CBSE Class 12 Physics Communication Systems Important Questions Worksheet

Communication Systems MCQ Questions with Answers Class 12 Physics 

Question- The particle nature of light is not confirmed by

(a) photoelectric effect

(b) scattering of X-rays by electrons

(c) diffraction of electrons

(d) compton effect

Answer-(c)

Question- The photoelectric current does not depend upon the
(i) frequency of incident light
(ii) work function of the metal
(iii) stopping potential
(iv) intensity of incident light

(a) (i) and (iv) only

(b) (ii) and (iii) only

(c) (iii) only

(d) (ii) only

Answer-(c)

Question-A discharge takes place between the two electrodes on applying the electric field to the gas in the discharge tube. The cause of this fluorescence was attributed to

(a) the radiations which appeared to be coming from the anode

(b) the radiation which appeared to be coming from the cathode

(c) the protons coming from the cathode

(d) the protons coming from the anode

Answer-(b)

Question-Photoelectric emmision occurs only when the incident light has more than a certain minimum

(a) power

(b) wavelength

(c) intensity

(d) frequency

Answer-(d)

Question- In photoelectric effect, electrons are ejected from metals, if the incident light has a certain minimum

(a) wavelength

(b) frequency

(c) amplitude

(d) angle of incidence

Answer-(b)

Question-Which of the following when falls on a metal will emit photoelectrons ?

(a) UV radiations

(b) Infrared radiation

(c ) Radio waves

(d) Microwaves

Answer-(a)

Question-Photons are deflected by

(a) electric field only

(b) magnetic field only

(c) electromagnetic field

(d) None of these

Answer-(d)

Question-The work-function of a metal is

(a) the minimum current required to take out electron from the metal surface

(b) the maximum frequency required to take out electron from the metal surface

(c) the minimum amount of energy required to take out the electron from the metal surface

(d) None of these

Answer-(c)

Question-The work function of a metal is independent of
(i) nature of the surface of the metal
(ii) dimensions of the metal
(iii) properties of the metal
(iv) abundance of the metal

(a) (i) only

(b) (i) and (iii)

(c) (ii) and (iii)

(d) (ii) and (iv)

Answer-(d)

Question-Which of the following metals is not sensitive to visible light?

(a) Caesium

(b) Sodium

(c) Rubidium

(d) Cadmium

Answer-(d)

Question-Cathode ray consists of

(a) photons

(b) electrons

(c) protons

(d) α-particles

Answer-(b)

Question- A photosensitive substance emits _____when illuminated by light.

(a) only protons

(b) only neutrons

(c) electrons and protons

(d) only electrons

Answer-(d)

Question-In optical fibre refractive index of core is

(a) less than R.I of cladding

(b) more than R.I of cladding

(c) equal to R.I of cladding

(d) halfed to R.I of cladding

Answer-(b)

Question-Telephony is an example of ________ mode of communication

(a) point-to-point

(b) broadcast

(c) both (a) and (b)

(d) None of these

Answer-(a)

Question-Sky wave propagation is not possible for waves of frequency > 30 MHz because

(a) these waves do not have much energy to reach ionosphere

(b) they are not reflected by ionosphere

(c) they get absorbed by troposphere

(d) they get reflected by stratosphere

Answer-(b)

More Question

1. What are the essential elements of a communication system? Also draw a neat block diagram of a communication system. 

2. Give the functions of the following

(a) Transmitter

(b) Transducer

3.What are the basic modes of communication? Give an example for each.

4. What is meant by amplification? Why amplification is necessary in communication systems ? 

5. Define the following terms.
(a) Range
(b) Repeater 

6 Define the following terms. (a)Range (b) Repeater

7 What is modulation? What are the different types of modulation?

8 Select the required bandwidth for transmitting speech, music, and TV signals from the following frequencies. (a) 20kHz (b) 6MHz (c) 2.8kHz

9 Draw diagram showing a fundamental sine wave and its harmonics.

10 Find the bandwidth of transmission medium required from the following for coaxial cable, and optical communication using fibres , satellite communication Uplink Downlink for BW 750 MHz and excess of 100GHz.5.9-6.4 GHz, 3.7-4.2GHZ

11 What are the different modes of propagation of electromagnetic waves? What is ground wave propagation? What are the limitations of ground wave propagation?

