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What is Ionosphere? The ionized region of the earth’s upper atmosphere extending from about 40 km to the height of a few earth radii above the surface of the earth, is referred to as the ionosphere. The ionosphere is made up of electrons, and positive and negative ions in the background of neutral particles of the atmosphere. The concentration of the neutral particles is much greater than that of the charged species, so that the ionosphere is a weakly ionized medium.  The direct interactions of the atmospheric constituents, such as oxygen and nitrogen, with X-ray and ultraviolet radiation from the sun produce electrons and positive ions simultaneously in the ionosphere . A few electrons that are so produced, adhere to the neighbouring neutral particles to create negative ions. Also some electrons recombined with neighbouring positive ions to reproduce the original neutral particles. In this way the generation and the loss of electrons and ions in the ionosphere occur si

           Propagation of Radio waves Radio waves launched from a transmitting antenna travel outward and are not markedly affected by the surrounding atmosphere, rain, snow etc. These waves readily pass through nonmetallic substances but are greatly attenuated by metals. Their movement in free space is best, and no material medium is required for their propagation. Electromagnetic waves with frequencies extending from about 10 kHz to 300 GHz are classed as radio waves. These wave are subdivided into smaller ranges for convenience, as shown in Band -- Frequency range -- wavelength range – Principle of application Very low frequency (VLF) – 10 to 30 kHz – 30 to 10 km – Direct long range communication Low frequency (LF) – 30 to 300 kHz – 10 to 1 km – Marine, navigational aids Medium frequency (MF) – 300 kHz to 3 MHz – 1km to 100m – Broadcasting High frequency (HF) – 3 to 30 MHz – 100 to 10m – all types of communication Very high frequency (VHF) – 30 to 300 MHz

                      WHAT IS  RADAR? The word radar is coined from the phrase “Radio detection and Ranging”. It is basically an echo-ranging system in which electromagnetic energy in the from of high-power short-duration pulse are sent out at distant objects or targets. The reflected signal or the echo is received at the   transmitting station and analysis   to obtain information on the location of the targets. Essentially, the radar system consists of a transmitter and receiver, both connected to a directional antenna through a switching arrangement, called a duplexer. A small portion of the pulse power generated by the transmitter excites the duplexer which disconnects the receiver from the antenna and connects the transmitter to the antenna. In most systems, the antenna scans   i,e.,   rotates to direct the radiated beam as desired. The scanning speed is small compared to the speed of light in free space, so that the antenna does not rotate significantly in the time

The Twin Paradox The relativistic effects give rise to a famous so-called      paradox      of     two persons, say A and B, who are twins, born at the same time. Suppose that A flies away in a space ship at a speed comparable to the speed of light, leaving B stationary on the ground.  B sees A moving fast, so that A’s clock appears to go slower from B’s point of view. If A comes back, he will find his twin brother B older than himself. The paradox arises from the contention that all motion is relative; so either twin could consider the other to be moving.  Thus each should find the other younger, which is a logical contradiction. This contention presupposes that the twins situations are symmetrical and interchangeable. But this is not true. For the twins to meet and make the comparison, A has to turn around to come back.  When the turns around, he feels the acceleration, while B feels nothing. Thus the situation is not symmetrical: there is a difference between the

SOME INVENTIONS THAT CHANGED THE WORLD BLOWN GLASS (c.100 B.C.E): “Blowing allowed for previously unparalleled versatility and speed of manufacture”. CLAW HAMMER (C.79): “ A   worker may be the hammer’s master, but the hammer still prevails….” CANNON (C.1128): “ shells are made of cast iron… and are sent flying toward the enemy camp from an eruptor.” POCKET WATCH (C.1508): “Peter Henlein… makes from a little iron a pocket clock with a lot of wheels”. COMBINATION LOCK (C.1550): “we ride nonpolluting bicycles to save mankind, but we lock them because we can’t trust mankind.” MICROSCOPE (c.1590): “Nature composes some of her loveliest poems for the microscope and the telescope.” NEWSPAPER (c.1605): “a news paper consists of just the same number of words, whether there be any news in it or not.” SPINNING MULE (c.1779) Clothing, woven cloth, spinning wheel, flying shuttle, spinning jenny, powered loom. SAFETY MATCH (18