My Science School

Scientists have developed techniques through which gases like nitrogen, oxygen, hydrogen, and helium can be converted into liquids. These techniques involve cooling of the gas to a certain temperature called the critical temperature and then it is compressed to a very high pressure. Due to this cooling and compression the molecules of the gas come closer and the gas gets converted into a liquid. Air is a mixture of nitrogen, oxygen and other gases and can be liquefied by cooling it to about -200°C at normal atmospheric pressure. Under high pressure it can be liquefied at about -141°C.

The technique used for the liquefaction of air is shown above. Through this technique the air from the atmosphere is compressed to a high pressure. This air is then allowed to expand rapidly. As a result, the air gets cooled to a very low temperature. Its heat is lost due to the sudden expansion. This cool air is compressed further by which it gets converted into liquid.

Liquid air is very cool. It is a mixture of liquid oxygen which boils at -183 °C, liquid nitrogen which boils at -196°C and liquid argon which boils at -186 °C. It is bluish in colour and is kept in special vacuum flasks. It is mainly used in research laboratories to produce low temperatures.

Liquid hydrogen boils at -253°C. It is cooler than liquid air. Liquid helium is still cooler. It boils at -269°C. All the liquid gases should be handled with care. If they fall on your skin they may damage the body cells. If a rubber tube is inserted in liquid air it becomes as hard as a wooden stick. 

           Ordinary water does not remove dirt from things because grease and water do not mix. So soap is one of the most common cleansing agents used all over the world. People use soaps and detergents to clean their skin, clothes, utensils and many other objects. How does soap remove dirt?

          Soap is basically a fatty acid salt which can be obtained by boiling fats or oils together with an alkali. When oil is allowed to react with caustic soda solution, the chemical reaction produces soap and glycerin. Both are separated. When soap is applied on a cloth, its molecules break into fatty acid ions and sodium ions. Fatty acid ions are repelled by water but are attracted towards greasy dirt particles. They surround each grease molecule and remove it from the surface of the cloth. These are carried away by the water and consequently the cloth gets cleaned. Other actions, such as agitating, squeezing or rubbing and rinsing help loosen dirt and grease so that water may carry them away.

          Today, chemical cleaners called detergents are more and more in use instead of ordinary soaps. Detergents clean better than soaps in hard water, (the ‘hardness’ of the water is caused by the presence of calcium and magnesium salts. Soap does not make much lather in hard water) but they do not, by themselves, make suds. Suds are not necessary for cleaning but substances that make suds are added to detergents.

          Many substances are added to a crude soap to make it suitable for use as toilet soap. Coconut oil is added to make it lather quickly. Dyes, perfumes, water softeners and germicides, which are tiny substances that kill germs, are also added. 

               Optical microscopes cannot magnify more than about 2500 times because the light rays can not produce a sharp image. The electron microscope is such a powerful instrument which can magnify minute objects by as much as a million times. It is used to study micro-organisms such as viruses, tissues and bacteria. We know that light travels in the form of waves. Similarly, waves are also associated with the moving electrons. These are known as matter waves. Electron microscope was constructed by making use of matter waves associated with electrons. Wavelengths of light waves are longer than that of the waves associated with electrons. Due to this reason an electron microscope has a higher resolving power and greater magnifications as compared to an optical microscope.

               The electron microscope works like an optical microscope with a condenser and objective and eyepiece (projector) lenses. The lenses are powerful magnets or electrodes.

               In an electron microscope a beam of electrons is focussed onto the object. With the help of electromagnetic lenses an enlarged image of the object is produced on a fluorescent screen. This image is photographed on a photographic film or plate. With the help of this photograph the object structure is studied in detail. Most of the big research laboratories make use of electron microscopes.

 

 

            Photostat or xerography is a means of copying documents, letters or pages of books without using liquid inks. This could be called ‘dry writing’.

            A photostat machine makes use of static electricity. It relies upon the special properties of an element called selenium. When the light falls on selenium’s surface its electrical resistance drops sharply.

            To copy a page, the operator places it face down, upon a horizontal glass window. The button is pressed on the photocopier and a bright light comes on to light up the page. Its image is projected onto a highly polished selenium-coated cylindrical drum through a lens. The drum is charged with static electricity.

            The place where the light reflected from the white parts of the page falls on the selenium drum, the electrical resistance of the drum drops. Selenium’s charge leaks away to the ground. The light does not reach the drum from the black areas of the page. The drum on these areas retains the electrostatic charge. Now the drum is covered with a special powdered black ink. The ink adheres to the drum where there is still an electrostatic charge and so the image of the document gets formed in powdered ink on the selenium drum.

