals cannot answer questions about the colours they perceive, scientists have had to develop experiments to find out whether animals can be trained to make choices on the basis of colour If an animal’s food is always placed under a red square instead of a green square and if the animal consistently looks under the red square when it is hungry, scientists conclude that the animal can distinguish between red and green. Since monkeys and apes can be trained in this way and, in addition, their retinal cells contain colour-sensitive chemicals, researchers are convinced that these primates have colour vision.

Non primate mammals tend to be insensitive to colour differences, but it is not certain whether this means that they cannot perceive them or that they do not regard them as important. Cats, for example, are commonly assumed to be colour-blind, but claims have been made that they can be trained to discriminate between some colours, among them blue and green, by first linking the colours with position. In any case, their sensitivity to colour is not great.

Birds have good colour discrimination, somewhat similar to that of humans. Many of their behaviour patterns for example, the identification of their mates or of their prey are based on the recognition of colour Fish can also discriminate among colours Bees have colour vision similar to human colour vision, except that it includes ultraviolet wavelengths too short for humans to perceive, and it excludes the red end of the spectrum, visible to humans.

Techniques for Reproducing Colour

When early humans painted pictures on cave walls, they used the pigments in coloured earth and clay to give colour to their creations. Red, black, and yellow pigments were the easiest to find. Then humans learned how to create new colours by mixing these three pigments. Other pigments, it was discovered, produced other colours, such as orange, brown, and blue-black.

As humans learned how to make pottery and carvings and to weave fabrics, they also learned how to apply colour to these new objects. Pigments had to be applied to pottery and carvings in a form that would adhere to their surfaces. The materials and techniques originally employed were closely related to those that had been used in painting pictures on flat surfaces. Naturally available pigments were mixed to get new colours

From the beginning, however, making dyes for woven fabrics involved chemical processes. At first, parts of plants were boiled to separate coloured chemicals. The cloth was then soaked in the resulting coloured solutions.

Today chemical reactions are used in various ways to produce new dyes. The changes in colour that result from these reactions are a consequence of changes in chemical structure and cannot be explained by the laws of colour mixing.

Modern techniques for printing pictures in full colour can become quite complicated. They are based on colour separation at one stage and subtractive colour mixing at another and may also include additive colour mixing. The three basic colours of colour printing yellow, cyan, and magenta can be mixed in various proportions to duplicate almost any colour

The first step in colour printing is to scan the original photograph or piece of artwork with a light beam that gets split into three beams after it has passed through, or has been reflected from, the original document. Each beam then strikes a photocell that is covered with a filter coloured to match one of the additive primary colours In this way each area of the original is separated into its three colour components. Four computers are used to correct the colour via electric currents that are fed into the computers from the photocells. Each computer corresponds to one of the additive primary colours and one computer is reserved for the black component, which is computed from the other three signals. Exposing lights manipulate the modified currents from the computers and then expose the corrected colour separations on film or paper.

In half-tone printing, the colour is applied as an array of tiny dots. Where both cyan and magenta ink are applied, some of the dots will be cyan and some magenta. Printed side by side, both colours are reflected, and they mix additively to form blue. Where yellow and magenta are reflected in equal strength, they mix additively to form red. Intermediate shades can be formed by varying either the size or the number of the dots and by printing the dots on top of one another.

Colour television also works by first separating colours into their additive primaries and then recombining them by additive mixing of coloured dots. In some cameras, mirrors separate the light into three beams. The first beam passes through a blue filter, the second through a green filter, and the third through a red filter. Three camera tubes then record the colour information in the form of electromagnetic signals, which are beamed to the television receiver. Consumer cameras may use a single detector chip, known as a charge-coupled device (CCD). Red, green, and blue filters are affixed directly to the chip.

