A l

2. The variation in electric and magnetic fields would lead to producing a wave
consisting of oscillating electric field E and magnetic field B perpendicular to each
other and also perpendicular to the direction of propagation of the wave. Such
waves which actually propagate in space even without any material medium are
called “electromagnetic waves”. e.g. radio waves, microwaves, infrared rays,
ultraviolet rays, X-rays, etc.
E
B
Y
X
Z
Envelope of E
Envelope of B
Direction of Propagation

3. Let a plane electromagnetic wave propagate along positive X-axis. Then the
propagating wavefront will be in Y—Z plane. ABCD is a portion of wavefront at any
time t. The electric and magnetic field vectors at time t will be zero to the right
of ABCD. To the left of ABCD, they will depend on x and t but not on Y and Z. Since
we are considering a plane wave.
Consider a closed
surface ABCDEFGH.
This surface does not
enclose any charge,
therefore by Gauss’s
theorem

4. As electric field does not depend on Y and Z.

5. i.e., component of electric field along the direction of
propagation is constant. As a constant field can not produce a
wave, this implies that In a similar manner it may be shown
that the component of magnetic field along the direction of
propagation of wave is zero, i.e., This shows that the electric
and magnetic fields have no component along the direction of
propagation. Thus, in an electromagnetic wave field vectors are
perpendicular to the direction of propagation of wave, i.e.,
electromagnetic waves are transverse in nature.
Hence, we conclude that in an electromagnetic wave,
both electric and the magnetic fields are perpendicular to the
direction of the wave propagation, that is, electromagnetic
waves are transverse in nature.

6. Suppose that a sinusoidal electromagnetic wave is propagating in free space along
the positive direction of X-axis with wave number k and angular frequency ω.
Then, the magnitudes of E and B, acting along Y- and Z-axis respectively, vary
with x and t can be written as
and,
where E0 and B 0 are the maximum values (amplitudes) of E and B respectively.
Now,
Where λ is wavelength and v is frequency of the wave.
Contd.

7. Where c is the speed of the electromagnetic wave, which is the speed of
light in free space. Now, from equation (i), we have
And from equation (ii), we have
Making these substitution in the relation , we have
Contd.

8. Since E and B are in phase, we can write
At any point in space. Thus, the ratio of the magnitudes of electric and
magnetic fields equals the speed of light in free space.

9. A moving charge moving with constant velocity produces
both electric and magnetic fields, the fields will not
change with time and no electromagnetic wave can be
produced. If, however, the motion of the charge is
accelerated, the electric and the magnetic wave will
change with space and time; it then produces
electromagnetic waves. Hence, we conclude that an
accelerated charge emits electromagnetic waves.

11. • Discovered by Paul Villard in 1900
• These are the most energetic photons, having no defined lower limit to their
wavelength.
•Wavelength range - 1 X 10-14 to 1 X 10-10 m
•Frequency range - 3 X 1022 to 3 X 1018 Hz
•Production – By transitions of atomic nuclei and decay of certain elementary
particles.
•Properties –
 Chemical reaction on photographic plates;
Fluorescence;
Ionisation;
Diffraction;
Highly-penetrating;
Charge less;
Harmful to body.
• Uses – Provide information about structure of atomic nuclei.
Paul Villard

12. •Wavelength range - 1 X 10-11 to 3 X 10-8 m
•Frequency range - 3 X 1019 to 1 X 1016 Hz
•Production – By sudden deceleration of high-speed electrons at high-atomic
number target, and (with discrete wavelengths) also by
electronic transitions among the innermost orbits of atoms.
•Properties – All properties of Gamma Rays but less penetrating.
• Uses – Reveal structures of inner atomic electron shells and crystals, help in
medical diagnosis.
X- ray of human hand,
one of the use of X rays.

13. •Wavelength range - 1 X 10-8 to 4 X 10-7 m
•Frequency range - 3 X 1016 to 8 X 1014 Hz
•Production – By sun, arc, vacuum spark
and ionised gases.
•Properties – All properties of Gamma
Rays but less penetrating, produce photo-
electric effect, absorbed by atmospheric
ozone, harmful to human body.
• Uses – In detection of invisible writing,
forged documents, fingerprints and to
preserve foodstuffs.
The amount of penetration of UV
relative to altitude in Earth's ozone

14. •Wavelength range - 4 X 10-7 to 8 X 10-7 m
•Frequency range - 8 X 1014 to 4 X 1014 Hz
•Production – Radiated by excited atoms
in gases and incandescent
bodies.
•Properties – Reflection. Refraction,
interference, diffraction,
polarisation, photo-electric
effect, photographic action
and sensation of sight.
• Uses – Reveals the structure of
molecules and arrangement of
electrons in external shells of
atoms.
Light dispersion by a prism.

