When the spectrum obtained from the Sun is examined, it consists of large number of dark lines (line absorption spectrum). These dark lines in the solar spectrum are known as Fraunhofer lines.

An electromagnetic waves are the waves that are radiated by an accelerated charge which propagates through space as coupled electric and magnetic fields, oscillating perpendicular to each other and to the direction of propagation of the wave.

The displacement current can be defined as the current which comes into play in the region in which the electric field or the electric flux is changing with time.

Dielectric constant (relative permittivity of the medium) is ε_r = 2.25
Magnetic permeability is μ_r = 2.5
Refractive index of the medium,
n = \(\sqrt { { \varepsilon }_{ r }{ \mu }_{ r } } =\sqrt { 2.25\times 2.5 } \) = 2.37

i) Maxwell's prediction was experimentally confirmed by Heinrich Rudolf Hertz in 1888. The experimental set up used is shown in Figure.
ii) It consists of two metal electrodes which are made of small spherical metals. These are connected to larger spheres and the ends of them are connected to induction coil with very large number of turns. This is to produce very high electromotive force (emf).
iii) Since the coil is maintained at very high potential, air between the electrodes gets ionized and spark (spark means discharge of electricity) is produced.
iv) The gap between electrode (ring type - not completely closed and has a small gap in between) kept at a distance also gets spark. This implies that the energy is transmitted from electrode to the receiver (ring electrode) as a wave, known as electromagnetic waves.

v) If the receiver is rotated by 90° - then no spark is observed by the receiver. This confirms that electromagnetic waves are transverse waves as predicted by Maxwell.
vi) Hertz detected radio waves and also computed the speed of radio waves which is equal to the speed of light (3 x 10⁸m S^-1).

μ_r = 1.0,  ε_r = 2.25
Refractive index of the medium \(n= {\sqrt{\varepsilon_{r} \mu_{r}}} =\sqrt{1 \times2.25}=1.5\)
Speed of electromagnetic waves in this medium,
\(V_{m}=\frac{c}{n}=\frac{3 \times 10^{8}}{1.5}\)
V_m = 2 x 10⁸ m/s

Power of the pulse P = 60 x 10^-3 W
Time internal t = 10^-6 S
Energy U = Power x time
U = p x t
= 60 x 10^-3 x 10^-6
U = 60 x 10^-9 J
Lineral momentum, \(\mathrm{P}=\frac{\text { Energy }}{\text { speed }}=\frac{U}{C} \ \)
\(\mathrm{P}=\frac{60 \times 10^{-9}}{3 \times 10^{8}} \)
P = 20 x 10^-17 kg ms^-1

Frequency of the electromagnetic waves given, f = 2450 MHz
The corresponding angular frequency is
ω = 2πf = 2 x 3.14 x 2450 x 10⁶
= 15,386 x 10⁶ Hz
= 1.54 x 10¹⁰ s^-1
The required magnetic field, B = \(\frac { { m }_{ e }\omega }{ |q| } \)
Mass of the electron, m_e = 9.11 x 10^-31 kg
Charge of the electron,
q = -1.60 x 10^-19C
⇒ |q| = 1.60 x 10^-19 C
B = \(\frac { (9.11\times { 10 }^{ -31 })(1.54\times 10^{ 10 }) }{ (1.60\times 10^{ -19 }) } \)  = 8.7683 x 10^-2T
B = 0.08768 T
This magnetic field can be easily produced with a permanent magnet. So, electromagnetic waves of frequency 2450 MHz can be used for heating and cooking food because they are strongly absorbed by water molecules.

(i) Electromagnetic waves are produced by any accelerated charge
(ii) Electromagnetic waves do not require any medium for propagation. So electromagnetic wave is a non mechanical wave.
(iii) Electromagnetic waves are transverse in nature. This oscillating electric field vector, oscillating magnetic field vector and propagation vector (gives direction of propagation) are mutually perpendicular to each other. 
(iv) Electromagnetic waves travel with speed which is equal to the speed of light in vacuum or free space,
\(c =\frac{1}{\sqrt{\varepsilon_{o} \mu_{o}}} \)
\(=3 \times 10^{8} \mathrm{~ms}^{-1} \)
where ε_o is the permittivity of free space or vacuum and μ_o is the permeability of free space or vacuum.
(v) In a medium with permittivity ε and permeability μ, the speed of electromagnetic wave is less than speed in free space or vacuum, that is, v < c.
In a medium of refractive index, \(\mu=\frac{c}{v} \)
n \(=\frac{\frac{1}{\sqrt{\varepsilon_{o} \mu_{o}}}}{\frac{1}{\sqrt{\varepsilon \mu}}} =\sqrt{\varepsilon_{r} \mu_{r}} \)
where ε_r is the relative permittivity of the medium (also known as dielectric constant) and μ_r is the relative permeability of the medium
(vi) Electromagnetic waves are not deflected by electric field or magnetic field.
(vii) Electromagnetic waves can exhibit interference, diffraction and polarization.
(viii) Like other waves, electromagnetic waves also carry energy, linear momentum and angular momentum.
(ix) If the electromagnetic wave incident on a material surface is completely absorbed, then the energy delivered is U and momentum imparted on the surface is \(p=\frac{U}{C}\)
(x) If the incident electromagnetic wave of energy U is totally reflected from the surface, then the momentum delivered to the surface is
\(\Delta p=\frac{U}{C}-\left(-\frac{U}{C}\right)=2 \frac{U}{C}\)