A colour picture tube contains three electron guns, one for each colour The back of the television screen is coated with tiny dots of chemicals called phosphors. When a phosphor is hit by an electron, it gives off wavelengths of light. Most colour television sets contain three kinds of phosphors that give off blue, green, and red light, respectively. A screen called a shadow mask lies behind the phosphor layer. The electron gun that receives the blue signal fires electrons toward the phosphors. The shadow mask screens the red-emitting and green-emitting phosphors from these electrons so that only the blue-emitting phosphors are activated. Similarly, the electron gun that receives the red signal is screened from all but the red-emitting phosphors, and the one that receives the green signal is screened from all but the green-emitting phosphors. The colours mix additively to form a wide range of colours

Colour photography is a blend of chemistry and the additive and subtractive principles of colour mixing. A typical colour film consists of three layers of chemicals, each of which is sensitive to one of the additive primaries. The top layer contains chemicals that react to blue light only. Below it lies a yellow filter that absorbs all blue light passing through the top layer. Green and red light pass through the top layer and the filter to reach the middle layer, which is sensitive to green and blue. Since the yellow filter has stopped all the blue light, only green light affects the middle layer. Red passes through this layer to the bottom layer, which is sensitive to red and blue and partly to green. Since the blue light and the green light have already been filtered out, only the red light can cause a chemical change in the bottom layer. In this way, three records are made of the scene, each containing information contributed by one of the additive primaries.

The three records may be combined into a colour transparency, which is dyed in appropriate combinations of the subtractive colour primaries magenta, cyan, and yellow. White light may be shined through the transparency and the resulting image projected onto a screen. If the transparency has been dyed with cyan and magenta only, the cyan absorbs red light, the magenta absorbs green light, and the light transmitted and projected onto the screen is blue. If the transparency has been dyed with yellow and a small amount of magenta, no blue can pass through it, and some but not all of the green is absorbed, leaving a mixture of wavelengths that produces a sensation of orange.

A photographic technique called holography uses laser light to produce a three-dimensional image of a subject. A record called a hologram is made of the interference pattern between laser light that has bounced off the subject and light coming directly from the laser. When laser light is shined through the hologram, light deflected by the hologram reconstructs the image of the subject. If an observer walks around the hologram and views it from different angles, he can see the changing perspective just as if the original subject were still there. Techniques are being developed to obtain holographic images using laser light of three different colours, so that a full-colour image can be produced. A problem not yet overcome is that ghost images appear to the side of the actual image. Ultimately, holography may be used to produce three-dimensional television.

Response to Colour

On the whole, people tend to regard blue and green as cool, quiet colours, while yellow and, especially, red are considered warm colours Individual colour preferences may be based on this general difference in responses to colour Tests have shown that the colour preferences of children tend to shift from warmer to cooler colours as they grow older. Other tests have shown that blue is the most widely preferred colour, with red, green, violet, orange, and yellow as runners-up. However, variations on these colours have produced different rankings. Yellowish green usually ranks below a relatively pure green. It has also been shown that many people prefer pure, or saturated, colours

Assisted by Roy S. Berns, Director of the Rochester Institute of Technology’s Munsell Colour Science Laboratory in New York.

BIBLIOGRAPHY FOR COLOUR

Anderson, L.W. Light and Color, rev. ed. (Raintree, 1988).

De Grandis, Luigina. Theory and Use of Color (Prentice, 1986).

Falk, D.S. and others. Seeing the Light: Optics in Nature, Photography Color, Vision, and Holography (Harper, 1986).

Hoban, Tana. Of Colors and Things (Greenwillow, 1989).

Kuehni, R.G. Colour, Essence and Logic (Van Nostrand Reinhold, 1983).

McLaren, K. The Colour Science of Dyes and Pigments, 2nd ed. (Hilger, 1986).

Nassau, Kurt. The Physics and Chemistry of Color (Wiley, 1983).

Norman, R.B. Electronic Color: The Art of Color Applied to Graphic Computing (Van Nostrand Reinhold, 1990).

Rossotti, Hazel. Colour (Princeton Univ. Press, 1985).

Thorell, L.G. and Smith, W.J. Using Computer Color Effectively (Prentice, 1990).

Williamson, S.J. and Cummins, H.Z. Light and Color in Nature and Art (Wiley, 1983).