12 Waves of which frequency range propagate through sky wave. Briefly explain what is sky wave propagation? Name the phenomenon by which the wave gets reflected towards the earth.

13 Give the range of frequencies for which the ionosphere acts as a reflector.

Communication Systems MCQ Questions with Answers Class 12 Physics 

Question- There is a need of translating the information contained in our original low frequency baseband signal into ...X... or ...X... frequencies before transmission . Here, X and Y refere to
(a) low, radio
(b) high, radio
(c) low, audio
(d) high, video
Answer-(b) 

Question-The phenomenon by which light travels in an optical fibres is
(a) reflection
(b) refraction
(c) rotal internal reflection
(d) transmission
Answer-(c) 

Question-Bandwidth of optical fibre communication is
(a) 106 to 109 Hz
(b) 1013 to 1015 Hz
(c) 109 to 1011 Hz
(d) none of these
Answer-(b)

Question-Communication is the process of
(a) keeping in touch
(b) exchange information
(c) broad casting
(d) entertainment by electronics
Answer-(b) 

Question-Ground wave propagation is possible for
(a) low radio frequency over a short range
(b) high radio frequency over a short range
(c) high radio frequency over a long range
(d) low radio frequency over a short range.
Answer-(a) 

Question- Buffer amplifier is used at the transmitting end to
(a) feed carrier frequency to master oscillator
(b) amplify carrier frequency
(c) mix modulating signal with carrier frequency
(d) isolate master oscillator from other stages of transmitter.
Answer-(d) 

Question-Electromagnetic waves of audible frequency ranges from
(a) 10 Hz to 10,000 Hz
(b) 20 Hz to 20,000 Hz
(c) 30 Hz to 30,000 Hz
(d) None of these.
Answer-(b) 

Question-The purpose of a detector at the receiving end is
(a) to amplify signal
(b) to reduce its frequency level
(c) to modulate signal
(d) to demodulate signal
Answer-(d) 

Question-Long range transmission of TV-signal is done by
(a) space-wave
(b) sky waves
(c) ground wave
(d) artificial satellite.
Answer-(d) 

Question- Picture signal of TV-signal is
(a) amplitude modulated
(b) frequency modulated
(c) phase modulated
(d) pulse modulated
Answer-(a)

Question-Ground waves are polarised
(a) parallel to the earth's surface
(b) normal to the earth's surface
(c) at an angle 45° from earth's surface
(d) in any direction.
Answer-(b) 

Question-Long distance short-wave radio broadcasting uses
(a) ground wave
(b) ionospheric wave
(c) direct wave
(d) sky wave
Answer-(c) 

Question-Sky wave propagation is not possible for frequencies
(a) equal to 30 MHz
(b) less than 30 MHz
(c) greater than 30 MHz
(d) None of these
Answer-(c) 

Question-During the process of transmission and reception the signal gets deteriorated due to
(a) noise introduced in the system
(b) distortion in the system
(c) both (a) and (b)
(d) neigher (a) nor (b)
Answer-(c)

Question-Global communication is achieved by using
(a) single geostationary satellite
(b) minimum two geostationary satellite 180° apart
(c) minimum three geostationary satellite 120° apart
(d) minimum four geostationary satellite 90° apart
Answer-(c)

Question- Long range propagation is not possible by space wave propagation because
(a) height of troposhere is quite small
(b) height of troposhere is large
(c) troposphere absorbs transmitted wave
(d) None of these.
Answer-(a)

Question-Which of the following is the element of a communication system?
(a) Transmitter
(b) channel
(c) Receiver
(d) All of the above
Answer-(d) 

Question-Intensity of electric field obtained at receiver antenna for a space wave propagation is
(a) directly proportional to the perpendicular-distance from transmitter to antenna
(b) inversely proportional to the perpendicular-distance from transmitter to antenna
(c) directly proportional to the square perpendiculardistance from transmitter to antenna
(d) inversely proportional to the square perpendiculardistance from transmitter to antenna
Answer-(d) 

