(i) When the spectrum obtained from the Sun is examined, it consists of large number of dark lines . These dark lines in the solar spectrum are called Fraunhofer lines.
(ii) The Absorption spectra for various materials are compared with the Fraunhofer lines in the solar spectrum, which helps in identifying elements present in the Sun's atmosphere.

(a) Microwaves:
It is produced by special vacuum tubes such as klystron, magnetron and gunn diode. The frequency range of microwaves is 10⁹ Hz to 10¹¹ Hz. These waves undergo reflection and can be polarised.
Uses:
It is used in radar system for aircraft navigation, speed of the vehicle, microwave oven for cooking and very long distance wireless communication through satellites.
(b) X-rays:
lt is produced when there is sudden stopping of high speed electrons at high-atomic number target, and also by electronic transitions among the innermost orbits of atoms. The frequency range of X-rays is from 10¹⁷ Hz to 10¹⁹ Hz. X-rays have more penetrating power than ultraviolet radiation.
Uses:
X-rays are used extensively in studying structures of inner atomic electron shells and crystal structures. It is used in detecting fractures, diseased organs, formation of bones and stones, observing the progress of healing bones. Further, in a finished metal product, it is used to detect faults, cracks, flaws and holes.
(c) Radio waves:
It is produced by accelerated motion of charges in conducting wires. The frequency range is from few Hz to 10⁹ Hz. It obeys reflection and diffraction.
Uses:
It is uses in radio and television communication systems and also in cellular phones to transmit voice communication in the ultra high frequency band.
(d) Visible light:
Visible light is produced by incandescent bodies and also it is radiated by excited atoms in gases. The frequency range is from 4 x 10¹⁴ Hz to 8 x 10¹⁴ Hz. It obeys the laws of interference, diffraction and can be polarised. It exhibits photo-electric effect also.
Uses:
It can be used to study the structure of molecules, arrangement of electrons in external shells of atoms and it causes sensation of vision.

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

Amplitude of magnetic field B = 10^-5 T
Frequency, f = 10¹⁰ HZ
(i) Wavelength of the wave,
\({\lambda}=\frac{c}{f} =\frac{3 \times 10^8}{ 10^{10}} \)
= 3 x 10^{8 - 10}
Wavelength = 3 x 10^-2 m
(ii) Electric field E(x,t)\(\hat i\)
Angular frequency \(\omega =2 \pi f \)
\(\omega=2 \times 3.14 \times 10^{10}=6.28 \times 10^{10} rads^{-1}\)
\(k=\frac{2 \pi}{\lambda} =\frac{2 \times 3.14}{3 \times 10^{-2}} \)
\(=\frac{6.28}{3 \times 10^{-2}}=\frac{628}{3}=2.09 \times 10^{2} \)
k = 2.09 x 10²
(iii) E_o = B_oC
E_o = 10^-5 x 3 x 10⁸
E_o = 3 x 10³ V m^-1
The required expression for electric field is
\(\vec{E}(x, t)=E_o \sin \left(\frac{2 \pi}{\lambda} x-2 \pi f t\right) \hat{i} N C^{-1} \)
\(\vec{E}(x , t)=3 \times 10^{3} \sin \left(2.09 \times 10^{2} \mathrm{x}-6.28 \times 10^{10} \mathrm{t}\right) \hat(-{k}) N C^{-1} \)

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.

Potential difference between the plates of the capacitor,
\(V=V_{\max } \sin 2 \pi f t\) 
\(=230 \sqrt{2} \sin (2 \pi \times 50 t)\) 
\(\therefore V=325 \sin 100 \pi t\) 
d = 1 mm = 1 x 10^–3 m
A = 20 cm² = 20 x 10^–4 m²
Displacement current, \(i_{d}=\epsilon_{0} \frac{d \Phi_{E}}{d t}=\epsilon_{\circ} \frac{d(\mathrm{EA})}{d t}\) 
\(\therefore i_{d}=\frac{\epsilon_{0} A}{d}\left[\frac{d V}{d t}\right] \quad\left[\because E=\frac{V}{d}\right]\) 
\(=\frac{\epsilon_{0} A}{d}(325)(100 \pi) \cos 100 \pi t\)
\(=\left(\begin{array}{l} 8.85 \times 10^{-12} \times 20 \times 10^{-4} \times 325 \\ \times 100 \times 3.14 \times \cos (100 \pi \times 1) \end{array}\right) /\left(1 \times 10^{-3}\right)\) 
\(\begin{aligned}=1.81 \times 10^{-6} \mathrm{~A}=1.81 \mu \mathrm{A}[\because \cos (100 \pi \times 1)=1] \end{aligned}\)

(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
\(\Delta p=\frac{U}{C}-\left(-\frac{U}{C}\right)=2 \frac{U}{C}\)

(i) IR radiation:
(i) It is used to produce dehydrated fruits, in green houses to keep the plants warm, heat therapy for muscular pain or sprain, TV remote as a signal carrier, to look through haze fog or mist.
(ii) It is used in night vision or infrared photography.
(ii) Microwaves:
It is used in radar systems for aircraft navigation, speed of the vehicle microwave oven for cooking and very long distance wireless communication through satellites.
(iii) UV radiation:
It is used to destroy bacteria in sterilizing the surgical instruments, burglar alarms, to detect the invisible writing, finger prints and also in the study of atomic structure.