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A communication system is a way of transfering information from one source to another.

The information signals that are mostly used in communication are electrical in nature. The basic components of communication system are:

1. Transmitter of information signal

2. Carrier of signal

3. Receiver of signal

(a) Low radio frequency at low range

The part of the communication system which converts signal for propagation to send on communication channel to receiver is Transmitter.

The answer is (c) 2πRh

The answer is (b) γ-ray

The answer is (a) 2/3

The answer is (d) > 1

Radiowaves of frequency 1.5 MHz to 30 MHz.

The answer is (c) \(\vec{E} \times \vec{B}\)

Modulation is a process through which audio, video, image or text information is added to an electrical or optical carrier signal to be transmitted over a telecommunication or electronic medium. Modulation enables the transfer of information on an electrical signal to a receiving device that demodulates the signal to extract the blended information.

The concepts that seeded nanotechnology were first discussed in 1959 by renowned Physicist Richard Feynman in which he described the possibility of synthesis via direct manipulation of atoms.

Nanotechnology is the engineering of functional systems at the molecular scale. One nanometer (nm) is one billionth or 10^-9, of a meter. By comparison, typical carbon-carbon bond lengths or spacing between these atoms in a molecule are in the range 0.12 – 0.15 nm and a DNA double-helix has a diameter around 2 nm. On the other hand, the smallest cellular life forms, the bacteria of the genus Mycoplasma are around 200 nm in length. By convention, nanotechnology is taken as the range 1 to 100 nm following the definition used by the National Nanotechnology Initiative in the US. The lower limit is set by the size of atoms (Hydrogen has the smallest atoms, which are approximately a quarter of a nm diameter) since nanotechnology must build its devices from atoms and molecules. Nanofibers are used in several areas and in different products, in everything from aircraft wings to tennis rackets. Inhaling airborne nanoparticles and nanofibers may lead to a number of pulmonary diseases eg. fibrosis

Nanostructers in nature : Average height of a man 1750,000,000 nm is too long compared to human hair with a diameter 50,000 nm. The size of red blood cells and E-coli bacteria are 5000 nm and 2000 nm respectively. Transistor printed an IC is around 90 nm and size of virus is about 50 nm. The width of a typical DNA is around 2.0 nm.

In electronics and telecommunication, modulation is process of varying one or more properties of periodic waveform, called the carrier signal, with modulating signal that typically contains information to be transmitted. Most radio systems in 20th century used frequency modulation (FM) or amplitude modulation (AM) to make the carrier carry radio broadcast.

In general Modulation is a process of conveying message signal. For example, a digital bit stream or an analog signal, inside another signal that can be physically transmitted. Modulation of a sine waveform transforms a narrow frequency range baseband message signal into a moderate to high frequency range passband signal, one that can pass through a filter.

Demodulation: Demodulation is extracting the original information-bearing signal from a modulated carrier wave. A demodulator is an electronic circuit (or computer programme) that is used to recover the information content from the modulated carrier wave. There are many types of modulation so there are many types of demodulators. The signal output from a demodulator may represent sound (an analog audio signal), images (an analog video signal) or binary data (a digital signal).

Differences between modulation and demodulation:

1. Modulation is the process of influencing data information on the carrier, while demodulation is the recovery of original information at the distant end from the carrier.

2. Modem is equipment that performs both modulation and demodulation.

3. Both process aim to achieve transfer information with minimum distortion, minimum loss and efficient utilization of spectrum.

4. Even though there are different methods for modulation and demodulation process, each has its own advantages and disadvantages. For example AM is used in shortwave and radio wave broadcasting. FM is mostly used in high-frequency radio-broadcasting and pulse modulation is known for digital signal modulation.

Sky wave propagation.

Nanotechnology made possible because of the instruments used to see things on the nanoscale. Some instruments are:

1. Optical microscope

2. Electron microscope

3. Scanning probe microscope

4. Atomic force microscope

Parallel to Z-axis.

Below 100 nm.

i. Radio waves (short wavelength or high frequency end)

ii. Radio waves (short wavelength or high frequency end)

iii. Visible region (yellow light)

The electromagnetic spectrum is generally divided into seven regions, in order of decreasing wavelength:

1. Radiowaves (10³ m)

2. Microwaves (10^-2 m)

3. Infrared rays (10^-5 m)

4. Visible rays (0.5 × 10^-6 m)

5. Ultraviolet rays (~ 10^-8 m)

6. X-rays (~ 10^-10 m)

7. Gamma rays (~ 10^-12 m).

 The given range corresponds to X-rays. X-rays are used for detection of fractures, formations of stones etc. in human bodies. They are also used to study crystal structure of solids.