Question-The electron density in all the layers of ionosphere
(a) is the same
(b) decreases with altitude
(c) increases with altitude
(d) sometimes decreases sometimes increases
Answer-(c)

Communication System

We live in the world of information. Information needs to be communicated from one entity to another entity.
This act of sending and receiving message from one place to another place, successfully, is called communication.
The word successful in the above definition, implies many things like 

The growing needs of human beings in the field of communication imposed demands on Evolution in communication
The table below shows us how physical messengers who travelled from one place to another changed to the 

As we see here, the basic elements of communication includes transmitter, Channel and the receiver. The transmitter and the receiver may be located geographically at different places. The Channel connects the transmitter and the receiver. 

Information Source – The source produces signal of the information which needs to be communicated.
Signal – Information in electrical form suitable for transmission is called signal.
Transmitter – Converts the source signal into suitable form for transmission through the channel.
Channel – The channel connecting the transmitter and the receiver is a physical medium. The channel can be in the form of wires, cables or wireless.
Noise – When the transmitted signal propagates along the channel, it may get distorted due to channel imperfection.
Thus, noise refers to unwanted signals that tend to disturb the process of communication from the transmitter to the receiver.
Receiver – Due to noise and other factors, the corrupted version of signal arrives at the receiver. The receiver has to reconstruct the signal into recognizable form of the original message for delivering it to the user. The signal at the receiver forms the output. 

Modes of communication 

Point to point communication – There is a single link between the transmitter and the receiver.
Communication takes place between single transmitter and receiver
Example – Telephone 

Broadcast mode – There are large number of receivers though information is sent by a single transmitter.
Example – Television and Radio 

Communication – Terminology
1. Transducer – Any device which converts energy in one form to another form is called transducer.
Electrical transducer: A device that converts some physical variable like pressure, displacement, force,temperature, into corresponding variations in electrical signal. Hence, the output of this would be an electrical signal. 

2. Signal Types – Information in electrical form suitable for transmission called signal, is of two types 

Analog signal –
o Continuous variations of voltage and current. Hence, single valued functions of time.
o Sine wave is a fundamental analog signal
o Example – Sound and picture signals in television 

Digital signal –
o Digital step value based
o Binary system where 0 represents low level and 1 represents high level is used
o Universal digital coding methods like BCD – Binary Coded Decimal and ASCII – American Standard
Code of Information Interchange is used in common 

Signal propagation – Terminology
1. Attenuation – The loss of strength of the signal while propagating through a medium is known as attenuation.
2. Amplification – The process of increasing the amplitude of the signal by using an electronic circuit is called amplification. This also increases the strength of the signal. Hence, it compensates the attenuation of the signal.
3. Range – It is the largest distance between the source and the destination upto which the signal is received with sufficient strength.
4. Bandwidth – It refers to the frequency range for which the equipment operates.
5. Modulation – If the information signal is of low frequency, it cannot be transmitted to long distances.
Hence, at transmission point, it is super imposed on high frequency wave. This high frequency wave acts as a carrier of information. This is modulation  Sinusoidal wave modulation-

There are 3 types of modulation, namely 1. Amplitude modulation 2.Frequency modulation and 3.Phase modulation
Amplitude modulation –
The amplitude of the carrier wave is varied in accordance with the information signal

Phase modulation –
The phase of the carrier wave is varied in accordance with the information signal

Pulse wave modulation-
There are 3 types of pulse wave modulation – namely (a) Pulse amplitude modulation (b) Pulse width modulation (c) Pulse position modulation

6. Demodulation – The process of retrieval of information from the carrier wave at the receiver is termed as demodulation. This is a reverse of modulation.

7. Repeater – A repeater is a combination of receiver and a transmitter.
A repeater picks up the signal from the transmitter, amplifies and retransmits it to the receiver. Thus repeaters are used to extend the range of communication system
Example – Communication satellite is a repeater station in space.

Propagation of electromagnetic waves
While communication using radio waves, the transmitter antenna radiates electromagnetic waves. These waves travel through the space and reach the receiving antenna at the other end. We have considered below some of the wave propagation methods in brief.