             Now a sheet of plain white paper is pressed close to the drum. The paper develops a charge opposite to that of the drum by induction. This charge attracts the ink powder from the drum. The ink jumps from the selenium drum to the paper, thus transferring the image to the paper. The paper is heated before it leaves the machine. This melts the ink which sticks permanently to the paper, giving a reproduction of the original document.

            In another type of electrostatic copiers, the image of the page is projected directly on to the paper, which charges its surface. The paper then passes through a bath of toner and the particles cling to the charged parts of the paper to produce the copy.

 

           Solar energy comes from the Sun to the Earth in the form of light and heat. It can be converted into electricity by solar cells. Today, scientists are engaged in developing new techniques for the benefit of mankind.

           Experiments have shown that when sunlight falls on certain metals like potassium and silicon, electrons are emitted from their surfaces. These electrons are known as photoelectrons and this phenomenon is called photoelectric effect. Photoelectrons so emitted can be used to produce electric current.

           Photoelectric effect has been used for the making of solar cells. A solar cell, in fact, is a device that directly converts solar energy into electric energy. These wafers of silicon element are used for making solar cells. Generally, a four centimetre long and two centimetre broad and 0.14 millimetre thick silicon wafer is used for one solar cell. When sunlight falls on this wafer it gets converted into electric energy. Each solar cell is capable of producing about a half volt of electricity.

           To produce useful quantities of electric current, it is necessary to join together a great number of solar cells. The cells are joined together in big panels and are placed in the sunlight. A panel of about 20,000 solar cells can produce 500 watts of electric power. The electricity produced thus can be used either immediately or can be used to charge electric storage batteries for later use. Materials like cadmium sulphide and gallium arsenide are also being used for making solar cells.

           Solar cells have several different uses. In sunny places, solar panels are used to provide boost power to telephone signals. In space where the supply of solar energy is plenty, solar cells are particularly useful. Spacecrafts, space-laboratories and satellites have a number of solar panels to provide power for their equipments.

          The biggest solar energy furnace of the world is in California, USA. It uses almost 2000 mirrors, which focus the sun rays on to a boiler unit on top of a tower. This produces steam which is used to drive electricity generating turbines. In India, The Bharat Heavy Electricals Ltd. (BHEL) and Central Electronics Ltd. (CEL) are making cells that have found many applications.

 

A camera is the instrument that will give us picture of a person or a scene called a photograph that can be kept for sweet memory. It has become one of the most important means of communication and expression in modern times. Basically, a camera is a dark box fitted with a lens on one side. Just behind the lens there is an aperture which controls the amount of light admitted. A shutter exposes the film to the light for a short interval. A photosensitive film is mounted on the opposite side of the lens. This film has a layer of silver bromide coated on it.

The object, whose photograph is to be taken, is focused on the film. This is done with the help of another lens called a view–finder. On opening the shutter the light from the object enters the box through the lens. An inverted image of the object is recorded on the photo film.

The exposed film is taken out of the camera and developed by a chemical process. For developing the film a solution of quinol and metal is used. During this process the portion on which more light has fallen becomes darker and the image of the object appears in reverse tones. The developed film is put in the hypo solution for fixing the image. That is how the negative of the object is obtained. In the negative white portions appear dark and black portions appear white.

Now with the help of an enlarger, a print or positive is made from the negative. The light coming through the negative is focused on the photosensitive bromide paper. This paper, after developing and fixing, takes the shape of a positive. That is how we get the photograph of an object.

There are a lot of developments taking place in the field of photography. Nowadays, the techniques of making coloured photographs are being used on a large scale. Instant cameras take photographs in a few seconds. These instant cameras are called Polaroid cameras. Some special cameras produce motion pictures like TV and video cameras, the pictures are made electrically. Holography has given birth to three dimensional photography in which we can see the length, breadth and thickness of the object.

          We make use of dry cells in our torches, transistors and cameras. As soon as we press the switch of the torch the filament of the bulb gets heated up and it begins to glow. Similarly, when we switch on our transistors sound is produced. These devices get their energy through the electric current produced by the dry cells fitted in them. There are three main types of dry cells: carbon-zinc, alkaline and mercury.