Posted 2012/05/20 by Stelios in Education

Tagged with , , , , , ,

WATER BUGS   Leave a comment

 

DEFINITION: any of various hemipteran insects that live in fresh waters, including the back swimmers and the water boatmen.

Even though they breathe air, several kinds of insects can also live underwater and are able to fly, crawl, or swim at will. Called water bugs, such insects belong to the order of true bugs Hemiptera.

One of the most common water bugs is the water boatman, the only water bug that can take flight directly from the water. It usually stays anchored to the bottom feeding on algae and organic debris and keeps a supply of air stored under its wings.

The back swimmer is named for its habit of rowing itself through the water upside down. It stores air in two grooves on its under surface. Each of its three pairs of legs serves a different purpose. The front pair captures small creatures for food; the middle pair holds on to objects; and the long, flat hind pair, fringed with stiff hairs, acts as oars that row with powerful sweeping strokes. Before it can take flight, the back swimmer must crawl onto the land, turn over, and wait for its wings to dry.

The grasping forelegs of the water scorpion resemble scorpion claws, and its scorpion like tail is used as a breathing tube. It seldom swims, preferring to hunt for small fish and insects from a fixed station. A bubble of air trapped underneath sensory hairs on its abdomen imparts stimuli that tell the bug whether it is walking down deeper into the water or heading upward.

The water strider, or pond skater, has a long narrow body and six spidery legs. Unlike the other water bugs, it never ventures underwater. Its underside is covered with a water-repellent coat of hair that keeps it afloat. Its middle legs propel it across the surface, its hind legs act as rudders and brakes, and its short forelegs are used exclusively for catching prey.

American cockroach (or water bug), insect (Periplaneta americana) of the family Blattidae.

The giant water bug is the largest species and can grow up to 4 inches (10 centimetres) long. These bugs have two retractable appendages that, when held together, form a breathing tube. The bugs are strong flyers and formidable hunters. They paralyse their prey by injecting a poison through the beak, and in this manner they can overpower fish twice their own size. The American cockroach (Periplaneta americana), a large insect in the order Orthoptera, is often called water bug.

Water boatmen belong to the family Corixidae; back swimmers, Notonectidae; water scorpions, Nepidae; water striders, Gerridae; giant water bugs, Belostomatidae.

Posted 2012/05/16 by Stelios in Education

Tagged with , , ,

VIRUSES   Leave a comment

DEFINITION:1 orig., venom, as of a snake 2 a) any of a kingdom (Virus) of prokaryotes, usually ultra microscopic, that consist of nucleic acid, either RNA or DNA, within a case of protein: they infect animals, plants, and bacteria and can reproduce only within living cells so that they are considered as being either living organisms or inert chemicals b) a disease caused by a virus 3 anything that corrupts or poisons the mind or character; evil or harmful influence 4 an unauthorized, disruptive set of instructions placed in a computer program, that leaves copies of itself in other programs and disks.

1981: US AIDS diagnosed. A new fatal, infectious disease was diagnosed in 1981. Called Acquired Immunodeficiency Syndrome (AIDS), it began appearing in major cities among homosexual men and intravenous drug users. Other high-risk groups were haemophiliacs and other recipients of blood or blood products, babies born of AIDS-infected women, bisexual men, and prostitutes and their customers. AIDS was soon recognized as a worldwide health emergency: a fatal disease with no known cure that quickly became an epidemic. It was especially widespread in Africa, the apparent land of its origin.

By 1983 the virus that causes the disease had been isolated. Some medicines, notably AZT (azidothymidine), slowed the disease’s progress for a few months or more; but the spread of AIDS continued relentlessly, with more than 3,000 new cases being reported each month by 1991.

The federal government had committed more than 1.6 billion dollars to research, while the homosexual community and other special interest groups sought more federal funding and greater assistance from the health insurance industry. Educational programs on safe sexual practices, such as the use of condoms, seemed the best means of slowing the epidemic. Meanwhile, more than 70,000 persons in the United States had died from AIDS by the end of the decade.

