COMMUNICATION SYSTEM Quiz-18

As per analysis for previous years, it has been observed that students preparing for NEET find Physics out of all the sections to be complex to handle and the majority of them are not able to comprehend the reason behind it. This problem arises especially because these aspirants appearing for the examination are more inclined to have a keen interest in Biology due to their medical background. Furthermore, sections such as Physics are dominantly based on theories, laws, numerical in comparison to a section of Biology which is more of fact-based, life sciences, and includes substantial explanations. By using the table given below, you easily and directly access to the topics and respective links of MCQs. Moreover, to make learning smooth and efficient, all the questions come with their supportive solutions to make utilization of time even more productive. Students will be covered for all their studies as the topics are available from basics to even the most advanced. .

Q1. Arrange the following communication frequency bands in the increasing order of frequencies 1. AM broadcast 2. Cellular mobile radio 3. F.M. broadcast 4. Television UHF 5. Satellite communication
•  1 3 4 2 5
•  1 2 3 4 5
•  5 2 4 3 1
•  1 3 2 4 5
Solution

Q2.The principle used in the transmission of signal through an optical fibre is
•  Total internal reflection
•  Reflection
•  Refraction
•  Dispersion
Solution
The optical fibres are used to transmit light signals from one place to another without any practical loss in the intensity of light signal. It works on the principle of total internal reflection.

Q3. Electromagnetic waves with frequencies greater than the critical frequency of ionosphere cannot be used for communication using sky wave propagation, because
•   The refractive index of the ionosphere becomes very high for f>fc
•  The refractive index of the ionosphere becomes very low for f>fc
•  The refractive index of the ionosphere becomes very high for f<fc
•  None of the above
Solution
The sky waves are the radiowaves of frequency range between 2 MHz to 30 MHz. The ionosphere reflects these radiowaves back to earth during their propagation through atmosphere.

The refractive index of ionosphere is less than its free space value. That is, it behaves as a rarer medium. Therefore, the wave will turn away from the normal when it enters the ionosphere. As we go deep into the ionosphere (electron density N is large), the refractive index keeps on decreasing. The refraction or bending of the beam will continue till it reaches critical angle after which it will be reflected back. If the frequency is too high, then after a certain value, the electron density N may never be so high as to produce enough bending for attainment of critical angle or condition of reflection. This is called critical frequency (fc ). For frequencies higher than this value, the refractive index of the ionosphere becomes very high, so they cross the ionosphere and do not return back to the earth.

Q4. Which fibers are less expensive and simple to construct?
•  Single-mode step index fiber
•  Multi-mode step index fiber
•  All are equally expensive
Solution
Multi-mode step index fibers are less expensive and easy to construct

Q5. Intelsat satellite is used for
•  Intercontinental communication
•  None of the above
Solution
Intelsat satellite is used for intercontinental communication

Q6. For good demodulation of AM signal of carrier frequency f, the value of RC should be
•  RC=1/f
•  RC<1/f
•  RC≥1/f
•  RC>>1/f
Solution
For good demodulation, 1/f<<RC or RC>>1/f

Q7.In hydrogen spectrum, the wavelength of HÎ± line is 656 nm, whereas in the spectrum of a distant galaxy, HÎ± wavelength is 706 nm. Estimated speed of the galaxy with respect to earth is
•  2×108 ms-1
•  2×107 ms-1
•  2×106 ms-1
•  2×105 ms-1
Solution
The wavelength of light emitted by a moving object is shifted. This effecd is called the Doppler’s shift given by Î»=Î»0 (1+v/c) …(i) Where Î»is perceived wavelength, v the velocity and c the speed of light. From Eq. (i) v=c((Î»-Î»0)/Î»0 ) Given, Î»0=656 nm,Î»=706 nm, c=3×108 ms-1 v=3×108 ((706-656)/656) v=3×108×50/656≈2×107 ms-1

Q8.The wavelength of red light from He-Ne laser is 633 nm in air but 474 nm in the aqueous humor inside the eye ball. Then the speed of red light through the aqueous humor is
•  3×108 m/s
•  1.34×108 m/s
•  2.25×108 m/s
•  2.5×108 m/s
Solution
Î¼∝ 1/v∝1/Î» ∴V∝Î» V1/V2 =Î»1/Î»2 ⇒(3×108)/V2 =633/474⇒V2=3×108×474/633 =2.25×108 m/s

Q9.In the communication systems, AM is used for broadcasting because
•  Its use avoids receiver complexity
•  It is more noise immune than other modulation system
•  It requires less transmitting power
•  No other modulation system can give the necessary bandwidth for faithful transmission
Solution
In the communication system, amplitude modulation is used for broadcasting to avoide receiver complexity.

Q10. In an FM system a 7 kHz signal modulate 108 MHz carrier so that frequency deviation is 50 kHz. The carrier swing is
•  7.143
•  8
•  0.71
•  350
Solution
Carrier swing =(Frequency deviation)/(Modulation frequency)=50/7=7.143

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