NCERT Solutions for CBSE Class 12 Physics ‒ Chapter 15: Communication Systems

NCERT Solutions for 12th Physics, Chapter 15 ‒ Communication Systems are available here. Problems from this chapter of NCERT textbook are frequently asked in CBSE board exams. In order to score well is CBSE Class 12 Physics board exam 2018; you must prepare all the questions and solutions for this chapter very well.

NCERT Solutions for CBSE Class 12 Physics ‒ Chapter 15: Communication Systems are given below:

Question 15.1: Which of the following frequencies will be suitable for beyond-the horizon communication using sky waves?

(a) 10 kHz

(b) 10 MHz

(c) 1 GHz

(d) 1000 GHz

Solution 15.1: (b)

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Question 15.2: Frequencies in the UHF range normally propagate by means of:

(a) Ground waves

(b) Sky waves

(c) Surface waves

(d) Space waves

Solution 15.2: (d)

Question 15.3: Digital signals

(i) do not provide a continuous set of values,

(ii) represent values as discrete steps,

(iii) can utilize binary system, and

(iv) can utilize decimal as well as binary systems.

Which of the above statements are true?

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(a) (i) and (ii) only

(b) (ii) and (iii) only

(c) (i), (ii) and (iii) but not (iv)

(d) All of (i), (ii), (iii) and (iv).

Solution 15.3: (c)

Question 15.4: Which of the above statements are true?

(a) (i) and (ii) only

(b) (ii) and (iii) only

(c) (i), (ii) and (iii) but not (iv)

(d) All of (i), (ii), (iii) and (iv).

Solution15.3: (c)

Question 15.4: Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for line-of-sight communication? A TV transmitting antenna is 81m tall. How much service area can it cover if the receiving antenna is at the ground level?


No, it is not necessary.

Service Area Covered = 𝜋d2 = 3.14 x 162 x 6.4 x 106 = 3258 sq km.

Question 15.5: A carrier wave of peak voltage 12V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75%?


Modulation Index = Am/Ac

0.75 = Am/12

Am = 9 V

Question 15.6: A modulating signal is a square wave, as shown in Fig. 15.14.

The carrier wave is given by c (t) = 2 sin (8 π t) volts.

(i) Sketch the amplitude modulated waveform

(ii) What is the modulation index?



(ii) Modulation Index = Am/Ac = 1 / 2.

Question 15.7: For an amplitude modulated wave, the maximum amplitude is found to be 10V while the minimum amplitude is found to be 2V. Determine the modulation index, μ.


Modulation Index = Am/Ac = (M1 – M2) / (M1 + M2) = (10 – 2) / (10 + 2) = 2 / 3

If M2 = 0,

Modulation Index = 1.

Question 15.8: Due to economic reasons, only the upper sideband of an AM wave is transmitted, but at the receiving station, there is a facility for generating the carrier. Show that if a device is available which can multiply two signals, then it is possible to recover the modulating signal at the receiver station.


Let the received signal be A1cos(wc + wm)t

Carrier wave = Ac cos ωc t

Multiplying above two signals

A1ACcos(ωc + ωm)t cosωct = [A1Ac/2].[cos(2ωc + ωm)t + cosωmt]

Using a low-pass filter, the modulating signal = [(𝐴1𝐴𝑐)/

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