PHOTOCELL, IT'S WORKING AND IT'S USES

A photocell is a technological application of the photoelectric effect. It is a device whose electrical properties are affected by light. It also some times called an electric eye. A photocell consists of a semi cylindrical photo sensitive metal plate c (emitter) and a wire loop A (collector) supported in an evacuated glass or quartz bulb. It is connected to the external circuit having a high tension battery B and micro ammeter as shown in the figure.

Sometime instead of the plate  C, a thin layer of photosensitive material is pasted on the inside of the bulb. A part of the bulb is left clean for the light to enter it. When light of suitable wavelength falls on the emitter C, photo electrons are emitted. These photo electrons are drawn to the collector A. Photo current of the order of a few micro ampere can be normally obtained from a photo cell.

A photocell converts a change in intensity of illumination into a change in photo current. This current can be used to operate control systems  and in light measuring devices. A photocell of lead  sulphide sensitive to infrared radiation is used in electric ignition circuits.

In scientific work, photo cells are used whenever it is necessary to measure the the intensity of light. Light meters in photographic cameras make use of photo cells to measure the intensity of incident light. The photocells, inserted in the door light electric circuit, are used as automatic door opener. A person approaching a doorway may interrupt a light beam which is incident on a photocell. The abrupt change id photo current may be used to start a motor which opens the door or rings an alarm. They are used in the control of a counting devices which records every interruption of the light beam caused by a person or object passing across the beam. So photocells help count the person entering an auditorium, provided they enter the hall one by one. They are used for detection of traffic law defaulters: an alarm may be surrounded whenever a beam of (invisible) radiation is intercepted.

In burglar alarm, (invisible) ultraviolet light is continuously  made to fall on a photocell installed at the door. A person entering the door interrupts the beam falling on the photocell. The abrupt change is photo current is used to start  an electric bell ringing. In fire alarm, a number of photocells are installed at suitable places in a building. In the event of breaking out of fire, light radiations fall upon the photocell. This completes the electric circuit through an electric bell or a siren which starts operating as a warning signal.

Photocells are used in the reproduction of sound in motion pictures and in television camera for scanning and telecasting scenes. They are used in industries for detecting minor flaws or holes in metal sheets. 

PARTICLE MODEL OF LIGHT

P a R T i C l E    M o d E l    o F     L i G H t 

Newton's fundamental contributions to mathematics, mechanics, and gravitation often blind us to his deep experimental and theoretical study of light. He made pioneering contributions in the field of optics. He further developed the corpuscular model of light proposed by Descartes. It presumes that light energy is concentrated in tiny particles called corpuscles. He further assumed that corpuscles of light were mass less elastic particles. With his understanding of mechanics, he could come up with a simple model of reflection and refraction. It is a common observation that a ball bouncing from a smooth plane surface obeys the laws of reflection. When this is an elastic  collision, the magnitude of the velocity  remains the same. As the surface is smooth, there is no force acting parallel to the surface, so the component of momentum in this direction also remains the same. Only the component perpendicular to the surface, i.e., the normal component of the momentum, gets reversed in reflection. Newton argued that smooth surfaces like mirrors reflect the corpuscles in a similar manner.

In order to explain the phenomena of refraction. newton postulated that the speed of the corpuscles was greater in water or glass than in air. How ever, later on it was discovered that the speed of light is less in water or glass than in air.

In the field of optics, Newton the experimenter, was greater than Newton the theorist. He himself observed many phenomena, which were difficult to understand in terms of particle nature of light. For example, the colours observed due to a thin film of oil on water. Properly of partial reflection of light is yet another such example. Everyone who looked into the water in a pond sees image of the face in it, but also sees the bottom of the pond. Newton argued that some of the corpuscles, which fall on the water, get reflected and some get transmitted. But the property could distinguish these two kinds of corpuscles ? Newton had to postulate some kind of unpredictable, chance phenomenon, which decided whether an individual corpuscle would be reflected or not. in explaining other phenomena, how ever, the corpuscles were presumed to behave as if they are identical, Such a dilemma does not occur in the wave picture of light. An incoming wave can be divided into two weaker waves at the boundary between air and water.

HISTORICAL NOTE OF DETERMINANTS

The Chinese method of representing the coefficients of the unknown of several linear equations by using rods on a calculating board naturally led to the discovery of simple method of elimination. the arrangement of rods was precisely that of the numbers is determinants. the Chinese, therefore early developed the idea of subtracting   columns and rows as in simplification of a determinant 'Mikami, China, pp 30, 93.

