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PAPER 2003

Q 1. Write short notes on any two of the following :

(a)Microwave Oven

(b) Optic Fibre

(c) Biotechnology

I INTRODUCTION

Biotechnology, the manipulation of biological organisms to make products that benefit human beings. Biotechnology contributes to such diverse areas as food production, waste disposal, mining, and medicine.
Although biotechnology has existed since ancient times, some of its most dramatic advances have come in more recent years. Modern achievements include the transferal of a specific gene from one organism to another (by means of a set of genetic engineering techniques known as transgenics); the maintenance and growth of genetically uniform plant- and animal-cell cultures, called clones; and the fusing of different types of cells to produce beneficial medical products such as monoclonal antibodies, which are designed to attack a specific type of foreign substance.

II HISTORY
The first achievements in biotechnology were in food production, occurring about 5000 BC. Diverse strains of plants or animals were hybridized (crossed) to produce greater genetic variety. The offspring from these crosses were then selectively bred to produce the greatest number of desirable traits (see Genetics). Repeated cycles of selective breeding produced many present-day food staples. This method continues to be used in food-production programs.

Corn (maize) was one of the first food crops known to have been cultivated by human beings. Although used as food as early as 5000 BC in Mexico, no wild forms of the plant have ever been found, indicating that corn was most likely the result of some fortunate agricultural experiment in antiquity.

The modern era of biotechnology had its origin in 1953 when American biochemist James Watson and British biophysicist Francis Crick presented their double-helix model of DNA. This was followed by Swiss microbiologist Werner Arber's discovery in the 1960s of special enzymes, called restriction enzymes, in bacteria. These enzymes cut the DNA strands of any organism at precise points. In 1973 American geneticist Stanley Cohen and American biochemist Herbert Boyer removed a specific gene from one bacterium and inserted it into another using restriction enzymes. This event marked the beginning of recombinant DNA technology, commonly called genetic engineering. In 1977 genes from other organisms were transferred to bacteria. This achievement eventually led to the first transfer of a human gene, which coded for a hormone, to Escherichia coli bacteria. Although the transgenic bacteria (bacteria to which a gene from a different species has been transferred) could not use the human hormone, they produced it along with their own normal chemical compounds.

In the 1960s an important project used hybridization followed by selective breeding to increase food production and quality of wheat and rice crops. American agriculturalist Norman Borlaug, who spearheaded the program, was awarded the Nobel Peace Prize in 1970 in recognition of the important contribution that increasing the world's food supply makes to the cause of peace.

III CURRENT TRENDS
Today biotechnology is applied in various fields. In waste management, for example, biotechnology is used to create new biodegradable materials. One such material is made from the lactic acid produced during the bacterial fermentation of discarded corn stalks. When individual lactic acid molecules are joined chemically, they form a material that has the properties of plastics but is biodegradable. Widespread production of plastic from this material is expected to become more economically viable in the future.
Biotechnology also has applications in the mining industry. In its natural state, copper is found combined with other elements in the mineral chalcopyrite. The bacterium Thiobacillus ferrooxidans can use the molecules of copper found in chalcopyrite to form the compound copper sulfate (CuSO4), which, in turn, can be treated chemically to obtain pure copper. This microbiological mining process is used only with low-grade ores and currently accounts for about 10 percent of copper production in the United States. The percentage will rise, however, as conventionally mined high-grade deposits are exhausted. Procedures have also been developed for the use of bacteria in the mining of zinc, lead, and other metals.
The field of medicine employs some of the most dramatic applications in biotechnology. One advance came in 1986 with the first significant laboratory production of factor VIII, a blood-clotting protein that is not produced, or has greatly reduced activity, in people who have hemophilia. As a result of this condition, hemophiliacs are at risk of bleeding to death after suffering minor cuts or bruises. In this biotechnological procedure, the human gene that codes for the blood-clotting protein is transferred to hamster cells grown in tissue culture, which then produce factor VIII for use by hemophiliacs. Factor VIII was approved for commercial production in 1992.

