Electromagnetic waves:

Electromagentic waves is most common source of energy in Universe because the radiations (light, X-rays, etc.) emitted by Sun and Stars is nothing but electromagnetic waves.

Properties or characteristics of electromagnetic waves:

Properties or characteristics of electromagnetic waves are as follows:

(a) Electromagnetic waves propagate in the form of time varying electric field (E) and magnetic field (B) such that E and B are perpendicular to each other and both E and B are perpendicular to direction of propagation of wave,

(b) Electromagnetic waves are transverse waves as vibrations of wave (E and B) are perpendicular to direction of wave,

(c) unlike elastic waves (e.g., sound waves) electromagnetic waves do not require medium for propagation,

(d) electromagnetic waves can be produced by accelerated electric charges,

(e) the relation between velocity, wavelength, and frequency is given by standard wave equation, c = νλ, where c = velocity of light, ν = frequency of electromagnetic wave, and λ = wavelength of electromagnetic wave,

(f) the velocity of electromagnetic waves in vacuum is given by c = 3 lakh kilometre per second = 3 × 10⁸ m/s,

(g) electromagnetic waves obey the principle of superposition of waves,

(h) electromagnetic waves exert pressure on the surface on which they are incident,

(i) the low frequency electromagnetic waves are unaffected by external electric and magnetic fields,

(j) the energy of electromagnetic wave is divided or distributed equally between the electric and magnetic field vectors,

(k) according to quantum theory, a particle is associated with every wave; and particle associated with electromagnetic wave is called photon. The energy of photon is given by the expression E = hν, where h = Planck’s constant = 6.63 × 10^–34 J.s and ν = frequency of photon or electromagnetic wave,

(l) SIU of frequency of electromagnetic waves is Hz or hertz,

(m) electromagnetic waves can be polarised and electric field in this wave (i.e., E) is responsible for its polarisation.

Propagation constant (k):

The propagation constant of electromagnetic wave is given by the following expression:

k = 2π/λ

Angular frequency (ω):

Angular frequency of electromagnetic waves is given by the following expression:

ω = 2πν

Electromagnetic spectrum:

The orderly distribution (i.e., sequential arrangement) of electromagnetic waves according to their wavelengths (or frequencies) in the form of distinct groups having different properties is called the electromagnetic spectrum.

Gamma Rays (γ-rays):

The high energy electromagnetic waves having the wavelengths in the range of 10^–12 m to 10^–10 m are called gamma rays. OR The electromagnetic waves whose frequencies range from 3 × 10¹⁸ Hz to 5 × 10²⁰ Hz are called gamma rays (γ-rays).

Properties of gamma rays:

Properties of gamma ray are as follows:

(a) these rays are highly penetrating and so they can penetrate through thick iron and lead blocks,

(b) they possess moderate ionising power,

(c) they affect photographic plate,

(d) they kill living cells on which they are incident,

(e) they can produce fluorescence in the substances like willemite, zinc sulphide, etc.,

(f) they knock out electrons from the surface on which they are incident.

Uses of gamma rays:

Uses of gamma ray are as follows:

(a) they are used for insect disinfectations for wheat and flour,

(b) they are used for preservation of food,

(c) they are used in radiotheraphy for the treatment of cancer and tumours,

(d) they are used to produce nuclear reactions.

X–rays:

The high energy electromagnetic waves having the wavelengths in the range of 10^–11 m to 10^–8 m are called X–rays. X–rays are also called Rontgen rays as these rays were discovered by Wilhelm Rontgen. When cathode rays are suddenly stopped by an obstacle, X–rays are produced.

Properties of X–rays:

Properties of x-rays are as follows:

(a) they affect photographic plate,

(b) they are not deflected by electric or magnetic field,

(c) they ionise the gases through which they pass,

(d) they produce fluorescence in many substances like zinc sulphide, etc.,

(e) they have high penetrating power and they can penetrate through paper, thin sheet of metal, wood, skin, flesh, etc., but cannot penetrate through bones and heavy metals,

(f) they exhibit photoelectric effect,

(g) they can kill living plant and animal tissues, hence long exposure to X-rays is harmful.

Uses of X–rays:

Uses of X-rays are as follows:

(a) they are used to study the structure of crystals,

(b)they are used to distinguish real diamonds, gems from artificial ones,

(c) they are used to photograph the broken bones in human body,

(d) they are used to cure skin diseases and to destroy tumours,

(e) they are used to detect flaws or cracks in metals,

(f) they are used for detection of explosives, opium, etc.

Ultraviolet rays:

The electromagnetic waves having the wavelengths in the range of 10^–8 m to 4 × 10^–7 m are called ultraviolet rays.

Properties of ultraviolet rays:

Properties of ultraviolet rays are as follows:

(a) they affect photographic plates,

(b) they produce fluorescence in certain materials,

(c) they exhibit photoelectric effect,

(d) they cannot pass through glass but they can pass through quartz, fluorite, rock salt, etc.,

(e) they possess the property of synthesizing vitamin D, when skin is exposed to sunlight.

Uses of ultraviolet rays:

(a) they destroy bacteria and hence can be used to sterilize the surgical equipment,

(b) being invisible to human eye, they can be used in burglar alarms,

(c) they are used in high resolving power microscopes,

(d) they are used to study molecular structure,

(e) they are used to distinguish the real diamonds and gems from the fake ones,

(f) they are used in the analysis of chemical compounds.

Visible light:

The electromagnetic waves having the wavelengths in the range of 4 × 10^–7 m to 8 × 10^–7 m or frequencies in the range 4 × 10¹⁴ Hz to 8 × 10¹⁴ Hz are called visible rays or visible light.

