51.

A sound absorber attenuates the sound level by 20 dB. The intensity decreases by a factor of

• 1000

• 10000

• 10

• 100

What maximum frequency can be reflected from ionosphere?

• 5 MHz

• 6 GHz

• 5 KHz

• 500 MHz

A hemisphere is positively charged uniformly. The electric field at a point on a diameter away from the centre is directed

• perpendicular to the diameter

• parallel to the diameter

• at an angle tilted towards the diameter

• at an angle tilted away from the diameter

Assertion:  Sky wave signals are used for long distance radio communication. These signals are in general, less stable than ground wave signals. 

Reason:  The state of ionosphere varies from hour to hour, day to day and season to season.

• If both assertion and reason are true and reason is the correct explanation of assertion.

• If both assertion and reason are true but reason is not the correct explanation of assertion.

• If assertion is true but reason is false.

• If both assertion and reason are false.

Assertion:  Short wave communication over long distance is not possible via ground waves. 

Reason:  The ground waves can bend round the corners ofthe objects on earth.

• If both assertion and reason are true and reason is the correct explanation of assertion.

• If both assertion and reason are true but reason is not the correct explanation of assertion.

• If assertion is true but reason is false.

• If both assertion and reason are false.

For satellite communication which wave is used

• Space wave

• Sky wave

• Ground wave

• Microwave

If modulation index is 1/2 and power of carrier wave is 2 W. Then what will be the total power in modulated wave?

• 0.5 W

• 1 W

• 0.25 W

• 2.25 W

Long distance communication between two point or earth is achieved by

• Space wave communication

• Sky wave communication

• Satellite wave communication

• Line of sight transmission

Assertion: In a communication system based on amplitude modulation the modulation index is kept <1. 

Reason: It ensures minimum distortion of signal.

• If both assertion and reason are true and reason is the correct explanation of assertion.

• If both assertion and reason are true but reason is not the correct explanation of assertion.

• If assertion is true but reason is false.

• If both assertion and reason are false.

Voltage of modulating wave of 5 V with 10 MHz frequency was superimposed on carrier wave of frequency 20 MHz and voltage 20 V then the modulation index is

• 0.25

• 1.25

• 2.43

• 64.0