IV CONTROVERSIES
Some people, including scientists, object to any procedure that changes the genetic composition of an organism. Critics are concerned that some of the genetically altered forms will eliminate existing species, thereby upsetting the natural balance of organisms. There are also fears that recombinant DNA experiments with pathogenic microorganisms may result in the formation of extremely virulent forms which, if accidentally released from the laboratory, will cause worldwide epidemics. Some critics cite ethical dilemmas associated with the production of transgenic organisms.
In 1976, in response to fears of disastrous consequences of unregulated genetic engineering procedures, the National Institutes of Health created a body of rules governing the handling of microorganisms in recombinant DNA experiments. Although many of the rules have been relaxed over time, certain restrictions are still imposed on those working with pathogenic microorganisms.

Q2:
Give names of the members of the solar system. Briefly write down main characteristics of : a). Mars b). venus

Solar System, the Sun and everything that orbits the Sun, including the nine planets and their satellites; the asteroids and comets; and interplanetary dust and gas. The term may also refer to a group of celestial bodies orbiting another star (see Extrasolar Planets). In this article, solar system refers to the system that includes Earth and the Sun.

Planet, any major celestial body that orbits a star and does not emit visible light of its own but instead shines by reflected light. Smaller bodies that also orbit a star and are not satellites of a planet are called asteroids or planetoids. In the solar system, there are nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Planets that orbit stars other than the Sun are collectively called extrasolar planets. Some extrasolar planets are nearly large enough to become stars themselves. Such borderline planets are called brown dwarfs.

Mars:

Mars (planet), one of the planets in the solar system, it is the fourth planet from the Sun and orbits the Sun at an average distance of about 228 million km (about 141 million mi). Mars is named for the Roman god of war and is sometimes called the red planet because it appears fiery red in Earth’s night sky.

Mars is a relatively small planet, with about half the diameter of Earth and about one-tenth Earth’s mass. The force of gravity on the surface of Mars is about one-third of that on Earth. Mars has twice the diameter and twice the surface gravity of Earth’s Moon. The surface area of Mars is almost exactly the same as the surface area of the dry land on Earth. Mars is believed to be about the same age as Earth, having formed from the same spinning, condensing cloud of gas and dust that formed the Sun and the other planets about 4.6 billion years ago.

Venus:

Venus (planet), one of the planets in the solar system, the second in distance from the Sun. Except for the Sun and the Moon, Venus is the brightest object in the sky. The planet was named for the Roman goddess of beauty. It is often called the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. In ancient times the evening star was called Hesperus and the morning star Phosphorus or Lucifer. Because the planet orbits closer to the Sun than Earth does, Venus seems to either precede or trail the Sun in the sky. Venus is never visible more than three hours before sunrise or three hours after sunset.

Q 6 : Define any five of the following :

(i) Acoustic
Acoustics (Greek akouein, “to hear”), term sometimes used for the science of sound in general. It is more commonly used for the special branch of that science, architectural acoustics, that deals with the construction of enclosed areas so as to enhance the hearing of speech or music. For the treatment of acoustics as a branch of the pure science of physics, see Sound.

The acoustics of buildings was an undeveloped aspect of the study of sound until comparatively recent times. The Roman architect Marcus Pollio, who lived during the 1st century BC, made some pertinent observations on the subject and some astute guesses concerning reverberation and interference. The scientific aspects of this subject, however, were first thoroughly treated by the American physicist Joseph Henry in 1856 and more fully developed by the American physicist Wallace Sabine in 1900.

(ii) Quartz
Quartz, second most common of all minerals, composed of silicon dioxide, or silica, SiO2. It is distributed all over the world as a constituent of rocks and in the form of pure deposits. It is an essential constituent of igneous rocks such as granite, rhyolite, and pegmatite, which contain an excess of silica. In metamorphic rocks, it is a major constituent of the various forms of gneiss and schist; the metamorphic rock quartzite is composed almost entirely of quartz. Quartz forms veins and nodules in sedimentary rock, principally limestone. Sandstone, a sedimentary rock, is composed mainly of quartz. Many widespread veins of quartz deposited in rock fissures form the matrix for many valuable minerals. Precious metals, such as gold, are found in sufficient quantity in quartz veins to warrant the mining of quartz to recover the precious mineral. Quartz is also the primary constituent of sand.