1981: WORLD AIDS identified. A strange, new, and deadly disease made its appearance in 1980. Physicians in such large cities as Los Angeles, New York, and San Francisco noticed that homosexual men were dying from rare lung infections or from a cancer known as Kaposi’s sarcoma. By 1981 the disease was identified and given a name: AIDS, or acquired immunodeficiency syndrome.

The virus that causes AIDS, human immunodeficiency virus (HIV), was identified by Dr. Luc Montagnier of the Pasteur Institute in Paris in research done during the years 1981-84. The results of Dr. Montagnier’s studies were released in 1984. Since its discovery, AIDS has become one of the world’s major health problems. Within certain populations it has become an epidemic: male homosexuals, haemophiliacs, and intravenous drug users in the United States, for example, and heterosexual men and women in Sub-Saharan Africa. Many people were infected through blood transfusions before HIV screening was introduced. An individual infected with the virus may not show the symptoms of AIDS for several years, but the condition is eventually fatal.

The search for a successful vaccine was pursued in laboratories around the world, with no success by the early 1990s. Meanwhile, the disease continued to spread to different parts of the world. Already rife in the United States, Europe, and sub-Saharan Africa by the mid-1980s, it quickly spread to Central and East Asia. The disease also began to spread to larger portions of the heterosexual community throughout the world.

The composition of a virus is relatively simple, and its size is extremely small. It cannot even properly be called an organism because it is unable to carry on life processes outside a living cell of an animal, plant, or bacteria. Yet its method of entering and “enslaving” a living cell is so ingenious that the virus is humankind’s deadliest enemy and resists the most advanced efforts of modern science to eliminate it.

Millions of people throughout the world suffer each year from viral diseases such as polio, measles, chicken pox, mumps, acquired immunodeficiency syndrome (AIDS), and the common cold. Viruses also produce such illnesses as foot-and-mouth disease in livestock, distemper in dogs, panleukopenia in cats, and hog cholera. The viruses that infect bacteria are called bacteriophages.

Structure and Composition

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

Viruses are exceedingly small; they range in size from about 0.02 to 0.25 micron in diameter (1 micron = 0.000039 inch). By contrast, the smallest bacteria are about 0.4 micron in size. As observed with an electron microscope, some viruses are rod-shaped, others are roughly spherical, and still others have complex shapes consisting of a multi sided “head” and a cylindrical “tail.” A virus consists of a core of nucleic acid surrounded by a protein coat called a capsid; some viruses also have an outer envelope composed of fatty materials and proteins. The nucleic-acid core is the essential part of the virus it carries the virus’s genes. The core consists of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), substances that are essential to the transmission of hereditary information. The protein capsid protects the nucleic acid and may contain molecules that enable the virus to enter the host cell that is, the living cell infected by the virus.

Cycle and Patterns of Infection

Outside of a living cell, a virus is a dormant particle. It exhibits none of the characteristics generally associated with life namely, reproduction and metabolic processes such as growth and assimilation of food. Unlike bacteria and other micro-organisms, viruses remain dormant in body fluids. Thus, great numbers of viruses may be present in a body and yet not produce a disease because they have not invaded the body’s cells. Once inside a host cell, however, the virus becomes an active entity capable of taking over the infected cell’s metabolic machinery. The cellular metabolism becomes so altered that it helps to produce thousands of new viruses.

The virus’s developmental cycle begins when it succeeds in introducing its nucleic acid, and in some cases its protein coat, into a host cell. Bacteriophages attach to the surface of the bacterium and then penetrate the rigid cell wall, transmitting the viral nucleic acid into the host. Animal viruses enter host cells by a process called endocytosis. Plant viruses enter through wounds in the cell’s outer coverings through abrasions made by wind, for example, or through punctures made by insects.

Virion, an entire virus particle the extracellular infective form of a virus consisting of an outer protein shell (capsid) and an inner core of nucleic acid (either ribonucleic or deoxyribonucleic acid); in some, the capsid further enveloped by a fatty membrane.