 Wavelength 10^-2 m belongs to microwaves. It is used in RADAR.

(A) radio waves, microwaves, infra-red, γ- rays

The earths atmosphere is transparent to visible light and radio waves but absorbs X-rays. Therefore, X-ray astronomy is possible only from the satellites orbiting the earth.

The answer is (C) β-rays.

Correct  answer is (d) \(\sqrt{\frac{1}{LC}}\)

Radius, r = 5cm = 5 x 10^-2 m 

The rate okf electric frield, \(\frac{dE}{dt}\)= 10¹² vm^-1s^-1

Displacement current, I_d= ε_{0\(\frac{dφ_E}{dt}\)} = ε₀ \(\frac{d}{dt}\) (EA)

= ε₀(πr²) \(\frac{dE}{dt}\)

= 8.85 x 10^-12 x 3.14 x (5 x 10^-2)² x 10¹²

I_d = 0.069

I_d = 0.07 (or) 70 mA

EM waves exert radiation pressure. Tails of comets are due to solar radiation.

The orderly distribution of electromagnetic radiations of all types according to their wavelength or frequency into distinct groups having widely differing properties is called electromagnetic spectrum.

When an object bums, it emits colours. That is, it emits electromagnetic radiation which depends on temperature. If the object becomes hot then it glows in red colour. If the temperature of the object is further increased then it glows in reddish-orange colour and becomes white when it is hottest. The spectrum usually called as black body spectrum.

rays are used to detect forged documents. UV-rays.

The frequency range of X-rays is from 3 x 10¹⁸ Hz to 10¹⁶ Hz.

It is used in radio and television communication systems and also in cellular phones to transmit voice communication in the ultra high frequency band

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 and used in night vision or infrared photography.

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.

It is used to destroy bacteria, sterilizing the surgical instruments, burglar alarm, detect the invisible writing, finger prints and also in the study of molecular structure.

Ozone layer absorbs ultraviolet radiations from sun and prevents it from reaching the earth’s surface and hence is crucial for human survival, as ultraviolet radiations are harmful to human beings.

i. Stars and galaxies emit different types of waves. Radio waves and visible light can pass through the Earth’s atmosphere and reach the ground without getting absorbed significantly.

Thus, the radio telescopes and optical telescopes can be placed on the ground.

ii. All other type of waves get absorbed by the atmospheric gases and dust particles. Hence, the y-ray, X-ray, ultraviolet, infrared, and microwave telescopes are kept aboard artificial satellites and are operated remotely from the Earth.

iii. Even though the visible radiation reaches the surface of the Earth, its intensity decreases to some extent due to absorption and scattering by atmospheric gases and dust particles. Optical telescopes are therefore located at higher altitudes.

When the light emitted directly from a source is examined with a spectrometer, the spectrum obtained is Emission spectrum.

Correct answer is (a) λ_m >λ_v > λ_x

The velocity of light rays of different wavelengths in a vacuum is same and hence the ratio of their velocities is 1.

(c) An electromagnetic wave going through vacuum is described by  k/ω 

(a) An electromagnetic wave going through vacuum is described by E₀k = B₀ω. 

Correct answer is (A) \(\overline{E} \times \overline{B}\)

Correct answer is (A) h^1/2

(i) γ rays (or X-rays)

(ii) Ultraviolet rays

(iii) Infrared rays

Production :

Rays : (Radioactive decay of nuclei)

X-rays : (X-ray tubes or inner shell electrons)

UV- rays : (Movement from one inner energy level to another)

Infrared rays : (Vibration of atoms and molecules)

Hertz confirms the existence of electromagnetic waves.

He produced stationary electromagnetic waves.

Using ν= cλ, he found that the em-waves travelled with the same speed as the speed of light. 

Consider a plane perpendicular to the direction of propagation of the wave. An electric charge, on the plane, will be set in motion by the electric and magnetic fields of em wave, incident on this plane. This illustrates that em waves carry energy and momentum.

(i) X-rays are produced by bombarding a metal target by high energy- electrons. 

(ii) To study the atomic structures, treatment for certain forms of cancer.

(iii) Presence of mind, alertness, taking initiative, helpful and caring.

The magnitudes of conduction and displacement currents are zero.