(iii) Pollination
Pollination, transfer of pollen grains from the male structure of a plant to the female structure of a plant. The pollen grains contain cells that will develop into male sex cells, or sperm. The female structure of a plant contains the female sex cells, or eggs. Pollination prepares the plant for fertilization, the union of the male and female sex cells. Virtually all grains, fruits, vegetables, wildflowers, and trees must be pollinated and fertilized to produce seed or fruit, and pollination is vital for the production of critically important agricultural crops, including corn, wheat, rice, apples, oranges, tomatoes, and squash.

In order for pollination to be successful, pollen must be transferred between plants of the same species—for example, a rose flower must always receive rose pollen and a pine tree must always receive pine pollen. Plants typically rely on one of two methods of pollination: cross-pollination or self-pollination, but some species are capable of both.

Most plants are designed for cross-pollination, in which pollen is transferred between different plants of the same species. Cross-pollination ensures that beneficial genes are transmitted relatively rapidly to succeeding generations. If a beneficial gene occurs in just one plant, that plant’s pollen or eggs can produce seeds that develop into numerous offspring carrying the beneficial gene. The offspring, through cross-pollination, transmit the gene to even more plants in the next generation. Cross-pollination introduces genetic diversity into the population at a rate that enables the species to cope with a changing environment. New genes ensure that at least some individuals can endure new diseases, climate changes, or new predators, enabling the species as a whole to survive and reproduce.

Plant species that use cross-pollination have special features that enhance this method. For instance, some plants have pollen grains that are lightweight and dry so that they are easily swept up by the wind and carried for long distances to other plants. Other plants have pollen and eggs that mature at different times, preventing the possibility of self-pollination.

In self-pollination, pollen is transferred from the stamens to the pistil within one flower. The resulting seeds and the plants they produce inherit the genetic information of only one parent, and the new plants are genetically identical to the parent. The advantage of self-pollination is the assurance of seed production when no pollinators, such as bees or birds, are present. It also sets the stage for rapid propagation—weeds typically self-pollinate, and they can produce an entire population from a single plant. The primary disadvantage of self-pollination is that it results in genetic uniformity of the population, which makes the population vulnerable to extinction by, for example, a single devastating disease to which all the genetically identical plants are equally susceptible. Another disadvantage is that beneficial genes do not spread as rapidly as in cross-pollination, because one plant with a beneficial gene can transmit it only to its own offspring and not to other plants. Self-pollination evolved later than cross-pollination, and may have developed as a survival mechanism in harsh environments where pollinators were scarce.

(iv) Allele
All genetic traits result from different combinations of gene pairs, one gene inherited from the mother and one from the father. Each trait is thus represented by two genes, often in different forms. Different forms of the same gene are called alleles. Traits depend on very precise rules governing how genetic units are expressed through generations. For example, some people have the ability to roll their tongue into a U-shape, while others can only curve their tongue slightly. A single gene with two alleles controls this heritable trait. If a child inherits the allele for tongue rolling from one parent and the allele for no tongue rolling from the other parent, she will be able to roll her tongue. The allele for tongue rolling dominates the gene pair, and so its trait is expressed. According to the laws governing heredity, when a dominant allele (in this case, tongue rolling) and a recessive allele (no tongue rolling) combine, the trait will always be dictated by the dominant allele. The no tongue rolling trait, or any other recessive trait, will only occur in an individual who inherits the two recessive alleles.

(v) Optical Illusion

All genetic traits result from different combinations of gene pairs, one gene inherited from the mother and one from the father. Each trait is thus represented by two genes, often in different forms. Different forms of the same gene are called alleles. Traits depend on very precise rules governing how genetic units are expressed through generations. For example, some people have the ability to roll their tongue into a U-shape, while others can only curve their tongue slightly. A single gene with two alleles controls this heritable trait. If a child inherits the allele for tongue rolling from one parent and the allele for no tongue rolling from the other parent, she will be able to roll her tongue. The allele for tongue rolling dominates the gene pair, and so its trait is expressed. According to the laws governing heredity, when a dominant allele (in this case, tongue rolling) and a recessive allele (no tongue rolling) combine, the trait will always be dictated by the dominant allele. The no tongue rolling trait, or any other recessive trait, will only occur in an individual who inherits the two recessive alleles.