15. •Wavelength range - 8X 10-7 to 5 X 10-3 m
•Frequency range - 4 X 1014 to 6 X 1010 Hz
•Production – From hot bodies and by
rotational and vibration
transitions in molecules.
•Properties –Heating effect on thermopile
and bolometer, reflection,
refraction, diffraction,
penetration through fog.
• Uses – In green houses to keep the
plants warm and in warfare to
look through haze, fog or mist.
Human body radiating infrared
radiation.

16. •Wavelength range - 1X 10-3 to 3 X 10-1 m
•Frequency range – 3 X 1011 to 1 X 109 Hz
•Production – By oscillating currents in
special vacuum tubes and
by electromagnetic
oscillators in electric
circuits.
•Properties –Reflection, polarisation.
• Uses – In radar, long-distance wireless
communication via satellites and
in microwave ovens.
Microwaves used in preparation
of fool in ovens.

17. •Wavelength range - 1X 10-1 to 1 X 10-4 m
•Frequency range – 3 X 109 to 3 X 104 Hz
•Production – By oscillating electric
circuits.
•Properties –Reflection, diffraction.
• Uses – In radio and T.V. communication
systems.
•Wavelength range - 5X 106 to 1 X 106 m
•Frequency range – 60 to 50 Hz
•Production – Weak Radiation from a.c.
circuits.

18. Large parabolic antenna for
communicating with spacecraft
An antenna is a length of conductor
which acts as a conversion device. All
communication systems employ
antenna, both at the transmitter as
well as the receiver.
At the transmitter, the
antenna converts electrical signal
into electromagnetic waves which
are radiated in free space.
At the receiver, the antenna
intercepts the transmitted
electromagnetic waves and converts
them into electrical signals which
are fed to the input of the receiver.

19. The audio-frequency electrical signals cannot be transmitted as such over
long distances because of the following reasons: -
• For efficient transmission and reception, the transmitting and receiving antennas
should have heights roughly equal to a quarter wavelength of the signal. ( Required
height of antenna- 7.5 km.
• The energy carried by an audio (low-frequency) signal is too small and so power
radiated from transmitting antenna is insignificant.
• All audio signals have frequencies within a limited range of 20 Hz to 20 kHz.
Hence audio signals from different transmitting stations would overlap and create
confusion
To overcome these difficulties, the audio signal is superimposed
on a high-frequency wave, and the resulting wave so produced is transmitted. The
audio signal is called ‘modulating signal’ or ‘modulating wave’, the high frequency
wave is called ‘carrier wave’, and the resulting wave is called ‘modulated wave’. This
process is called ‘modulation’.

21. Amplitude modulation (AM) is
a technique used in electronic
communication, most commonly
for transmitting information via
a radio carrier wave. AM works by
varying the strength of the
transmitted signal in relation to
the information being sent. For
example, changes in signal
strength may be used to specify
the sounds to be reproduced by a
loudspeaker, or the light intensity
of television pixels.
An audio signal (top) may be carried
by an AM or FM radio wave

22. In telecommunications and
signal processing, frequency
modulation (FM) is the
encoding of information in
a carrier wave by varying
the instantaneous
frequency of the wave.
(Compare with amplitude
modulation, in which
the amplitude of the carrier
wave varies, while the
frequency remains constant.)
An audio signal (top) may be carried
by an AM or FM radio wave

23. Phase modulation (PM) is a modulation pattern
that encodes information as variations in
the instantaneous phase of a carrier wave.

25. The modulation index (or modulation depth) of
a modulation scheme describes by how much the
modulated variable of the carrier signal varies around its
unmodulated level. It is defined differently in each
modulation scheme.
Further, the modulation index of the three types of
modulation is described:-
 Amplitude modulation index
 Frequency Modulation Index
 Phase Modulation Index

26. The AM modulation index is the measure of the amplitude variation surrounding an
unmodulated carrier. As with other modulation indices, in AM this quantity (also
called "modulation depth") indicates how much the modulation varies around its
unmodulated level. For AM, it relates to variations in carrier amplitude and is defined
as:
where M and A are the message amplitude and carrier amplitude, respectively, and
where the message amplitude is the maximum change in the carrier amplitude,
measured from its unmodulated value. So if , carrier amplitude varies by 50% above
(and below) its unmodulated level; for , it varies by 100%. To avoid distortion,
modulation depth must not exceed 100 percent. Transmitter systems will usually
incorporate a limiter circuit to ensure this. However, AM demodulators can be
designed to detect the inversion (or 180-degree phase reversal) that occurs when
modulation exceeds 100 percent; they automatically correct for this defect.

27. Variations of a modulated signal with percentages of modulation are shown below. In
each image, the maximum amplitude is higher than in the previous image.

