The theory of relativity was put forth by Albert Einstein in two parts. The first part called the special relativity theory was published in 1905. It stipulates that the velocity of light in vacuum is constant irrespective of the relative velocity of the source and the observer. As a consequence, the measurement of length is dependent upon the relative velocities of the measuring rod and the observer.
For example, to a stationary observer on a railway platform, a train moving with a speed approaching that of light appears shrunk lengthwise. At the same time, to a passenger looking through the window on the train, the railway platform and the observer appear shrunk. But, to both the observer on the platform and the train passenger the immediate surroundings appear normal.
The second part of this theory published in 1915 is called the general relativity theory. It states that the gravitational attraction between two objects is not due to an attractive force but because of a curvature produced in space-time continuum by mass.
Remote sensing is a technique of collecting information about the earth’s natural resources using aircraft or satellites. The basic tool used here is photography — infrared and false colour photography in particular. Modern remote sensing satellites are equipped with special cameras which take photographs of any particular region of the earth’s surface in several colours simultaneously. These separate images can be electronically processed to produce multi-colour images and maps outlining areas of specific interest such as diseased or pest infected crops, forest areas, rich fishing areas in seas, even underground water and mineral reserves. Remote sensing is an indispensable tool for better monitoring and management of natural resources.
Scuba is an acronym for Self-Contained Underwater Breathing Apparatus. It consists of a portable breathing device that can be carried by free swimming divers. Basically it is nothing but a mouthpiece joined to one or two tanks of compressed air which are strapped to the diver’s back. The equipment makes it possible for divers to venture deeper into the waters than they would have been able to without it.
SETI is an acronym for Search for Extra Terrestrial Intelligence. The possibility that intelligent life might exist elsewhere in the universe arises from the fact that the universe is made up of billions and billions of galaxies each containing billions of stars. Even if small fractions of this large number of galaxies have planets with conditions suitable for the appearance of life, there could be several million planets with intelligent life capable of communicating with us using radio signals which are the simplest media of communication. Also radio waves are capable of travelling across astronomical distances carrying substantial information.
The solar wind is a stream of charged particles flowing out from the sun’s outer atmosphere or corona. It blows at speeds of about 400-500 kilometres per second and on an average takes 4-5 days to reach the vicinity of the earth. It consists mainly of elementary particles such as protons and electrons in roughly equal numbers, together with heavier particles such as alpha particles (helium nuclei). Violent events in the sun’s atmosphere such as flares and prominences send bursts of high speed particles singing through the slower moving wind. The sun loses more than a million tonnes of matter per second into the solar wind. This may appear to be alarming, but it is an infinitesimal quantity compared to the mass of the sun and so there is no danger of the fading away of the sun due to solar winds.
Sonar, an acronym for Sound Navigation and Ranging, is a remote sensing device used for detecting and locating sound waves under water. It has many uses. Naval uses include detection of submarines, and underwater navigation. Civilian uses include water depth measurement, locating fish schools and mapping the ocean floor. It is also used for locating objects such as pipe lines or ship wrecks lying on the ocean floor, for measuring water current profiles and determining characteristics of ocean bottom sediments.