Seki Kowa, the greatest of the Japanese Mathematicians of seventeenth century in his work 'Kai Fukudai no Ho'  in 1683 showed that he had the idea of determinants and of their expansion. But he used this device only in eliminating a quantity from two equations and not  directly in the solution of a set of simultaneous linear equations. 'T. Hayashi, " The Fakudoi and Determinants in Japanese Mathematics," in the proc. of the Tokyo Math. Soc., V,

Vendermonde  was the first to recognize determinants as independent functions. He may be called the formal founder. Laplace (1772), gave general method of expanding a determinant in terms of its complementary minors. In 1773 Lagrange treated determinants of the second and third orders and used them for purpose other than the solution of equations. In 1801, Gauss used determinants in his theory of numbers.


The next contributor was Jacques-Philippe- Marie Binet (1812) who stated the theorem relating to the product of two matrices of m-columns and n-rows, which for the special case of m=n reduces to the multiplication theorem more satisfactory then Binet's 

The greatest contributor to the theory was Carl Gustav Jacob Jacobi, after this the word determinats received its final acceptance.

QUATERNARY PERIOD

THE QUATERNARY PERIOD (1.6 million years ago present) forms the second part of Cenozoic era (65 million years ago present): it has been characterized by altering cold (glacial) and warm (inter glacial) periods. During cold periods, ice sheets and glaciers have formed repeatedly on northern and southern continents. The cold environment in North America and Eurasia, and to a lesser extent in South America and parts of Australia, have caused the migration of many life forms towards the Equator. Only the specialized ice age mammals such as Mammuthus and Coelodonta, with their thick wool and fact insulation, were suited to life in very cold climates. Humans developed throughout the Pleistocene period (1.6 million-10,000 years ago) in Africa and migrated northward into Europe and Asia. Modern humans, Homo sapiens, lived on the cold European continent 30,000 years ago and hunted mammals. The end of the last ice age and the climatic changes that occurred about 10,000 years ago brought extinction to many Pleistocene mammals, but enabled humans to flourish.

TERTIARY PERIOD

Following the demise of the dinosaurs at the end of the cretaceous period, the tertiary period (65-1.6 million years ago), which formed the first part of the Cenozoic era (65 million years ago present), was characterized by a huge expansion of mammal life, Placental mammals nourish  and maintain the young in the mother's uterus; only three orders of placental mammals existed during cretaceous times, compared with 25 orders during the tertiary period. One of these 25 included the first hominid, Australopithecus, which appeared in Africa. By the beginning of the tertiary period, the continents had almost reached their present  position. The  Tethys sea, which had separated the northern continents from Africa and India, began to close up, forming the Mediterranean sea and allowing migration  of terrestrial animals between  Africa and Western Europe. India's collision with Asia led to the formation of the Himalayas. During the middle part of the tertiary period, the forest dwelling and browsing mammals were replaced by mammals such as the horse, better suited to grazing the open savannahs that began to dominate. Repeated cool periods throughout the tertiary period established the Antarctic as an icy island continent. 

GUITARS

G u I T a R s

The guitars is a plucked stringed instruments. There are two types of guitar acoustic and electric. Acoustic guitars have hollow bodies and six or twelve strings. Plucking the strings produces vibrations that are amplified by their hollow bodies. Electric guitars usually have solid bodies and six strings. Pick ups placed under the strings convert their vibration into electric signals that are magnified by an amplifier, and sent to a loud speaker where they are converted into sounds. Electric bass guitars are very similar in structure to electric guitars and produce sound in the same way, but have four strings and play bass notes.

CRETACEOUS PERIOD

The Mesozoic era ended with the cretaceous period, which lasted from 146 to 65 million years ago. During this period, Gondwanaland and Laurasia were breaking up into smaller land masses that more closely resembled those of the modern continents. the climate remained mild and moist but the seasons becomes more marked. Flowering plants, including deciduous trees, replaced many cycads, seed ferns and conifers. Animal species become more varied, with the evolution of new mammals, insects, fish, crustaceans and turtles. Dinosaurs  evolved into a wide variety of species during Cretaceous times; more than half of all known dinosaurs including Iguanodon, Deinonychus, Tyrannosaurus and Hypsilophodon lived during this period. At the end of the Cretaceous period, however dinosaurs become extinct. The reason for this mass extinction is unknown but it is thought to have been caused by claimatic changes due to either a catastropic meteor impact with the Earth or extensive volcanic eruptions.

TRIASSIC PERIOD

The Triassic period (245-208 million years ago)  marked the beginning of what is known as the age of the Dinosaurs ( the Mesozoic era). During this period, the present day continents were massed together, forming one huge continent known as Pangaea. This land mass experienced extremes of climate, with lush green areas around the coast or by lakes and rivers, and arid deserts in the  interior. The only forms of plant life were non flowering plants, such as conifers, ferns, cycads, and ginkgos; flowering plants had not yet evolved. The principle forms of  animal life included primitive amphibians, rhynchosaurs ("beaked lizards"), and primitive crocodilians. Dinosaurs first appeared about 230 million years ago, at the beginning of the  Late Triassic period. The earliest known dinosaurs were the carnivorous (flesh eating) herrerasaurids and staurikosaurids,  such as Herrerasaurus and  Stauriksaurus. Early herbivorous (plant eating) dinosaurs first appeared in Late Triassic times include Plateosaurus and Technosaurus. By the end of the Triassic period, dinosaurs dominated Pangaea, possibly contributing to the extinction of many other reptiles.