Once inside the host cell, the virus’s genes usually direct the cell’s production of new viral protein and nucleic acid. These components are then assembled into new, complete, infective virus particles called virions, which are then discharged from the host cell to infect other cells.

In the case of bacteriophages, the new virions are usually released by bursting the host cell a process called lysing, which kills the cell. Sometimes, however, bacteriophages form a stable association with the host cell. The virus’s genes are incorporated into the host cell’s genes, replicate as the cell’s genes replicate, and when the cell divides, the viral genes are passed on to the two new cells.

In such cases no virions are produced, and the infecting virus seems to disappear. Its genes, however, are being passed on to each new generation of cells that stem from the original host cell. These cells remain healthy and continue to grow unless, as happens occasionally, something triggers the latent viral genes to become active. When this happens, the normal cycle of viral infection results: the viral genes direct viral replication, the host cell bursts, and the new virions are released. This pattern of infection is called lysogeny.

Closely related to lysogeny is the process known as transduction, whereby a virus carries bacterial genes from one host to another. This transduction process occurs when genes from the original host become incorporated into a virion that subsequently infects another bacterium.

Viral infections of plant and animal cells resemble those of bacterial cells in many ways. The release of new virions from plant and animal cells does not, however, always involve the bursting of the host cell as it does in bacteria. Particularly among animal viruses, the new virions may be released by budding off from the cell membrane, a process that does not necessarily kill the host cell.

In general, a viral infection produces one of four effects in a plant or animal cell: in apparent effect, in which the virus remains dormant in the host cell; cytopathic effect, in which the cell dies; hyperplastic effect, in which the cell is stimulated to divide before its death; and cell transformation, in which the cell is stimulated to divide, take on abnormal growth patterns, and become cancerous.

Cold sore (or fever blister, or Herpes simplex), a virus infection of the borders of the mouth, lips and nose, or genitals; marked by watery blisters; may be due to illness, emotional upset, or other stress.

Viral infections in animals can be localized or can spread to various parts of the body. Some animal viruses produce latent infections: the virus remains dormant much of the time but becomes active periodically. This is the case with the herpes simplex viruses that cause cold sores.

Natural Defences, Immunization, Treatment

Fever, a condition in which the body temperature rises above normal.

Animals have a number of natural defences that may be triggered by a viral infection. Fever is a general response; many viruses are inactivated at temperatures just slightly above the host’s normal body temperature. Another general response of infected animal cells is the secretion of a protein called interferon. Interferon inhibits the reproduction of viruses in non infected cells.

Fever and interferon production are general responses to infection by any virus. In addition, humans and other vertebrates can mount an immunological attack against a specific virus. The immune system produces antibodies and sensitized cells that are tailor-made to neutralize the infecting virus. These immune defenders circulate through the body long after the virus has been neutralized, thereby providing long-term protection against reinfection by that virus.

Such long-term immunity is the basis for active immunization against viral diseases. A weakened or inactivated strain of an infectious virus is introduced into the body. This virus does not provoke an active disease state, but it does stimulate the production of immune cells and antibodies, which then protect against subsequent infection by the virulent form of the virus.

Active immunizations are routine for such viral diseases as measles, mumps, poliomyelitis, and rubella. In contrast, passive immunization is the injection of antibodies from the serum of an individual who has already been exposed to the virus. Passive immunization is used to give short-term protection to individuals who have been exposed to such viral diseases as measles and hepatitis. It is useful only if provided soon after exposure, before the virus has become widely disseminated in the body.

The treatment of an established viral infection usually is restricted to relieving specific symptoms. There are few drugs that can be used to combat a virus directly. The reason for this is that viruses use the machinery of living cells for reproduction. Consequently, drugs that inhibit viral development also inhibit the functions of the host cell. Nonetheless, a small number of antiviral drugs are available for specific infections.

The most successful controls over viral diseases are epidemiological. For example, large-scale active immunization programs can break the chain of transmission of a viral disease. Worldwide immunization is credited with the eradication of smallpox, once one of the most feared viral diseases. Because many viruses are carried from host to host by insects or contaminated food, insect control and hygienic food handling can help eliminate a virus from specific populations.