(f) Ovulation
Unlike germ cells in the testis, female germ cells originate as single cells in the embryonic tissue that later develops into an ovary. At maturity, after the production of ova from the female germ cells, groups of ovary cells surrounding each ovum develop into “follicle cells” that secrete nutriment for the contained egg. As the ovum is prepared for release during the breeding season, the tissue surrounding the ovum hollows out and becomes filled with fluid and at the same time moves to the surface of the ovary; this mass of tissue, fluid, and ovum is known as a Graafian follicle. The ovary of the adult is merely a mass of glandular and connective tissue containing numerous Graafian follicles at various stages of maturity. When the Graafian follicle is completely mature, it bursts through the surface of the ovary, releasing the ovum, which is then ready for fertilization; the release of the ovum from the ovary is known as ovulation. The space formerly occupied by the Graafian follicle is filled by a blood clot known as the corpus hemorrhagicum; in four or five days this clot is replaced by a mass of yellow cells known as the corpus luteum, which secretes hormones playing an important part in preparation of the uterus for the reception of a fertilized ovum. If the ovum goes unfertilized, the corpus luteum is eventually replaced by scar tissue known as the corpus albicans. The ovary is located in the body cavity, attached to the peritoneum that lines this cavity.

(vii) Aqua Regia
Aqua Regia (Latin, “royal water”), mixture of concentrated hydrochloric and nitric acids, containing one part by volume of nitric acid (HNO3) to three parts of hydrochloric acid (HCl). Aqua regia was used by the alchemists (see Alchemy) and its name is derived from its ability to dissolve the so-called noble metals, particularly gold, which are inert to either of the acids used separately. It is still occasionally used in the chemical laboratory for dissolving gold and platinum. Aqua regia is a powerful solvent because of the combined effects of the H+, NO 3-, and Cl- ions in solution. The three ions react with gold atoms, for example, to form water, nitric oxide (NO), and the stable ion AuCl- 4, which remains in solution.

Q12: Differentiate between the following pairs :

(A) Lava and Magma

Lava, molten or partially molten rock that erupts at the earth’s surface. When lava comes to the surface, it is red-hot, reaching temperatures as high as 1200° C (2200° F). Some lava can be as thick and viscous as toothpaste, while other lava can be as thin and fluid as warm syrup and flow rapidly down the sides of a volcano. Molten rock that has not yet erupted is called magma. Once lava hardens it forms igneous rock. Volcanoes build up where lava erupts from a central vent. Flood basalt forms where lava erupts from huge fissures. The eruption of lava is the principal mechanism whereby new crust is produced (see Plate Tectonics). Since lava is generated at depth, its chemical and physical characteristics provide indirect information about the chemical composition and physical properties of the rocks 50 to 150 km (30 to 90 mi) below the surface.
Magma, molten or partially molten rock beneath the earth’s surface. Magma is generated when rock deep underground melts due to the high temperatures and pressures inside the earth. Because magma is lighter than the surrounding rock, it tends to rise. As it moves upward, the magma encounters colder rock and begins to cool. If the temperature of the magma drops low enough, the magma will crystallize underground to form rock; rock that forms in this way is called intrusive, or plutonic igneous rock, as the magma has formed by intruding the surrounding rocks. If the crust through which the magma passes is sufficiently shallow, warm, or fractured, and if the magma is sufficiently hot and fluid, the magma will erupt at the surface of the earth, possibly forming volcanoes. Magma that erupts is called lava.

(B) Ultraviolet and infrared

Ultraviolet Radiation, electromagnetic radiation that has wavelengths in the range between 4000 angstrom units (Å), the wavelength of violet light, and 150 Å, the length of X rays. Natural ultraviolet radiation is produced principally by the sun. Ultraviolet radiation is produced artificially by electric-arc lamps (see Electric Arc).

Ultraviolet radiation is often divided into three categories based on wavelength, UV-A, UV-B, and UV-C. In general shorter wavelengths of ultraviolet radiation are more dangerous to living organisms. UV-A has a wavelength from 4000 Å to about 3150 Å. UV-B occurs at wavelengths from about 3150 Å to about 2800 Å and causes sunburn; prolonged exposure to UV-B over many years can cause skin cancer. UV-C has wavelengths of about 2800 Å to 150 Å and is used to sterilize surfaces because it kills bacteria and viruses.