28. As in other modulation systems, this quantity indicates by how much the
modulated variable varies around its unmodulated level. It relates to variations in
the carrier frequency:
where is the highest frequency component present in the modulating
signal xm(t), and is the peak frequency-deviation—i.e. the maximum
deviation of the instantaneous frequency from the carrier frequency. If ,
the modulation is called narrowband FM, and its bandwidth is
approximately . If , the modulation is called wideband FM and its
bandwidth is approximately . While wideband FM uses more bandwidth, it
can improve the signal-to-noise ratio significantly; for example, doubling the
value of , while keeping constant, results in an eight-fold improvement in
the signal-to-noise ratio.(Compare this with Chirp spread spectrum, which uses
extremely wide frequency deviations to achieve processing gains comparable to
traditional, better-known spread-spectrum modes).

29. With a tone-modulated FM wave, if the modulation frequency is
held constant and the modulation index is increased, the (non-negligible)
bandwidth of the FM signal increases but the spacing between spectra
remains the same; some spectral components decrease in strength as others
increase. If the frequency deviation is held constant and the modulation
frequency increased, the spacing between spectra increases.
Frequency modulation can be classified as narrowband if the change in the
carrier frequency is about the same as the signal frequency, or as wideband
if the change in the carrier frequency is much higher (modulation index >1)
than the signal frequency. For example, narrowband FM is used for two way
radio systems such as Family Radio Service, in which the carrier is allowed
to deviate only 2.5 kHz above and below the center frequency with speech
signals of no more than 3.5 kHz bandwidth. Wideband FM is used for FM
broadcasting, in which music and speech are transmitted with up to 75 kHz
deviation from the center frequency and carry audio with up to a 20-kHz
bandwidth.

30. As with other modulation indices, this quantity indicates
by how much the modulated variable varies around its
unmodulated level. It relates to the variations in the phase
of the carrier signal:
where is the peak phase deviation.

31. Demodulation is the act of
extracting the original
information-bearing signal from a
modulated carrier wave.
A demodulator is an
electronic circuit (or computer
program in a software-defined
radio) that is used to recover the
information content from the
modulated carrier wave.
A Demodulator

32. Atmospheric Window are of
three significant types:-
 Radio Window
 Infrared Window
 Optical Window

33. The radio window is the range of frequencies of electromagnetic
radiation that the earth's atmosphere lets through. The wavelengths in the
radio window run from about one centimeter to about eleven- meter waves.
Opacity of the Earth's atmosphere

34. The infrared atmospheric window is the overall dynamic property
of the earth's atmosphere, taken as a whole at each place and occasion of
interest, that lets some infrared radiation from the cloud tops and land-sea
surface pass directly to space without intermediate absorption and re-
emission, and thus without heating the atmosphere.
It cannot be defined simply as a part or set of parts of the
electromagnetic spectrum, because the spectral composition of window
radiation varies greatly with varying local environmental conditions, such as
water vapour content and land-sea surface temperature, and because few or no
parts of the spectrum are simply not absorbed at all, and because some of the
diffuse radiation is passing nearly vertically upwards and some is passing
nearly horizontally. A large gap in the absorption spectrum of water vapor, the
main greenhouse gas, is most important in the dynamics of the window. Other
gases, especially carbon dioxide and ozone, partly block transmission.

35. As the main part of the 'window' spectrum, a clear electromagnetic spectral
transmission 'window' can be seen between 8 and 14 µm. A fragmented part of the
'window' spectrum (one might say a louvered part of the 'window') can also be seen
in the far infrared between 0.2 and 5.5 µm.

36. The infrared absorptions of the principal natural greenhouse gases are mostly
in two ranges. At wavelengths longer than 14 µm (micrometers), gases such
as CO2 and CH4 (along with less abundant hydrocarbons) absorb due to the
presence of relatively long C-H and carbonyl bonds, as well as water (H2O)
vapor absorbing in rotation modes. The bonds of H2O and NH3 absorb at
wavelengths shorter than 8 µm. Except for the bonds in O3, no bonds between
carbon, hydrogen, oxygen and nitrogen atoms absorb in the interval between
about 8 and 14 µm, though there is weaker continuum absorption in that
interval
Without the infrared atmospheric window, the Earth would become much too warm
to support life, and possibly so warm that it would lose its water, as Venus did early
in solar system history. Thus, the existence of an atmospheric window is critical to
Earth remaining habitable planet.

37. Optical Window means a
(usually at least mechanically flat,
sometimes optically flat, depending on
resolution requirements) piece of
transparent (for a wavelength range of
interest, not necessarily for visible light)
optical material that allows light into
an optical instrument. A window is
usually parallel and is likely to be anti
reflection coated, at least if it is
designed for visible light. An optical
window may be built into a piece of
equipment (such as a vacuum chamber)
to allow optical instruments to view
inside that equipment.