          A carbon-zinc dry cell consists of a cylindrical vessel made of zinc. A carbon rod with a brass cap is placed at the centre of this vessel. Zinc acts as the negative electrode and carbon as the positive electrode. A paste of ammonium chloride, zinc chloride, manganese dioxide and carbon is filled in the vessel around the carbon rod. The paste is further surrounded by another paste of plaster of Paris, ammonium chloride and zinc chloride. Plaster of Paris makes the paste hard. The zinc vessel is closed with a layer of pitch. Finally, the vessel is wrapped with a thin cardboard and we have a carbon-zinc dry cell. This cell is a lechlanche type of primary cell in which the electrolyte ammonium chloride is dissociated into positive and negative ions. These are attracted towards the electrodes and produce voltage. When they are connected with a wire, current begins to flow. In this reaction hydrogen gas is also produced and it is converted into water by manganese dioxide. These cells produce electricity till the whole of the manganese dioxide is used up to convert hydrogen into water. Manganese dioxide, after the reaction with hydrogen, gets converted into manganese oxide. After this conversion, no electricity is produced by the cell.

          An alkaline dry cell battery is more powerful and lasts eight times more than a carbon-zinc cell. It also has a carbon electrode and a zinc casing electrode. The electrolyte is a strong alkali solution which has potassium hydroxide. Alkaline dry cells are used mainly for portable radios.

          In a mercury dry cell, the voltage remains constant till the end of the life of the battery. A mercuric oxide electrode is used. The other electrode is the zinc casing. The electrolyte is potassium hydroxide.

          Why these cells are called dry cells? It is simply because its electrolyte is not in a solution form but in a paste form. It would become useless the moment the electrolyte dries up. Dry cells are adversely affected by high temperatures. Hence they should be stored in a cool place, away from direct sunlight. It is better to take out the cells from the torchlight, camera or transistor if they are not being used because moisture leaks out from the cell and it swells up. 

An airconditioner is an electrically operated device used to keep houses, offices, and laboratories cool during summer and warm during winter. It not only controls temperature but also regulates humidity.

Today, airconditioners of all types and sizes are available. The big airconditioner plants are capable of cooling or heating an entire building.

          In general, an airconditioner keeps the temperature between 20°C and 25.5°C and relative humidity around 35-70%. An airconditioner plant consists of a compressor and a cooling liquid like the Freon gas. The cooling liquid evaporates in the cooling coil. This vapour is then carried to the electrically-operated compressor. It then goes to the condenser where it is cooled by air or water as it passes through the radiator. Here the vapour changes to a liquid giving off heat in the process. The compressor thus serves to transfer heat from one place to another. A fan sends fresh air into the room which keeps the temperature of the room to the desired level. The airconditioner has certain substances which remove the moisture from the room. It also has filters to remove the dust particles from the air. This is how an airconditioner controls the temperature and humidity and keeps the air clean. Some airconditioners have attachments so when turned on we get hot air in the winters.

          Many new buildings, factories and homes are now being designed to include air-conditioning. Ships, aeroplanes, cars, offices, restaurants, theatres, shops and space vehicles make use of this steady flow of comfortable, purified air.

 

          A refrigerator is a cabinet used for keeping food and other substances cool and fresh. It has solved the problem of storing food for short periods, particularly in hot climates. The freezing compartment can store foods for months together.

          The working principle of a domestic refrigerator is shown in the figure. It is a compression and absorption refrigerator. Compression-and-absorption systems cool by changing a refrigerant from a liquid state to a gaseous state and back into the liquid state again.

          It consists of a storage tank, filled with Freon gas, the refrigerant control device, the evaporator, the compressor and the condenser. When the machine is switched on, Freon gas leaves the storage tank under high pressure. It travels through pipes of the refrigerant control device. These pipes act as evaporator. The liquid Freon absorbs heat from the pipes and gets evaporated. As the heat is drawn out, the temperature inside the cabinet and that of the food materials falls. Freon gas is now again converted into liquid by the condenser and the compressor. This cycle continues and the temperature inside the cabinet goes on falling. When the temperature inside the cabinet reaches an optimum value, a thermostat control cuts off the electric power and the machine stops. Again the temperature starts rising. When it reaches a certain mark, thermostat control again starts the machine. In this way a constant temperature is maintained and the food materials are preserved.

          Home refrigerators are sealed, airtight, leak proof, heavily insulated cabinets that maintain cooling temperatures between 0 and 4°C. Most of them have freezing compartments that can maintain temperatures between -18°C and -14°C. Either electricity or gas may be used to power refrigerators. 

          A vacuum cleaner is an electrically operated machine which is used to clean carpets, rooms, floors, book shelves and electronic instruments. It can conveniently remove dust and bits of paper. It can be handled by a single person and has proved very useful in cleaning homes, factories, stores, curtains, carpets, walls and cars.