The earth's atmosphere protects living organisms from the sun's ultraviolet radiation. If all the ultraviolet radiation produced by the sun were allowed to reach the surface of the earth, most life on earth would probably be destroyed. Fortunately, the ozone layer of the atmosphere absorbs almost all of the short-wavelength ultraviolet radiation, and much of the long-wavelength ultraviolet radiation. However, ultraviolet radiation is not entirely harmful; a large portion of the vitamin D that humans and animals need for good health is produced when the human's or animal's skin is irradiated by ultraviolet rays.

When exposed to ultraviolet light, many substances behave differently than when exposed to visible light. For example, when exposed to ultraviolet radiation, certain minerals, dyes, vitamins, natural oils, and other products become fluorescent—that is, they appear to glow. Molecules in the substances absorb the invisible ultraviolet light, become energetic, then shed their excess energy by emitting visible light. As another example, ordinary window glass, transparent to visible light, is opaque to a large portion of ultraviolet rays, particularly ultraviolet rays with short wavelengths. Special-formula glass is transparent to the longer ultraviolet wavelengths, and quartz is transparent to the entire naturally occurring range.

In astronomy, ultraviolet-radiation detectors have been used since the early 1960s on artificial satellites, providing data on stellar objects that cannot be obtained from the earth's surface. An example of such a satellite is the International Ultraviolet Explorer, launched in 1978.

INFRARED RADIATION
Infrared Radiation, emission of energy as electromagnetic waves in the portion of the spectrum just beyond the limit of the red portion of visible radiation (see Electromagnetic Radiation). The wavelengths of infrared radiation are shorter than those of radio waves and longer than those of light waves. They range between approximately 10-6 and 10-3 (about 0.0004 and 0.04 in). Infrared radiation may be detected as heat, and instruments such as bolometers are used to detect it. See Radiation; Spectrum.

Infrared radiation is used to obtain pictures of distant objects obscured by atmospheric haze, because visible light is scattered by haze but infrared radiation is not. The detection of infrared radiation is used by astronomers to observe stars and nebulas that are invisible in ordinary light or that emit radiation in the infrared portion of the spectrum.

An opaque filter that admits only infrared radiation is used for very precise infrared photographs, but an ordinary orange or light-red filter, which will absorb blue and violet light, is usually sufficient for most infrared pictures. Developed about 1880, infrared photography has today become an important diagnostic tool in medical science as well as in agriculture and industry. Use of infrared techniques reveals pathogenic conditions that are not visible to the eye or recorded on X-ray plates. Remote sensing by means of aerial and orbital infrared photography has been used to monitor crop conditions and insect and disease damage to large agricultural areas, and to locate mineral deposits. See Aerial Survey; Satellite, Artificial. In industry, infrared spectroscopy forms an increasingly important part of metal and alloy research, and infrared photography is used to monitor the quality of products. See also Photography: Photographic Films.

Infrared devices such as those used during World War II enable sharpshooters to see their targets in total visual darkness. These instruments consist essentially of an infrared lamp that sends out a beam of infrared radiation, often referred to as black light, and a telescope receiver that picks up returned radiation from the object and converts it to a visible image.

(C) Fault and Fold
Fold (geology), in geology, bend in a rock layer caused by forces within the crust of the earth. The forces that cause folds range from slight differences in pressure in the earth’s crust, to large collisions of the crust’s tectonic plates. As a result, a fold may be only a few centimeters in width, or it may cover several kilometers. Rock layers can also break in response to these forces, in which case a fault occurs. Folds usually occur in a series and look like waves. If the rocks have not been turned upside down, then the crests of the waves are called anticlines and the troughs are called synclines (see Anticline and Syncline).

Fault (geology), crack in the crust of the earth along which there has been movement of the rocks on either side of the crack. A crack without movement is called a joint. Faults occur on a wide scale, ranging in length from millimeters to thousands of kilometers. Large-scale faults result from the movement of tectonic plates, continent-sized slabs of the crust that move as coherent pieces (see Plate Tectonics).

