1.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

2. Let a plane electromagnetic wave propagate along positive X-axis. Then the propagating wavefront will be in YZ 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 Gausss theorem 3. As electric field does not depend on Y and Z. 4. 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. 5. 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 . 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 is wavelength and v is frequency of the wave. Contd. 6. 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. 7. 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. 8. 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. 9. 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 10. 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. 11. 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 12. 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. 13. 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. 14. 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. 15. 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. 16. 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. 17. 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. 18. 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 19. 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 20. Phase modulation (PM) is a modulation pattern that encodes information as variations in the instantaneous phase of a carrier wave. 21. 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 22. 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. 23. 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. 24. 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-deviationi.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). 25. 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. 26. 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. 27. 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 28. Atmospheric Window are of three significant types:- Radio Window Infrared Window Optical Window 29. 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 30. 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. 31. As the main part of the 'window' spectrum, a clear electromagnetic spectral transmission 'window' can be seen between 8 and 14 m. 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 m. 32. The infrared absorptions of the principal natural greenhouse gases are mostly in two ranges. At wavelengths longer than 14 m (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 m. Except for the bonds in O3, no bonds between carbon, hydrogen, oxygen and nitrogen atoms absorb in the interval between about 8 and 14 m, 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. 33. 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.