## COMMUNICATION SYSTEM Quiz-16

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. The k line of singly ionized calcium has a wavelength of 393.3 nm as measured on earth. In the spectrum of one of the observed galaxies, this spectral line is located at 401.8 nm. The speed with which the galaxy is moving away from us, will be
•  6480 kms-1
•  3240 kms-1
•  4240 kms-1
•  None of the above
Solution
Here, Î»0=393.3 nm Î»=401.8 nm Red shift, z=(Î»-Î»0)/Î»0 =(401.8-393.3)/393.3=8.5/393.3=0.0216 Now z=v/c then v=zc=0.0216×3×108=6.48×106 ms-1=6480 km s-1

Q2.A laser is a coherent source because it contains
•  Many wavelengths
•  Uncoordinated wave of a particular wavelength
•  Coordinated wave of many wavelengths
•  Coordinated waves of a particular wavelength
Solution
Coordinated waves of a particular wavelength

Q3. In amplitude modulation, carrier wave frequencies are
•   Lower compared to those in frequency modulation
•  Higher compared to those in frequency modulation
•  Same as in frequency modulation
•  Lower sometimes and higher sometimes to those in frequency modulation
Solution
Lower compared to those in frequency modulation

Q4. The mobile telephones operate typically in the range of
•  1-100 MHz
•  100-200 MHz
•  1000-2000MHz
•  800-950MHz
Solution
800-950MHz

Q5.An oscillator is producing FM waves of frequency 2 kHz with a variation of 10 kHz. What is the modulating index?
•  0.20
•  5.0
•  0.67
•  1.5
Solution
The modulation index is given as Î²=∆f/fm Where, ∆f is frequency deviation, fm the highest modulating frequency. Given, ∆f=10 kHz=10×103 Hz, fm=2 kHz=2×103 Hz ∴ Î²=10×103/2×103 =5

Q6. The population inversion in helium-neon laser is produced by
•  Photon excitation
•  Electron excitation
•  Inelastic atomic collisions
•  Chemical reaction
Solution
The population inversion in helium-neon laser is produced by electron excitation

Q7.A transmitter supplies 9 kW to the aerial when unmodulated. The power radiated when modulated to 40% is
•  5 kW
•  9.72 kW
•  10 kW
•  12 kW
Solution
Pt=Pc [1+m2/2]=9[1+(0.4)2/2] =9[1+0.16/2] (∵m=40%=0.4) =9(1.08)=9.72 kW

Q8.A wave is represented as e=10sin⁡(108 t+6 sin⁡ 1250t) then the modulating index is
•  10
•  1250
•  108
•  6
Solution
Comparing with standard equation

Q9.In a diode AM-detector, the output circuit consists of R=1 kÎ©and C=10 pF. A carrier signal of 100 kHz is to be detected. Is it good
•  Yes
•  No
•  Information is not sufficient
•  None of these
Solution
For demodulation 1/fc << RC
1/fc =1/(100×103 )=10-5 s RC=103×10×10-12 s=10-8 s We see that 1/fc here is not less than RC as required by the above condition. Hence, this is not good

Q10. If Î¼1 and Î¼2 are the refractive indices of the material of core and cladding of an optical fiber, then the loss of light due to its leakage can be minimised by having
•  Î¼1>Î¼2
•  Î¼1<Î¼2
•  Î¼1=Î¼2
• None of these
Solution
Working of optical fiber is based on total internal reflection. Hence, Î¼1>Î¼2

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