(D) Caustic Soda and Caustic Potash

Electrolytic decomposition is the basis for a number of important extractive and manufacturing processes in modern industry. Caustic soda, an important chemical in the manufacture of paper, rayon, and photographic film, is produced by the electrolysis of a solution of common salt in water (see Alkalies). The reaction produces chlorine and sodium. The sodium in turn reacts with the water in the cell to yield caustic soda. The chlorine evolved is used in pulp and paper manufacture.

Caustic soda, or sodium hydroxide, NaOH, is an important commercial product, used in making soap, rayon, and cellophane; in processing paper pulp; in petroleum refining; and in the manufacture of many other chemical products. Caustic soda is manufactured principally by electrolysis of a common salt solution, with chlorine and hydrogen as important by-products.
Potassium hydroxide (KOH), called caustic potash, a white solid that is dissolved by the moisture in the air, is prepared by the electrolysis of potassium chloride or by the reaction of potassium carbonate and calcium hydroxide; it is used in the manufacture of soap and is an important chemical reagent. It dissolves in less than its own weight of water, liberating heat and forming a strongly alkaline solution.

(E) S.E.M. and T.E.M.


Q15: Laser

I INTRODUCTION

Laser, a device that produces and amplifies light. The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. Laser light is very pure in color, can be extremely intense, and can be directed with great accuracy. Lasers are used in many modern technological devices including bar code readers, compact disc (CD) players, and laser printers. Lasers can generate light beyond the range visible to the human eye, from the infrared through the X-ray range. Masers are similar devices that produce and amplify microwaves.

II PRINCIPLES OF OPERATION
Lasers generate light by storing energy in particles called electrons inside atoms and then inducing the electrons to emit the absorbed energy as light. Atoms are the building blocks of all matter on Earth and are a thousand times smaller than viruses. Electrons are the underlying source of almost all light.

Light is composed of tiny packets of energy called photons. Lasers produce coherent light: light that is monochromatic (one color) and whose photons are “in step” with one another.

A Excited Atoms
At the heart of an atom is a tightly bound cluster of particles called the nucleus. This cluster is made up of two types of particles: protons, which have a positive charge, and neutrons, which have no charge. The nucleus makes up more than 99.9 percent of the atom’s mass but occupies only a tiny part of the atom’s space. Enlarge an atom up to the size of Yankee Stadium and the equally magnified nucleus is only the size of a baseball.

Electrons, tiny particles that have a negative charge, whirl through the rest of the space inside atoms. Electrons travel in complex orbits and exist only in certain specific energy states or levels (see Quantum Theory). Electrons can move from a low to a high energy level by absorbing energy. An atom with at least one electron that occupies a higher energy level than it normally would is said to be excited. An atom can become excited by absorbing a photon whose energy equals the difference between the two energy levels. A photon’s energy, color, frequency, and wavelength are directly related: All photons of a given energy are the same color and have the same frequency and wavelength.

Usually, electrons quickly jump back to the low energy level, giving off the extra energy as light (see Photoelectric Effect). Neon signs and fluorescent lamps glow with this kind of light as many electrons independently emit photons of different colors in all directions.

B Stimulated Emission
Lasers are different from more familiar sources of light. Excited atoms in lasers collectively emit photons of a single color, all traveling in the same direction and all in step with one another. When two photons are in step, the peaks and troughs of their waves line up. The electrons in the atoms of a laser are first pumped, or energized, to an excited state by an energy source. An excited atom can then be “stimulated” by a photon of exactly the same color (or, equivalently, the same wavelength) as the photon this atom is about to emit spontaneously. If the photon approaches closely enough, the photon can stimulate the excited atom to immediately emit light that has the same wavelength and is in step with the photon that interacted with it. This stimulated emission is the key to laser operation. The new light adds to the existing light, and the two photons go on to stimulate other excited atoms to give up their extra energy, again in step. The phenomenon snowballs into an amplified, coherent beam of light: laser light.

In a gas laser, for example, the photons usually zip back and forth in a gas-filled tube with highly reflective mirrors facing inward at each end. As the photons bounce between the two parallel mirrors, they trigger further stimulated emissions and the light gets brighter and brighter with each pass through the excited atoms. One of the mirrors is only partially silvered, allowing a small amount of light to pass through rather than reflecting it all. The intense, directional, and single-colored laser light finally escapes through this slightly transparent mirror. The escaped light forms the laser beam.

