Ch 13Electromagnetic Waves and Communication System — Class 11 Physics, concept-first.

We live in an information age that is built almost entirely on the physics of electromagnetic (EM) waves.

Four basic laws describe how static and changing electric and magnetic fields relate to the charges and currents that produce them.

Maxwell's theory makes a precise statement about what produces an EM wave: accelerated electric charges radiate EM waves. Consider a charge that oscillates back and forth with some fixed frequency.

EM waves share a common set of defining characteristics, all of which follow from Maxwell's equations.

The electromagnetic spectrum is the orderly, sequential arrangement of all EM waves according to their wavelength (equivalently, their frequency, since fixes one once the other is known) into distinct…

Radio waves are generated by the accelerated motion of electric charges in a conducting wire -- typically an oscillating circuit built from an inductor and a capacitor.

Microwaves were the first EM waves in this spectrum to be discovered experimentally, by Hertz in 1888, and they are produced by oscillator electric circuits containing a capacitor and an inductor, or…

Infrared waves were discovered by William Herschel in 1800. Every hot body emits infrared radiation, and it is a major component of the Sun's output -- about 60% of all solar radiation reaching the Ea…

Visible light is by far the most familiar form of EM wave, simply because it is the one band the human eye can detect directly -- which is exactly why this narrow wavelength range earns the name 'visi…

Ultraviolet (UV) rays were discovered by J. Ritter in 1801. They can be produced by a mercury vapour lamp, an electric spark, a carbon arc lamp, or by striking an electrical discharge in hydrogen or x…

X-rays were discovered by the German physicist W. C. Rontgen in 1895 while he was studying cathode rays (a stream of electrons emitted from the cathode inside a vacuum tube); they are accordingly also…

Gamma rays (-rays) were discovered by P. Villard in 1900. Unlike the other bands of the spectrum, which mostly originate from electron transitions in atoms or from accelerating charges in circuits, ga…

A television antenna picking up a signal from a broadcasting tower, or from a satellite, is a familiar everyday example of EM-wave communication in action: in radio-wave communication generally, an an…

Ground wave (also called surface wave) propagation is the mode in which a radio wave from a transmitting antenna travels near the surface of the Earth so as to reach a nearby receiving antenna.

Space wave propagation covers radio waves that reach the receiving antenna either directly, travelling in a straight line (a line-of-sight path) from the transmitting antenna, or indirectly, after ref…

Sky wave propagation is the mode in which radio waves from a transmitting antenna reach the receiving antenna only after being reflected back down by the ionosphere.

Communication, in its most general sense, is simply the exchange of information, and human beings have practised it since ancient times in many forms -- speaking, writing, singing, and even body langu…

Every communication system, no matter how simple or elaborate, is built from three basic (essential) elements: (a) a transmitter, (b) a communication channel, and (c) a receiver, exactly as sketched i…

A number of standard terms recur throughout the study of any electronic communication system, and it is worth defining each of them clearly before going further.

As noted in the previous section, an ordinary audio signal has a low frequency (below about 20 kHz), and low-frequency signals of this kind simply cannot be transmitted efficiently over large distance…