Ground or Surface wave propagation:

o In this mode of wave propagation, ground has a strong influence on propagation of signal waves from the transmitting antenna to the receiving antenna. The signal wave glides over the surface of the earth
o While propagating on the surface of the earth, the ground wave induces current in the ground. It also bends around the corner of the objects on the earth
o Due to this, the energy of the ground wave is gradually absorbed by the earth and the power of the ground wave decreases
o The power of the ground wave decreases with the increase in the distance from the transmitting station.
This phenomenon of loss of power of the ground wave is called attenuation
o The attenuation of ground waves increases very rapidly with the increase in its frequency
o Thus, ground wave communication is not suited for high frequency signal wave and for very long range communication
o To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength of the signal 

Sky waves: 

o The ionosphere plays a major role in sky wave propagation. We know that the earth’s atmosphere is divided into various regions like – Troposphere, Stratosphere, Mesosphere and Ionosphere.
o The ionosphere is also called as thermosphere as temperature increases rapidly here and it is the outermost part of the earth’s atmosphere.
o Above troposphere, we have various layers like D (part of stratosphere), E (part of stratosphere), F₁ (part of mesosphere), F₂ (part of ionosphere)
o The ionosphere is called so because of the presence of large number of ions or charged particles.Ionisation occurs due to the absorption of the ultraviolet and other high energy radiation coming from the sun, by the air molecues
o The phenomenon of bending of electromagnetic waves in this layer so that they are diverted towards the earth is helpful in skywavepropogation. This is similar to total internal reflection in optics
o The radiowaves of frequency range from 1710 kHz to 40 MHz are propagated in sky wave propagation 

Space waves: 

o The space waves travel in straight line from the transmitting antenna to the receiving antenna.
o Hence, space waves are used for line of sight communication such as television broadcast, microwave link and satellite communication
o The line of sight communication is limited by (a) the line of sight distance (b) the curvature of the earth
o At some point by the curvature of the earth, the line of sight propagation gets blocked.
o The line of sight distance is the distance between transmitting antenna and receiving antenna at which they can see each other. It is also called range of communication dM
o The range of space wave communication can be increased by increasing the heights of the transmitting antenna and receiving antenna.
o The maximum line of sight distance (range of communication) dM between two transmitting antenna of height hT and the receiving antenna of height hR above the earth is given by 

Modulation and its necessity
Any message signal, in general, is not a single frequency sinusoidal. But it spreads over a range of frequencies called the signal bandwidth.
Suppose we wish to transmit an electronic signal, in the audio frequency range, say 20 Hz to 20kHz range, over a long distance we need to consider factors like 

a. Size of antenna:
o Antenna is needed for both transmission and reception
o Antenna should have a size comparable to the wavelength of the signal, atleastλ/4 where λ is the wavelength of the signal
o In the above audio frequency range, if we consider frequency 15,000= ט Hz. Then λ = c / / 108* 3= ט 15,000 = 20,000 m
o Hence, antenna length = λ/4 = 20,000 / 4 = 5000 m.
o It is practically impossible to design an antenna of height 5000m
o So the transmission frequency should be raised in such a way that the length of the antenna is within 100m which is feasible for practical purpose
o This shows that there is a need for converting low frequency signal to high frequency before transmission 

b. Effective power radiated by the antenna:
o Effective power rated by the antenna = P = E/t
o Also, E = h ט = hc/λ
o Hence, P = E/t = hc/λ * c/λ
o Studies reveal that if l is the linear length of the antenna, then P is proporational to (l/λ)2
o Hence, for good transmission, high power and hence small wavelength and high frequency waves are required
o High frequency waves becomes inevitable in this case also 

c. Avoiding mixing of signals from different transmitters:
o When many transmitters are transmitting baseband information signals simultaneously, they all gets mixed up
o There is no way to distinguish between them
o Possible solution is communication at high frequencies and allotting a band of frequencies for each transmitter so that there is no mixing
o This is what is being dene for different radio and TV broadcast stations
Hence, we understand the necessity of modulation. 