History of Virus Research

Historic descriptions of viral diseases date back as far as the 10th century BC. The concept of the virus, however, was not established until the last decade of the 19th century, when several researchers obtained evidence that agents far smaller than bacteria were capable of causing infectious diseases.

Mosaic disease, highly infectious virus disease affecting many plants including cucumber, potato, tomato, bean, and turnip; dwarfs growth and mottles leaves.

The existence of viruses was finally proved when bacteriophages were discovered by independent researchers in 1915 and 1917. The question of whether viruses are actually micro-organisms (similar to very tiny bacteria) was resolved in 1935, when the virus responsible for causing mosaic disease in tobacco was isolated and crystallized; the fact that it could be crystallized proved that the virus was not a cellular organism.

Bacteriophages are a valuable research tool for molecular biologists. Studies of bacteriophages have helped to illuminate such basic biological processes as genetic recombination, nucleic-acid replication, and protein synthesis.

COCKROACHES   Leave a comment

 

DEFINITION: any of an order (Blattaria) of insects with long antennae and a flat, soft body: some species are common household pests.

The cockroach is considered one of the most obnoxious of household pests. This brown or black insect can be found in houses, apartment and office buildings, ships, trains, and air planes in many parts of the world. Domestic cockroaches, which are also called roaches, have a disagreeable odour They live in warm, dark areas. Their broad, flat bodies permit them to crawl in narrow cracks and along pipes. They hide in the daytime, coming out at night to feed. The diet of the cockroach, which includes both plant and animal products, ranges from food, paper, clothing, and books to dead insects. Although cockroaches can be difficult to eliminate entirely, a variety of common poisons and traps are effective in controlling their numbers. Cockroaches are believed to be able to transmit several different human diseases.

Cockroaches are among the oldest living insects. Fossil cockroaches that resemble today’s species are commonly found in Coal Age deposits from more than 320 million years ago. About 3,500 species have been identified. Although the most notable varieties are those that infest households in the temperate regions, most species are tropical. Some reach lengths of several inches, and many are colourful Several species of woodland cockroaches are found in temperate regions. These live amid decaying wood and other vegetation and do not enter houses.

The cockroach has long, powerful legs and can run very fast. Long antennae on the head are used for feeling in dark places. Most species have two pairs of wings that are larger in the males. The female cockroach carries her eggs in a leathery capsule called an ootheca that protrudes from the rear of the abdomen. Females of some common species lay 16 to 45 eggs at a time. The eggs take from 4 to 12 weeks to hatch. After the female deposits an egg case, soft, white young called nymphs emerge. After exposure to air, the nymphs harden and turn brown.

The scientific name of the German cockroach is Blattella germanica. A common household pest, it is light-brown with two dark stripes on the thorax segment just behind the head. Because it is only about half an inch (12 millimetres) long, it can easily enter or be transported into homes. Abundant around the water pipes of the Croton Aqueduct in New York City, this cockroach became known as the Croton bug.

American cockroach (or water bug), insect (Periplaneta americana) of the family Blattidae.

The American cockroach (Periplaneta americana), also called water bug, is 1.2 to 2 inches (30 to 50 millimetres) long, reddish brown, and lives outdoors or in dark, heated indoor areas, for example in basements and furnace rooms. The American cockroach, a native of tropical and subtropical America, has well-developed wings and can fly long distances.

Cockroaches belong to the order Orthoptera and to the family Blattidae. They are closely related to the grasshoppers, katydids, and crickets.

Posted 2012/05/09 by Stelios in Education

Tagged with ,

SEXUALLY TRANSMITTED DISEASES (Part 1 of 2)   Leave a comment

1981: AIDS diagnosed. A new fatal, infectious disease was diagnosed in 1981. Called Acquired Immunodeficiency Syndrome (AIDS), it began appearing in major cities among homosexual men and intravenous drug users. Other high-risk groups were haemophiliacs and other recipients of blood or blood products, babies born of AIDS-infected women, bisexual men, and prostitutes and their customers. AIDS was soon recognized as a worldwide health emergency: a fatal disease with no known cure that quickly became an epidemic. It was especially widespread in Africa, the apparent land of its origin.