Albert Einstein first proposed stimulated emission, the underlying process for laser action, in 1917. Translating the idea of stimulated emission into a working model, however, required more than four decades. The working principles of lasers were outlined by the American physicists Charles Hard Townes and Arthur Leonard Schawlow in a 1958 patent application. (Both men won Nobel Prizes in physics for their work, Townes in 1964 and Schawlow in 1981). The patent for the laser was granted to Townes and Schawlow, but it was later challenged by the American physicist and engineer Gordon Gould, who had written down some ideas and coined the word laser in 1957. Gould eventually won a partial patent covering several types of laser. In 1960 American physicist Theodore Maiman of Hughes Aircraft Corporation constructed the first working laser from a ruby rod.

III TYPES OF LASERS
Lasers are generally classified according to the material, called the medium, they use to produce the laser light. Solid-state, gas, liquid, semiconductor, and free electron are all common types of lasers.

A Solid-State Lasers
Solid-state lasers produce light by means of a solid medium. The most common solid laser media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the rods are fashioned into two parallel surfaces coated with a highly reflecting nonmetallic film. Solid-state lasers offer the highest power output. They are usually pulsed to generate a very brief burst of light. Bursts as short as 12 × 10-15 sec have been achieved. These short bursts are useful for studying physical phenomena of very brief duration.

One method of exciting the atoms in lasers is to illuminate the solid laser material with higher-energy light than the laser produces. This procedure, called pumping, is achieved with brilliant strobe light from xenon flash tubes, arc lamps, or metal-vapor lamps.

B Gas Lasers
The lasing medium of a gas laser can be a pure gas, a mixture of gases, or even metal vapor. The medium is usually contained in a cylindrical glass or quartz tube. Two mirrors are located outside the ends of the tube to form the laser cavity. Gas lasers can be pumped by ultraviolet light, electron beams, electric current, or chemical reactions. The helium-neon laser is known for its color purity and minimal beam spread. Carbon dioxide lasers are very efficient at turning the energy used to excite their atoms into laser light. Consequently, they are the most powerful continuous wave (CW) lasers—that is, lasers that emit light continuously rather than in pulses.

C Liquid Lasers
The most common liquid laser media are inorganic dyes contained in glass vessels. They are pumped by intense flash lamps in a pulse mode or by a separate gas laser in the continuous wave mode. Some dye lasers are tunable, meaning that the color of the laser light they emit can be adjusted with the help of a prism located inside the laser cavity.

D Semiconductor Lasers
Semiconductor lasers are the most compact lasers. Gallium arsenide is the most common semiconductor used. A typical semiconductor laser consists of a junction between two flat layers of gallium arsenide. One layer is treated with an impurity whose atoms provide an extra electron, and the other with an impurity whose atoms are one electron short. Semiconductor lasers are pumped by the direct application of electric current across the junction. They can be operated in the continuous wave mode with better than 50 percent efficiency. Only a small percentage of the energy used to excite most other lasers is converted into light.

Scientists have developed extremely tiny semiconductor lasers, called quantum-dot vertical-cavity surface-emitting lasers. These lasers are so tiny that more than a million of them can fit on a chip the size of a fingernail.

Common uses for semiconductor lasers include compact disc (CD) players and laser printers. Semiconductor lasers also form the heart of fiber-optics communication systems (see Fiber Optics).

E Free Electron Lasers.
Free electron lasers employ an array of magnets to excite free electrons (electrons not bound to atoms). First developed in 1977, they are now becoming important research instruments. Free electron lasers are tunable over a broader range of energies than dye lasers. The devices become more difficult to operate at higher energies but generally work successfully from infrared through ultraviolet wavelengths. Theoretically, electron lasers can function even in the X-ray range.

The free electron laser facility at the University of California at Santa Barbara uses intense far-infrared light to investigate mutations in DNA molecules and to study the properties of semiconductor materials. Free electron lasers should also eventually become capable of producing very high-power radiation that is currently too expensive to produce. At high power, near-infrared beams from a free electron laser could defend against a missile attack.

