Electromagnetic waves are those waves in which electric and magnetic field vectors changes sinusoidally and are perpendicular to each other as well as at right angles to the direction of propagation of wave.
The equation of plane progressive electromagnetic wave can be written as
E = Eo sin Ω (t – x / c) and B = Bo sin Ω (t – x / c). Where, Ω =2πv
Electromagnetic waves are produced by accelerated charge particles.
Properties of EM Waves
(i) These waves are transverse in nature.
(ii) These waves propagate through space with speed of light, i.e., 3 * 108 m / s.
(iii) The speed of electromagnetic wave,
c = 1 / √μo εo
where, μoo is permittivity of free space,
∴ c = Eo / Bo
where Eo and Bo are maximum values of electric and magnetic field vectors.
According to Maxwell, when a charged particle is accelerated, it produces electromagnetic wave. The total radiant flux at any instant is given by,
p = q2a2 / 6 πεoc2
(iv) The rate of flow of energy in an electromagnetic wave is described by the vector S called the poynting vector, which is defined by the expression,
S = 1 / μo E * B
SI unit of S is watt/m2.
(v) Its magnitude S is related to the rate at which energy is transported by a wave across a unit area at any instant.
(vi) The energy in electromagnetic waves is divided equally between electric field and magnetic field vectors.
(vii) The average electric energy density.
UE = 1 / 2 εo E2 = 1 / 4 εo E2o
(viii) The average magnetic energy density,
UB = 1 / 2 B2 / μo = 1 / B2o / μo
(ix) The electric vector is responsible for the optical effects of an electromagnetic wave.
(x) Intensity of electromagnetic wave is defined as energy crossing per unit area per unit time perpendicular to the directions of propagation of electromagnetic wave.
(xi) The intensity I is given by the relation,
I = < μ > c = 1 / 2 εo E2oc
(xii) The existence of electromagnetic waves was confirmed by Hertz experimentally in 1888.
Propagation of Electromagnetic Waves
In radio wave communication between two places. the electromagnetic waves are radiated out by the transmitter antenna at one place which travel through the space and reach the receiving antenna at the other place.
The arranged array of electromagnetic radiations in the sequence of their wavelength or frequency is called electromagnetic spectrum
Radio and microwaves are used in radio and TV communication,
Infrared rays are used to
(i) Treat muscular straw.
(ii) For taking photographs’ in fog or smoke.
(iii) In green house to keep plants warm.
(iv) In weather forecasting through infrared photography.
Ultraviolet rays are used
(i) In the study of molecular structure.
(ii) In sterilizing the surgical instruments.
(iii) In the detection of forged documents, Eringer prints.
X-rays are used
(i) In detecting faults, cracks, flaws and holes in metal products.
(ii) In the study of crystal structure.
(iii) For the detection of pearls in oysters.
γ – rays are used for the study of nuclear structure.
The gaseous envelope surrounding the earth is called earth’s atmosphere. It contain the following layers:-
(i) Troposphere- This region extends upto a height of 12 km from earth’s surface.
(ii) Stratosphere- This region extends from 12 km to 50 km. In this region, most of the atmospheric ozone is concentrated from 30 to 50 km. This layer is called ozone layer.
(iii) Mesosphere- The region extends from 50 km to 80 km.
(iv) Ionosphere- This region extends from 80 km to 400 km.
In ionosphere the electron density is very large in a region beyond 110 km from earth’s surface which extends vertically for a few kilometer.
This layer is called Kennelly Heaviside layer.
In ionosphere a layer having large electron density is found at height 250 km from earth’s surface, called Appleton layer.
There are four main layers in earth’s atmosphere having high density of electrons and positive ions, produced due to ionisation by the high energy particles coming from sun. star or cosmos. These layers play their effective role in space communication. These layers are D, E, F1 and F2.
(i) D-layer is at a virtual height of 65 km from surface of earth and having electron density = 109 m-3
(ii) E-layer is at a virtual height of 100 km, from the surface of earth, having electron density = 2 * 1011 m-3
(iii) F1-layer is at a virtual height of 180 km from the surface of earth, having electron density = 3 * 1011 m-3
(iv) F2 – layer is at a vertical height of about 300 km in night time and about 250 to 400 km in day time. The electron density of this layer is = 8 * 1011 m-3