By 1983 the virus that causes the disease had been isolated. Some medicines, notably AZT (azidothymidine), slowed the disease’s progress for a few months or more; but the spread of AIDS continued relentlessly, with more than 3,000 new cases being reported each month by 1991.

The federal government had committed more than 1.6 billion dollars to research, while the homosexual community and other special interest groups sought more federal funding and greater assistance from the health insurance industry. Educational programs on safe sexual practices, such as the use of condoms, seemed the best means of slowing the epidemic. Meanwhile, more than 70,000 persons in the United States had died from AIDS by the end of the decade.

1981: AIDS identified. A strange, new, and deadly disease made its appearance in 1980. Physicians in such large cities as Los Angeles, New York, and San Francisco noticed that homosexual men were dying from rare lung infections or from a cancer known as Kaposi’s sarcoma. By 1981 the disease was identified and given a name: AIDS, or acquired immunodeficiency syndrome.

The virus that causes AIDS, human immunodeficiency virus (HIV), was identified by Dr. Luc Montagnier of the Pasteur Institute in Paris in research done during the years 1981-84. The results of Dr. Montagnier’s studies were released in 1984. Since its discovery, AIDS has become one of the world’s major health problems. Within certain populations it has become an epidemic: male homosexuals, haemophiliacs, and intravenous drug users in the United States, for example, and heterosexual men and women in Sub-Saharan Africa. Many people were infected through blood transfusions before HIV screening was introduced. An individual infected with the virus may not show the symptoms of AIDS for several years, but the condition is eventually fatal.

The search for a successful vaccine was pursued in laboratories around the world, with no success by the early 1990s. Meanwhile, the disease continued to spread to different parts of the world. Already rife in the United States, Europe, and sub-Saharan Africa by the mid-1980s, it quickly spread to Central and East Asia. The disease also began to spread to larger portions of the heterosexual community throughout the world.

Dec. 1, 1993: AIDS awareness day. The sixth annual World AIDS Day was celebrated in many countries. The commemoration was started in 1988 by the World Health Organization. The day was partly a memorial for those who had died from AIDS (acquired immunodeficiency syndrome). Although the growth of the epidemic had peaked in the United States in the early 1990s, the disease was still spreading rapidly in Africa and parts of Asia. No cure had yet been found, nor was there a successful vaccine to protect against contracting the disease.

Dec. 1, 1993: AIDS awareness day. The sixth annual World AIDS Day was celebrated in many countries. The commemoration was started in 1988 by the World Health Organization. The day was partly a memorial for those who had died from AIDS. Although the growth of the epidemic had peaked in the United States in the early 1990s, the disease was still spreading rapidly in Africa and parts of Asia. No cure had yet been found, nor was there a successful vaccine to protect against contracting the disease.

April 7, 1994: Failure of AZT. A team of British and French medical researchers reported that AZT, a drug often used to fight AIDS, does not slow the onset of the disease in persons who have the HIV virus. Published in the journal Lancet as the Concorde Report, the document stated that AZT does prolong the lives of patients who have developed AIDS, but it does not impede the progression of HIV into full-blown AIDS. The report was based on tests conducted on HIV-infected patients in Ireland, France, and the United Kingdom.

Diseases that can be passed between people during sexual contact have plagued humankind throughout history. Until recently such a disease was called venereal disease, or VD. The preferred term now is sexually transmitted disease, or STD. The two main venereal diseases in the United States have traditionally been gonorrhoea and syphilis. Scientists now know that many other diseases can be passed during sex. More than 30 STDs have been identified.

AIDS (acquired immunodeficiency syndrome), incurable disease caused by a virus that damages the human body’s immune system; believed to be transmitted through sexual contacts, blood transfusions, or contaminated needles used for intravenous drug injections; often fatal; high percentage of victims are homosexuals or drug abusers.

