What is Ozone hole?
There is a thin layer of ozone that circles the earth at a height of 15-50 km. Its protective presence shields life by absorbing the high energy ultraviolet radiation of the sun. In 1987, a thinning of this layer was first observed over Antarctica. This was referred to as the ‘ozone hole’.The hole is formed by the destruction of ozone molecules by certain man-made substances that liberate chlorine atoms when exposed to ultraviolet radiation in the stratosphere. The chlorine atoms then react with ozone molecules breaking them up. In recent years the ozone hole has been reported over the Arctic as well. Of the many chemicals blamed for causing the ozone hole, the chlorofluorocarbons, widely used by the refrigeration industry have been held primarily responsible.
It is a thin, cylindrical cable made of glass fibre used for transmitting light signals. Light is propagated down an optical fibre by refraction and internal reflection. There are two types of fibres, one in which the central core has uniform refractive index and the other where there is a gradual change in the refractive index decreasing outward from the centre of the fibre. The core of an optical fibre is surrounded by a layer called the ‘cladding’ which has a uniform refractive index but of magnitude less than that of the core. The outside of an optical fibre is generally coated with a dark coloured plastic sleeving which protects the fibre.
In nuclear reactors a number of radioactive waste products are produced as the fuel is used up. Prominent among them are plutonium, strontium, cesium and barium which are produced by fission of uranium and neutron irradiation.
Nitrogen is essential for plants and animal life as it is a constituent of major bimolecules like proteins and nucleic acids. It is available in plenty in the atmosphere. However, for plants to be able to use it efficiently, it has to be converted into some nitrogenous compounds. This conversion is called nitrogen fixation. Nitrogen in the combined form is also necessary for the manufacture of explosives and chemical fertilizers.
A change in the structure of a gene resulting in an altered gene product, that is a protein, is called mutation. Most mutations have harmful effects in an organism. But a few may result in changes which can make that organism better adapted to its environment. In that case, natural selection acts to increase the frequency of the advantageous trait leading to evolution.
It is a phenomenon caused by disturbances in the earth’s magnetic field. It gives rise to spurious currents in submarine cables and interferes with radio communication. The disturbance in earth’s magnetic field is caused by the sudden short-lived brightening of the sun’s surface called solar flare.
MRI or the Magnetic Resonance Imaging is a technique for obtaining images of organs for medical diagnosis without having to resort to surgery. The technique uses the phenomenon of Nuclear Magnetic Resonance (NMR). Through MRI it is possible to differentiate between the various types of soft tissues. This gives it an edge over X-ray photography. For obtaining the images, the property of certain substances which selectively absorb energy when irradiated by electromagnetic radiation is used. The nuclei of certain atoms behave like tiny magnets. When substances with such atoms are placed in a magnetic field and continuously irradiated by electromagnetic waves of varying frequency, there comes a frequency called resonance frequency when a part of the electromagnetic energy is absorbed by the substances. The value of the resonance frequency is proportional to the intensity of the applied magnetic field and the nature of the substance that exhibits nuclear magnetism.
LPG, an acronym for Liquefied Petroleum Gas, is the most popular cooking fuel used in urban households. It is a mixture of gases, liquefied by compression, consisting of volatile and flammable hydrocarbons such as ethane, propane, butane and pentane. It is obtained either from crude oil or from natural gas. Crude oil, recovered from underground oil deposits, is a mixture of heavy hydrocarbons. In a petroleum refinery, this oil is heated and turned into a mixture of gases and liquids. This mixture is then passed into a huge tower called fractionating column which is hotter at the bottom and cooler at the top. The heavier hydrocarbons with large number of carbon atoms condense at higher temperatures at the bottom whereas the lighter components with fewer carbon atoms condense at a lower temperature at the top.
sandwiched between two thin glass sheets coated with a transparent conducting material such as tin oxide which acts as the electrodes. On the top and bottom of the glass sheets are placed two polarizing filters or polarizers turned 90° with respect to each other. Since in the liquid crystal film the polarized light is entering from the top by 90°, the light can pass through the second polarizer also, and when reflected from a mirror placed below the second polarizer, the display panel appears clear. When a small voltage is applied across one or more electrodes which are laid on the glass in the shape of alphabets, digits or symbols, the molecular alignment in the respective regions of the liquid crystal film is altered and they no more rotate the polarized light. This makes the characters on the display unit appear black on a clear background. Liquid crystal displays are quick acting and consume negligible power.