          The inventors of household appliances in the late 1800s sometimes succeeded by trial and error method. The first electric cleaners actually blew out a stream of air to dislodge dust from carpets. Unfortunately dust went and settled down on everything else! In 1901 an inventor named H.C. Both had another idea. Why not make the cleaner suck and not blow? To see if the idea worked, he lay down on the floor and began sucking through a handkerchief. It worked, and H.C. Both built the first modern vacuum cleaner.

          A vacuum cleaner consists of a small electric motor to which a small fan is attached. The fan rotates at a very high speed. It sucks the dust in from one side and throws it out from the other. A cloth bag is fitted in the machine to collect the dirt. This bag is made in such a way that it allows the air to push through while even very tiny dust particles are trapped in. When the cleaner is turned on, the fan inside spins very fast. And as the cleaner’s nozzle is pushed along on the carpet, air rushes in taking the dust and dirt from the carpet along. These get trapped in the cloth bag and the carpet becomes clean.

          In fact, when the fan rotates, a partial vacuum is created inside the machine. That is why it is called a vacuum cleaner. Because of the partial vacuum, the pressure of the outside air pushes dust and small light-weight materials into the bag.

          Modern industrial vacuum cleaners are able to suck up rubbish including wood shavings, broken glass and even liquid. 

The term laser stands for “light amplification by stimulated emission of radiation”. A laser produces a very thin but very powerful beam of light, so powerful that it can drill a hole through the hardest substance – diamond. Unlike the light from a torch or a fluorescent tube, a laser beam does not spread much. Laser light contains light of only one colour, unlike sunlight which is a mixture of seven colours. Laser beams are highly monochromatic and coherent. The density of their energy also remains the same even over long distances. If comparisons can be made, ordinary light is like a crowd while the laser light is a well ordered army platoon.

The first laser was made in 1960 by an American scientist named Theodore H. Maiman. It was a ruby laser which produced a red beam of light, million times brighter than the sunlight. Today we have different types of lasers which produce different wave-lengths of different powers. Some of the well-known lasers are Ruby, Nel, Yah, He-Ne, Argon, Diode laser etc. A laser has three main parts. They are (1) the medium – the material that produces the beam (such as a ruby crystal or a gas); (2) the power source that energizes the medium and; (3) the resonator to make the beam more powerful (usually mirrors that reflect the light backwards and forwards to build up its strength).

Lasers are used by doctors to perform delicate surgeries, such as re-attaching a detached retina of the eye. The laser treatment is painless and the patient remains conscious during the operation. Laser beams are being used in the treatment of cancer and to stop tooth decay. A laser can be used by a surveyor as an infinitely tall ‘ranging pole’, from which to take bearings and measurements. The laser beam is fired vertically into the sky, and it provides an easily detected fixed point for other surveyors, even if hills and forests are in the way. Builders of skyscrapers use laser beams like plumb-lines to check that the building is vertical. Unlike an ordinary plumb-line, a laser beam cannot be blown by the wind. Because the laser beam is so straight and narrow that it makes an excellent measuring instrument for astronomers. The Apollo astronauts who landed on the Moon in 1969 left behind a laser reflector. Laser pulses were sending from the Earth to hit the reflector, and the beam bounced between the Earth and the Moon accurate to within 15 cms.

Laser beams are used by soldiers as range-finders, target designators, and guided missiles. Experiments are being conducted with lasers as space weapons, for destroying satellites and ballistic missiles.

Lasers are also being used in several fields of research such as Roman effect and interferometry. Lasers are being used in communications, computers, welding and drilling of metals, photography, stone removal from kidneys without operation, heart surgery, meteoreology etc. 

          John A. Larson, a medical student from California University developed a machine in 1921 which can detect lies with a fair degree of accuracy. This machine is known as ‘polygraph’ or a lie detector. In 1972, the American inventor Allan Bell developed another improved model of a lie dictator which was able to detect slight trembling in the voice which often occurs when a person tells a lie. Today, it is widely used in crime detection. How does this machine detect lies?

          When someone tells a lie, there occur certain physiological changes in his body. His heart begins to beat faster leading to rapid perspiration. His blood pressure and breathing pattern change. His voice also starts trembling. This machine is designed to record all these changes.

          When a person undergoes a lie-detector test, various parts of the machine are attached to his body by wires. The interrogator asks the person all relevant questions. The physiological changes taking place while questioning are recorded by a pen recorder fitted in the instrument. Finally, these observations are analyzed.

          However, the polygraph is not wholly dependable. Its accuracy is estimated to be about 80%. In fact, a lot depends on the skill of the examiner in this regard. Some people, who are truly unaware of the fact that they are lying, cannot be caught by the lie-detector. Moreover, the hardened criminals can control their emotions and render this machine ineffective. Because of these reasons the results of a lie-detector test cannot be accepted as evidence in the court of law. However, it is being used by the police departments in almost all the countries.

