Assertion & Reason

Question 11 Mark

Assertion : Short wave bands are used for transmission of radio waves to a large distance.
Reason : Short waves are reflected by ionosphere.

Answer

(a) Having the range of wavelength from $30 \mathrm{~km}$ to $30 \mathrm{~cm}$ are known as short wave. These waves are used for radio transmission and for general communication purpose to a longer distance from ionosphere. lonosphere is the outermost region of atmosphere extending from height of $80 \mathrm{~km}$ to 400 $\mathrm{km}$ approximately, above the surface of earth. Therefore, both the assertion and reason are true and reason is the correct explanation of assertion.

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Question 21 Mark

Assertion : Diode lasers are used as optical sources in optical communication.
Reason : Diode lasers consume less energy.

Answer

(b) In optical communication, diode laser is used to generate analog signals or digital pulses for transmission or digital pulses for transmission trough optical fibres. The advantage of diode lasers are their small size and low power input.

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Question 31 Mark

Assertion : In high latitude one sees colourful curtains of light hanging down from high altitudes.
Reason : The high energy charged particles from the sun are deflected to polar regions by the magnetic field of the earth.

Answer

(a) Microwave communication is preferred over optical communication because microwaves provide large number of channels and wider band width compared to optical signals as information carrying capacity is directly proportional to band width. So, wider the band width, greater the information carrying capacity.

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Question 41 Mark

Assertion : Television signals are received through sky-wave propagation.
Reason : The ionosphere reflects electromagnetic waves of frequencies greater than a certain critical frequency.

Answer

(d) TV signals (frequency greater than $30 \mathrm{MHz}$ ) cannot be propagated through sky wave propagation.Above critical frequency, an electromagnetic wave penetratates the ionosphere and is not reflected by it.

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Question 51 Mark

Assertion : A dish antenna is highly directional.
Reason : This is because a dipole antenna is omni directional.

Answer

(b) A dish antenna is a directional antenna because it can transmit or sec.

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Question 61 Mark

Assertion : Fax is a modulating and demodulating device.
Reason : lt is necessary for exact reproduction of a document.

Answer

(e) The electronic reproduction of a document at a distance plane is known as FAX modulation and demodulation is done by modem.

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Question 71 Mark

Assertion : Satellite is an ideal platform for remote sensing.
Reason : Satellite in polar orbit can provide global coverage or continuous coverage of the fixed area in geostationary configuration.

Answer

(a) The remote sensing is done through a satellite. A remote sensing satellite files in a polar orbit at an altitude of $918 \mathrm{~km}$, around the earth, in such away that it passes over a given location on the earth at the same local time.

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Question 81 Mark

Assertion : Microwave propagation is better than the sky wave propagation.
Reason : Microwaves have frequencies 100 to $300 \mathrm{GHz}$, which have very good directional properties.

Answer

(a) Microwaves have got good directional properties. Due to it, the microwaves can be directed as beam signals in a particular direction, much better than radio waves, because microwaves do not bend around the corners of any obstacle coming in their way.

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Question 91 Mark

Assertion : The television broadcasting becomes weaker with increasing distance.
Reason : The power transmitted from TV transmitter varies inversely as the distance of the receiver

Answer

(c) As the distance increases, TV signals becomes weaker. So assertion is true. The power transmitted from TV transmitter is inversely proportional to the square of the distance of the receiver. That's why reason is false.

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Question 101 Mark

Assertion : The surface wave propagation is used for medium wave band and for television broadcasting.
Reason : The surface waves travel directly from transmitting antenna to receiver antenna through atmosphere.

Answer

(a) Both assertion and reason are true and reason is the correct explanation of assertion. (For more detail, refer theory).

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Question 111 Mark

Assertion : The electromagnetic waves of shorter wavelength can travel longer distances on earth's surface than those of longer wavelengths.
Reason : Shorter the wavelength, the larger is the velocity of wave propagation.

Answer

(c) The electromagnetic waves of shorter wavelength do not suffer much diffraction from the obstacles of earth's atmosphere so they can travel long distance.Also, shorter the wavelength, shorter is the velocity of wave propagation.

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Question 121 Mark

Assertion : The electrical conductivity of earth's atmosphere decreases with altitude.
Reason : The high energy particles (i.e. $\gamma$-rays and cosmic rays) coming from outer space and entering our earth's atmosphere cause ionisation of the atoms of the gases present there and the pressure of gases decreases with increase in altitude.

Answer

(e) The electrical conductivity of earth's atmosphere increases with height so assertion is false.When high energy particles enters in earth's atmosphere. They ionises the gases present in atmosphere. Also as we go up, the air thins out gradually and air pressure decreases.

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