The names of such STDs as acquired immunodeficiency syndrome (AIDS) and gonorrhoea are known to most people; however, other STDs such as trichomoniasis and genital candidiasis may not be as familiar. Some STDs affect only a few people or do not cause life-threatening problems. Other STDs, such as gonorrhoea and chlamydial infections, affect many people or cause severe health damage.

STDs are a major health problem throughout the world. In the United States STDs strike an estimated 20 million people each year, or an average of one person every 1.5 seconds. About one half of STD patients are under the age of 25. Nearly 2.5 million teenagers are infected with an STD each year.

Pelvic inflammatory disease (PID), general, acute inflammation of the pelvic cavity in women.

Sterility, in biology, the inability to produce offspring; one cause is the production of non functioning sex cells.

The health problems caused by STDs seem endless. The diseases can cause arthritis, sterility, nervous system damage, heart disease, and death. Women and infants suffer the most damage from STDs. For example, each year more than 1 million women suffer from pelvic inflammatory disease resulting from gonorrhoeal or chlamydial infections. About 200,000 of these women become sterile each year. More than 300,000 babies are injured or die each year from STDs.

Transmission

Mucous membrane (or mucosa), membrane that secretes mucus and lines the mouth, nose, throat, windpipe, lungs, eyelids, and the alimentary canal.

STDs are caused by a variety of organisms that include bacteria, protozoa, viruses, and very small insects such as Phthirus pubis, or pubic lice. These organisms usually live in the warm and moist parts of the body called mucous membranes. The penis, vagina, rectum, mouth, and eyes have mucous membranes. These organisms usually invade a person through the mucous membranes during sexual contact. Some STDs are passed by deep kissing or skin-to-skin contact, though these transmission methods are not common.

Anyone can get an STD regardless of age, sex, race, social class, or whether the person is heterosexual, homosexual, or bisexual. Exposure to STD organisms results from participating in certain sexual or drug-use behaviours

People with many sexual partners have the greatest risk of contracting an STD. The risk increases with each new partner. The risk is even greater if any of the partners have several sex partners.

It is virtually impossible to get an STD from such things as door knobs, toilet seats, drinking glasses, or whirlpool baths. Light and air destroy STD organisms very quickly. Some STDs such as gonorrhoea, syphilis, and genital herpes are practically always spread by sexual contact. Such diseases as AIDS, hepatitis, and pediculosis pubis, however, can sometimes be acquired through non-sexual means. AIDS and some forms of hepatitis can be acquired from infected blood in intravenous drug needles and syringes. Pubic lice can be picked up from contaminated clothing or bedding that is infected with the lice or their eggs.

Most STDs can also be passed during pregnancy or birth from an infected woman to her baby. Women can develop some infections in the vagina without having sex. It is possible but not common for those infections to be passed to others during sex. Other vaginal infections are sexually transmitted, but the woman’s sex partners may not have symptoms.

Prevention

Most people have heard some information about STDs. In recent years public awareness has increased. The media have developed greater coverage of STDs, and more schools teach about STDs. There are news items on television and in newspapers and magazines about AIDS almost daily. This increased discussion has alerted people to how widespread STDs are, to STD health dangers, and to methods of preventing STDs. Hence, many people have become more cautious.

Medical personnel and public health officials believe that education is the key to controlling the spread of STDs. STDs are dangerous. Further medical advances may bring improved modes of treatment, but avoiding infection in the first place is vital. The actions of individual persons are the most important factors in halting the spread of these diseases. Many health educators emphasize the idea that the wisest teaching approach is to motivate people to practice responsible STD-preventive behaviour, such as sexual abstinence and sexual fidelity.

Communication between partners. Among people who are mature enough to have a sexual relationship, one of the most important things to do is communicate. A person should feel free to discuss concerns about getting an STD. The conversation can be started by one partner stating that he or she cares about the health and well-being of both persons. Persons deciding to have sex with a new partner should discuss ways of protecting each other.