ELECTROMAGNETIC WAVES

The Process of Emission of Electromagnetic Waves: -
The Hertzian dipole is shown in the figure.
Let the dipole moment, p, of this dipole at time,
t, be given by p = p0 cos wt. The electric field lines in the plane of the paper and magnetic field lines
perpendicular to the plane of the paper are shown in the figure. Figures ( a ) and ( b ) show the state of the dipole and the corresponding electric and magnetic field lines at times t = 0 and t = T / 8 respectively.
At time t = T / 4, the dipole moment becomes
zero. In this case, the electric and the magnetic
field lines form closed loops and are de-linked from the dipole as shown in the figure ( c ).
At time t = 3T / 8, the electric charges on the dipole get reversed and the electric and magnetic field lines get again linked with the dipole. Meanwhile, he field lines which had formed closed loops move forward and travel some distance as shown in figure ( d ). At t = T / 2, the situation is as shown in figure ( e ). So, during every t = T / 2 time, due to the oscillations of the dipole, closed loops of the electric and magnetic fields are continuously formed and are transmitted in space after getting dissociated from the dipole.
According to Maxwell’s the magnetic fields at all
points on the path of propagation of the electromagnetic wave do not come into existence
instantaneously, but the effect travels in free
space at the velocity of light. Hence the phase of
the oscillations continuously decrease
along the path of the wave. The position of the
fields at any particular instant is shown in the
figures.
In the region close to the oscillations of the
charges, the phase difference between the
E and B fields is equal to p / 2. Their magnitude quickly falls as per 1 / r3 ( where r is the distance from the source ). These components of the transmitted waves are called the inductive components.
At large distance, the phase difference between E and B is zero. Their magnitudes fall as per 1 / r. These components of the fields are known as radiated components. Thus, E and B fields oscillate in mutually perpendicular planes, perpendicular to the direction of propagation of the wave. Both E and B values increase from zero to maximum with the passage of time and then start decreasing and become zero again. Then, the direction of the fields get reversed, become maximum in the reverse direction and increase to zero. Thus oscillations of  the fields continue as the wave passes through any point.
The energy and frequency of the electromagnetic waves is respectively equal to the kinetic energy and frequency of oscillations of the charges oscillating between the two spheres
For electromagnetic waves, c ( velocity ) = l ( wavelength ) ´ f ( frequency ).
Seven years after Hertz’s experiment, Acharya Jagdishchandra Bose generated electromagnetic waves of wavelength 5 to 25 mm. At the same time, Italian scientist, Marconi, successfully transmitted electromagnetic waves upto a distance of several miles.