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.