Infrared rays:

The electromagnetic waves having the wavelengths in the range of 10^–6 m to 10^–4 m are called infrared rays. All hot bodies are sources of infrared rays.

Properties of infrared rays:

Properties of infrared rays are as follows:

(a) they obey the laws of reflection and refraction,

(b) they heat the object on which they are incident,

(c) they affect photographic plates,

(d) they produce interference and can be polarised,

(e) they are strongly absorbed by glass,

(f) they can penetrate through thick columns of fog and mist.

Uses of infrared rays:

Uses of infrared rays are as follows:

(a) Infrared rays are used in long distance photography and photography at night,

(b) they are used in the diagnosis of superficial tumours and varicose veins,

(c) they are used to cure infantile paralysis and to treat sprains, dislocations, and fractures,

(d) they are used in solar water heaters and solar cookers,

(e) they are used to snap thermograph of body that shows diseased parts of body,

(f) they are used to keep greenhouse warm,

(g) they are used in remote controls of TV, computer, etc.

Microwaves:

The electromagnetic waves having the frequencies in the range 5 × 10⁹ Hz to 1 × 10¹² Hz are called microwaves.

Properties of microwaves:

Properties of microwaves are as follows:

(a) they obey laws of reflection and refraction,

(b) they heat an object on which they are incident,

(c) they are produced by oscillator electric circuits containing a capacitor and an inductor, they can also be produced by special vacuum tubes called klystrons, magnetrons, and Gunn diodes.

Uses of microwaves:

Uses of microwaves are as follows:

(a) they are used to transmit the TV and mobile signals,

(b) they are used for cooking in microwave ovens,

(c) they are used in radar systems for the location of distant objects like ships, aeroplanes, etc., (d) they are used in the study of atomic and molecular structure.

Radio waves:

The electromagnetic waves having very long wavelengths ranging from few centimeters to few hundred kilometres are called radio waves.

Properties of radio waves:

Properties of radio waves are as follows:

(a) they obey laws of reflection and refraction,

(b) they are diffracted from large size obstacles coming in their path,

(c) they are produced by oscillator electric circuits containing an inductor and a capacitor.

Uses of radio waves:

Use of radio waves are as follows:

(a) they are used for wireless communication,

(b) they are used for radio and television broadcasting,

(c) they are also used with some limitations in transmission of signals of mobile phones.

Atmosphere:

Earth is surrounded by a thick shell of air and this shell of air is called atmosphere. Atmosphere extends from surface of earth to an altitude of about 400 kilometre. The density of air is maximum close to earth’s surface and and it decreases with height. This shell is further divided in subshells called troposphere, stratosphere, mesosphere, and ionosphere.

Troposphere:

A subshell of atmosphere that extends from earth’s surface to a height of about 12 kilometre is called troposphere. Troposphere is the source of air that we breath. Apart from air it contains dust, smoke, pollen grains, salt, and organic material. Its temperature ranges from 220 K to 280 K. Its density ranges from 1.0 to 0.1 kg/m³. Density decreases with distance.

Stratosphere:

A layer above the troposhere is called stratosphere. It is a subshell of atmosphere that extends from the height (measured from earth’s surface) of about 12 kilometre to 50 kilometre. The density of air in this layer ranges from 0.1 to 0.01 kg/m³ and temperature ranges from 220 K to 280 K. Density decreases with distance. Part of the stratosphere extending from 15 to 50 kilometre above the earth’s surface contains ozone and this part is called Ozone layer.

Mesosphere:

The layer above the stratosphere is called the mesosphere. It is a subshell of atmosphere that extends from the height (measured from earth’s surface) of about 50 kilometre to 80 kilometre. In this layer temperature ranges from 280 K to 180 K and it decreases with height.

Ionosphere:

The layer above the mesosphere is called the ionosphere. It is a subshell of atmosphere that extends from the height (measured from earth’s surface) of about 80 kilometre to 400 kilometre. This layer contains positive and negative ions, hence the name. This layer plays an important role in radio and telecommunications. Ground wave propagation : When the radio waves from the transmitting antenna propagate along the surface of the earth so as to reach the receiving antenna, the wave propagation is called ground wave or surface wave propagation. Radio waves having the frequencies less than 2 MHz (medium frequency band) are transmitted in this mode. This mode cannot be used for TV or FM radio transmission.

Space wave propagation:

When the radio waves from the transmitting antenna reach the receiving antenna either directly or after reflection from the ground or after reflection from troposphere, the wave propagation is called space wave propagation. These waves are also called as tropospheric waves. Their frequencies are in the range of 100 to 200 MHz. This mode is mainly used for transmission of TV signals.

Sky wave propagation:

When the radio waves from the transmitting antenna reach the receiving antenna directly or after reflection in the ionosphere, the wave propagation is called sky wave propagation. These waves are also called as sky waves. Their frequencies are in the range of 3 to 30 MHz.

Critical frequency:

It is the maximum value of the frequency of radio waves reflected back to the earth from the ionosphere, when the waves are directed normally to the ionosphere.

Skip distance or skip zone:

It is the shortest distance from a transmitter measured along the surface of the earth, at which a sky wave of fixed frequency (greater than critical frequency) will be returned to the earth.

Geostationary satellite:

Geostationary satellite is an artificial satellite that revolves around the earth with the time period of 24 hour and its direction of revolution is same as the that of rotation of earth. Consequently, this satellite appears to be stationary from the ground. Its height is approximately 36000 kilometre above the surface of earth. It is used in telecommunications. Minimum three geostationary satellites are required for transmission of communications signals all over the earth.