CARBONIFEROUS TO PERMIAN PERIODS

The carboniferous period (363-290 million years ago) takes its name from the thick, carbon rich layers now coal that were produced during this period as swampy tropical forests were repeatedly  drowned by shallow seas. The humid climate across northern and equatorial continents throughout Carboniferous times produced the first dense plant cover on Earth. During the early part of this period, the first reptiles appeared. Their development of a waterproof egg with a protective internal structure ended animal life's dependence on an aquatic environment. Towards the end of Carboniferous times, the Earth's continents Laurasia and Gondwanaland collided, resulting in the huge land mass of Pangaea. Glaciers smothered much of the southern hemisphere during the Permian period (290-245 million years ago), covering Antarctica, parts of Australia and much of South America, Africa and India. Ice locked up much of the world's water and large areas of the northern hemisphere experienced a drop in sea level. Away from the poles, deserts and a hot dry climate predominated. As a result of these conditions, the Permian period ended with the greatest mass extinction of life on Earth ever.

PRECAMBRIAN TO DEVONIAN PERIODS

When the Earth formed about 4,600 million years ago, its atmosphere consists of volcanic gases with little oxygen, making it hostile to most forms of life. One large  super continent, Gondwanaland, was situated over the southern polar region, While other smaller continents were spread over the rest of the World. Constant movement of the Earth's crustal plates carried continents across the Earth's surface. The first primitive life forms emerged around 3,400 million years ago in shallow, warm seas. The build up of oxygen began to form a shield of ozone around the Earth, protecting living organisms from the sun's harmful rays and helping to establish an atmosphere in which life could sustain itself. The first vertebrates appeared  about 470 million years ago, during the Ordovician period (510-439 million years ago), the first land plants appeared around 400 million years ago during the Devonian period (409-363 million years ago), and the first land animals about 30 million years later.

THE CHANGING EARTH

The Earth formed from a cloud of dust and gas drifting through space about 4,600 million years ago. Dense minerals sank to the center while lighter ones formed a thin rocky crust. How ever, the first  known life forms bacteria and blue green algae did not appear until about 3,400 million years ago, and it was only about 700 million years ago that more complex plants and animals began to develop. Since then, thousands of animal and plant species have evolved; some, such as the dinosaurs, survived for many million of years, while others died out quickly. The Earth itself is continually changing. Although continents neared their present locations about 50 million years ago, they are still drifting slowly over the planet's surface, and mountain ranges such as the Himalayas which began to form 40 million years ago are continually begin built up and worn away. Climate is also subject to change: the Earth has under gone a series of ice ages interspersed with warmer periods (the most recent glacial period was at its height about 20,000 years ago)

THE FIVE BASIC CONCEPTS OF OOP (OBJECT ORIENTED PROGRAMMING) WITH DEFINITION

The general concepts of OOP are given below.

1. Data abstraction
2. Data encapsulation
3. Modularity
4. Inheritance
5. Polymorphism

Data abstraction :  Abstraction refers to the act of representing essential features without including the background details or explanation.

Data encapsulation:  Placing data and functions together is central idea of object oriented programming. This is known as encapsulation

Modularity:  The act of partitioning a program into individual units is called modularity.

Inheritance: Inheritance is the capability of one class to inherit properties from another class.

Polymorphism: Polymorphism is the ability for a message or data to be processed in more than one form. The same operation differently depending upon the type of data it is working with.

LASER LIGHT

Imagine a crowded market place or a railway platform with people entering a gate and going towards all directions. Their footsteps are random and there is no phase correlation between them. On other hand, think of a large number of soldiers in a regulated march. Their footsteps are very well correlated. See figure here.

This is similar to the difference between light emitted by an ordinary source like a candle or a bulb and that emitted by a laser. The acronym LASER stands for light  Amplification by stimulated Emission of Radiation. Since its development in 1960, it has entered into all areas of science and technology. It has found applications in physics, chemistry, biology, medicine, surgery, engineering, etc. There are low power lasers, with a power of  0.5 mW, called pencil lasers, which serve as pointers. There are also lasers of different power, suitable for delicate surgery of eye or glands in the stomach. Finally, there are lasers which can cut or weld steel.

Light emitted from a source in the form of packets of waves. Light coming out from an ordinary source contains a mixture of many wavelengths. There is also no phase relation between the various waves. There fore, such light, even if it is passed through an aperture, spreads very fast and the beam size increases rapidly with distance. In the case of laser light, the wave length of each packet is almost the same. Also the average length of packet of waves is much larger. This means that there is better phase correlation over a longer duration of time. This results in reducing the divergence of laser beam substantially.