Band width
Bandwidth is also defined as the amount of data that can be transmitted in a fixed amount of time 

Signals – Bandwidth:
o The message signal can be voice, music, picture or computer data
o Each of the above have different frequency ranges
o The speech signals frequency range from 300Hz to 3100Hz. Hence, bandwidth = 3100 -300 = 2800 Hz
o Any music requires bandwidth of 20kHz because of high frequencies produced by musical instruments
o Video signals for transmission of picture requires 4.2 MHz of bandwidth
o The Television signal which contains both voice and picture is usually allocated a bandwidth of 6MHz bandwidth for transmission 

Transmission Medium – Bandwidth:
o Different types of transmission media offers different bandwidth
o Coaxial cables, widely used wire medium offers bandwidth of approximately 750 MHz
o Communication through free space using radio waves offers wide range from hundreds of kHz to few GHz
o Optical fibres are used in the frequency range of 1THz to 1000 THz (THz – Tera Hertz; 1THz = 1012Hz)
o As mentioned earlier, to avoid mixing of signals, allotting a band of frequencies to a specific transmitter is in practise
o The International Telecommunication Union administers this frequency allocation
o Services like FM Broadcast, Television, Cellular Mobile Radio and Satellite communication operate under fixed frequency bands Let us now consider amplitude modulation in detail. 

Amplitude modulation
As we know, in amplitude modulation, the amplitude of the carrier wave is varied in accordance with the amplitude of the information signal or modulating signal.
For sinusoidal modulating wave, 

Equation (6) shows that the amplitude modulated signal consists of
1. Carrier wave of frequency ω_c
2. Sinusoidal wave of frequency (ω_c-ω_m)
3. Sinusoidal wave of frequency (ω_c+ω_m)

The two additional waves are called side bands. The frequency of these bands are called side band frequencies
Frequency of lower side band = (ω_c-ω_m)
Frequency of upper side band = (ω_c+ω_m)

The band width of the AM wave is Frequency of lower side band minus Frequency of upper side band (ω_c+ω_m) - (ω_c-ω_m) = 2ω_m (Twice the frequency of modulating signal)
Graphical representation

The signal is passed through band pass filter centered at ω_c
This rejects the low and high frequencies. In the above case, the filter rejects d.c, ω_c,ω_m, 2ω_m,(ω_c– ω_m). The frequencies ω_c, (ω_c – ω_m) and (ω_c+ ω_m) are passed. This is amplitude modulated wave.
This wave cannot be passed as such. It needs to be amplified and then fed to an antenna of appropriate size for radiation.

Detection Process:
o It is then passed through the detector.
Hence, INPUT – Modulated carrier wave of frequencies ωc, (ωc+ωm) and (ωc-ωm)
OUTPUT – Original signal m(t) of frequency ωm

More Question

1 Name the waves that are used for line of sight communication and satellite communication.

2 Communication of which type of waves are limited to line of sight paths? Why?

3 Give an expression to find the maximum line of sight distance between the transmitting antenna and receiving antenna. 

4 A transmitting antenna at the top of a tower has a height of 32m and the height of the receiving antenna is 50m. What is the maximum distance between them for satisfactory communication in LOS mode? Given radius of earth is 6.4 X 10⁶m.

5 Give the diagram representing various modes of propagation of em waves.

6 What is the need of modulation? 

7  Explain how effective power radiated by an antenna depends upon the wavelength?

8 What is meant by Amplitude Modulation? Also give a plot of amplitude versus frequency for an Amplitude Modulated wave.

9 Give the differences between AM and FM. 

10 A message signal of frequency 10kHz and peak voltage of 10volts is used to modulate a carrier of frequency 1 MHz and Peak voltage of 20 volts. Determine (a) modulation index (b) the side bands produced. 

11 What is HTML?

12 What is E-mail? What is the use of it?

13 Explain how a document can be sent from one location to another using FAX.

14 What is a cell? What is the function of a cell?

15 In which region of em spectrum mobile telephony works? Define the term handover in mobile telephony 

16 Give a note on World Wide Web.