IV LASER APPLICATIONS

The use of lasers is restricted only by imagination. Lasers have become valuable tools in industry, scientific research, communications, medicine, the military, and the arts.

A Industry
Powerful laser beams can be focused on a small spot to generate enormous temperatures. Consequently, the focused beams can readily and precisely heat, melt, or vaporize material. Lasers have been used, for example, to drill holes in diamonds, to shape machine tools, to trim microelectronics, to cut fashion patterns, to synthesize new material, and to attempt to induce controlled nuclear fusion (see Nuclear Energy).
Highly directional laser beams are used for alignment in construction. Perfectly straight and uniformly sized tunnels, for example, may be dug using lasers for guidance. Powerful, short laser pulses also make high-speed photography with exposure times of only several trillionths of a second possible.

B Scientific Research
Because laser light is highly directional and monochromatic, extremely small amounts of light scattering and small shifts in color caused by the interaction between laser light and matter can easily be detected. By measuring the scattering and color shifts, scientists can study molecular structures of matter. Chemical reactions can be selectively induced, and the existence of trace substances in samples can be detected. Lasers are also the most effective detectors of certain types of air pollution. (see Chemical Analysis; Photochemistry).

Scientists use lasers to make extremely accurate measurements. Lasers are used in this way for monitoring small movements associated with plate tectonics and for geographic surveys. Lasers have been used for precise determination (to within one inch) of the distance between Earth and the Moon, and in precise tests to confirm Einstein’s theory of relativity. Scientists also have used lasers to determine the speed of light to an unprecedented accuracy.

Very fast laser-activated switches are being developed for use in particle accelerators. Scientists also use lasers to trap single atoms and subatomic particles in order to study these tiny bits of matter (see Particle Trap).

C Communications
Laser light can travel a large distance in outer space with little reduction in signal strength. In addition, high-energy laser light can carry 1,000 times the television channels today carried by microwave signals. Lasers are therefore ideal for space communications. Low-loss optical fibers have been developed to transmit laser light for earthbound communication in telephone and computer systems. Laser techniques have also been used for high-density information recording. For instance, laser light simplifies the recording of a hologram, from which a three-dimensional image can be reconstructed with a laser beam. Lasers are also used to play audio CDs and videodiscs (see Sound Recording and Reproduction).

D Medicine
Lasers have a wide range of medical uses. Intense, narrow beams of laser light can cut and cauterize certain body tissues in a small fraction of a second without damaging surrounding healthy tissues. Lasers have been used to “weld” the retina, bore holes in the skull, vaporize lesions, and cauterize blood vessels. Laser surgery has virtually replaced older surgical procedures for eye disorders. Laser techniques have also been developed for lab tests of small biological samples.

E Military Applications
Laser guidance systems for missiles, aircraft, and satellites have been constructed. Guns can be fitted with laser sights and range finders. The use of laser beams to destroy hostile ballistic missiles has been proposed, as in the Strategic Defense Initiative urged by U.S. president Ronald Reagan and the Ballistic Missile Defense program supported by President George W. Bush. The ability of tunable dye lasers to selectively excite an atom or molecule may open up more efficient ways to separate isotopes for construction of nuclear weapons.

V LASER SAFETY
Because the eye focuses laser light just as it does other light, the chief danger in working with lasers is eye damage. Therefore, laser light should not be viewed either directly or reflected.

Lasers sold and used commercially in the United States must comply with a strict set of laws enforced by the Center for Devices and Radiological Health (CDRH), a department of the Food and Drug Administration. The CDRH has divided lasers into six groups, depending on their power output, their emission duration, and the energy of the photons they emit. The classification is then attached to the laser as a sticker. The higher the laser’s energy, the higher its potential to injure. High-powered lasers of the Class IV type (the highest classification) generate a beam of energy that can start fires, burn flesh, and cause permanent eye damage whether the light is direct, reflected, or diffused. Canada uses the same classification system, and laser use in Canada is overseen by Health Canada’s Radiation Protection Bureau.

Goggles blocking the specific color of photons that a laser produces are mandatory for the safe use of lasers. Even with goggles, direct exposure to laser light should be avoided.

Last edited by Last Island; Sunday, December 30, 2007 at 07:35 PM.
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