If there are N atoms in a source, each emitting light with intensity I, then the total intensity produced by an ordinary source is proportional to NI, where as in a laser source, it is proportional to NNI (NN means N square). Considering that N is very large, we see that the light from laser can be much stronger than that from an ordinary source.

When astronauts of the Apollo  missions visited the moon, they place a mirror on its surface, facing the Earth. Then scientists on the Earth sent a strong laser beam, which was reflected by the mirror on the moon and received back on the Earth. the size of the reflected laser beam and the time taken for round trip were measured. This allowed a very accurate determination of (a) the extremely small divergence of a laser beam and (b) the distance of the moon from the earth.

FASTER AND SMALLER: THE FUTURE OF COMPUTER TECHNOLOGY

The integrated chip (IC) is at the heart of all computer system. In fact ICs are found in almost  all electrical devices like cars, televisions, CD players, cell phones etc. The miniaturisation that made the modern personal computer possible could never have happened without the IC. ICs are electronic devices that contain many transistors, resistors, capacitors, connecting wires - all in one package. You must have heard of the microprocessor.  The microprocessor is an IC that processes all information in a computer, like keeping track of what keys are pressed, running programmes, games etc. The IC was first invented by Jack Kilky at Texas instruments in 1958 and he was awarded Nobel Prize for this in 2000. ICs are produced on a piece of  semiconductors  crystal (or chip) by a process called photolithography. Thus the entire information technology (IT) industry hinges on semiconductors. Over the years, the complexity of ICs has increased while the size of its features continued to shrink. In the past five decades, a dramatic miniaturisation in computer technology had made modern day computers faster and smaller. In the 1970s, Gordon Moore, co-founder of INTEL, pointed out that the memory capacity of a chip (IC) approximately doubled every one and half years. This is popularly known as Moore's law. The number of transistors per chip has risen exponentially and each year computers are becoming more powerful, yet cheaper than the year before. It is intimated from current trends  that the computers available in 2020 will operate at 40 GHz (40,000 MHz) and would be smaller, more efficient and less expensive than present day computers. The explosive growth in the semiconductors industry and computer technology is best expressed by a famous quote from Gordon Moore: "if the auto industry advanced as rapidly as the semiconductors industry, a Rolls Royce would get half million miles per gallon, and it would be cheaper to throw it away than to park it."

ALBERT EINSTEIN ( 1879 - 1955 )

ALBERT EINSTEIN ( 1879 - 1955 ) Einstein, one of the greatest physicists of all time, was born in Ulm, Germany. In 1905, he  published three path breaking papers. In the first paper, he  introduced the notation of light quanta ( now called photons) and used it to explain the features of photoelectric effect. In second paper, he developed a theory of Brownian motion, confirmed experimentally a few years later and provided a convincing evidence of the atomic picture of matter. The third paper gave birth to the special theory of relativity. In 1916, he  published the general theory of relativity. Some of Einstein,s most significant later contribution's are: the notion of stimulated emission introduced in an alternative derivation of Planck's black body radiation law, static model of the universe which started modern cosmology, quantum statistics of a gas of massive bosons, and a critical analysis of the foundations of quantum mechanics. In 1921, he was awarded the Nobel Prize in physics for his contribution to theoretical physics and the photoelectric effect. 

THOMAS YOUNG ( 1773 - 1829 )

THOMAS YOUNG ( 1773 - 1829 ) English physicist, physician and Egyptologist. Young worked on a wide variety of scientific problems, ranging from the structure of the eye and mechanism of vision to the decipherment of the Rosetta stone. He revived the wave theory of light and recognised that interference phenomena provide proof of the wave properties of light.

CHRISTIAAN HUYGENS ( 1629-1695 )

CHRISTIAAN HUYGENS ( 1629-1695 ) Dutch physicist, astronomer, mathematician and the founder of the wave theory of light. His book, Treatise on light, makes fascinating reading even today. He brilliantly explained the double refraction shown by the mineral calcite in this work in addition to reflection and refraction. He was the first to analyse circular and simple harmonic motion and designed and built improved clocks and telescopes. He discovered the true geometry of Saturn's rings.

EARTH'S MAGNETISM AND DYNAMIC EFFECT

     


 A freely suspended magnetic needle comes to rest pointing approximate north - south direction. This is because of an external magnetic field - the magnetic field of earth. Earth behaves as a magnet whose field is similar to that of a huge bar magnet and is of the order of  10 raised to -5 T.

The source of geomagnetism is not an exact single one. A reasonable guess is that due to the electrical current produced by convective motion of metallic fluids ( consisting mostly of molten iron and nickel) in the outer core of the earth. This is known as the dynamic effect.