 

Holograms are amazing pictures that appear to have depth just like the real object. For example, if you look at a hologram of the front of a cube and then move to one side, you will see the sides of the cube just as if you were walking around it. Holograms are made with lasers.

An ordinary photograph records the variations in light intensity reflected from an object. Holography, however, records not only the intensity of light but also its phase. In an ordinary photograph, we get two dimensional images but in holography we get three dimensional images.

Holography is the means of making three dimensional pictures called holograms. It was invented by Dennis Gabon, a Hungarian born physicist who worked in Britain in the year 1948. He was awarded Nobel Prize for Physics in 1971. But the actual holography became possible only after laser was developed at the beginning of the 1960. The intense-coherent light of laser allowed high quality holograms to be made.

To make a hologram, the object is lit by light from a laser. This light reflects from the object and strikes a photographic plate placed nearby. At the same time, the laser beam is split so that it also reflects from mirrors and strikes the plate directly.

The plate is then developed. A black and white pattern appears on it, producing a holographic plate. When one side of this plate is lit up by a laser beam and it is viewed from the other side, an image of the object appears behind the plate. The image is in three dimensions just like the real object, but in the colour of the laser light. Projection holograms can be projected by lasers to appear in front of the holographic plate. There are also special holograms that can be seen by day light.

Holograms have proved very useful. They are used to record very small changes in the size of the objects. This is because they can record an object from many angles with a great accuracy. Scientists use them to study the growth of crystals or the build-up of dirt on old oil paintings. They can be used in computer memory systems to store information. In the field of industries holography is being used to prevent the pirating of the goods of big companies. In India also, some big companies are using the holograms to distinguish their original goods from the copied ones. 

          Man has been using artificial lights ever since he discovered fire. In the beginning, it used to be just the log fire, and then the wick on the edge of a stone bowl filled with animal’s fats and then the candles. A great step in this field was made by Thomas Alva Edison who invented the modern electric bulb in 1878. And by 1930, the scientists of America and Europe had developed the wonderful fluorescent tube.

          A fluorescent tube light consists of a long glass tube coated with a fluorescent material. Two tungsten electrodes are fitted at the end of this tube. The tube is evacuated and filled with a little amount of mercury and argon gas. When the tube is connected with the electric line, tungsten electrodes get heated up and start producing electrons. These electrons collide with mercury atoms and produce ultraviolet and visible light. Ultraviolet light, on colliding with fluorescent material, is turned into visible light. The colour of this light depends upon the fluorescent material.

         These tubes are coated with calcium tungstate to produce blue light. A coating of calcium silicate produces pink colour while that of zinc silicate produces green light.

         A fluorescent light is definitely an improvement on the ordinary bulb. Its electric consumption is very low but the light-producing efficiency is better than the bulb.

 

          A planetarium is a special type of building where a dumbbell shaped planetarium projector is used to recreate the night sky on the inside of the dome-shaped ceiling. It is a very complex instrument fitted with lenses, prisms and mirrors. It can not only show the stars almost exactly as they appear in the real sky, but can also show their movements from season to season or as they appear from different places. It can also duplicate the movements of the Sun, Moon, planets and other heavenly bodies in the sky. Modern planetariums are also equipped with slide projectors that can simulate events like space travel and landing on other planets. The planetarium is used to educate not only astronomers and navigators but also students about the Universe. It also provides spell bounding spectacle to a lay person.

         The first planetarium was built by Walter Bauersfeld of the Zeiss Optical Company of Jena (Germany) in 1923. The apparatus was kept at the centre of the dome. It could reproduce the real sky. The motions of the heavenly bodies taking place in one year could be reduced to an interval varying from several seconds to half an hour.

         Planetariums are useful because it is very difficult to learn about heavenly bodies just by looking at the sky since it is a very time consuming process to track the heavenly bodies due to pollution, clouds, etc. In a planetarium the motions of heavenly bodies taking place in a year can be watched in an interval of a few seconds.

          Today all big cities of the world have these dome-shaped buildings equipped with planetarium projectors to educate people. The biggest planetarium is in Moscow. It is located in a building with a diameter of 25.15 meters. The planetarium in San Francisco, USA, contains about 25,000 mechanical parts and weighs 2.5 metric tons. In our country there are planetariums in cities like Bombay, Calcutta, Jaipur, Delhi and Bangalore and thousands of people visit them everyday.