The electric field associated with an electromagnetic wave in va

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 Multiple Choice QuestionsMultiple Choice Questions

41.

If E and B represent electric and magnetic field vectors of an electromagentic wave, the direction of propagation of the wave is along

  • E × B

  • B × E

  • E

  • B


42.

Out of the following statements which is not true ?

  • Infrared radiations arise due to minor electron transitions in atoms

  • Infrared radiations are used for long distance photography

  • Sun is the natural source of infrared radiation

  • Infrared radiations are detected by using a bolometer


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43.

The electric field associated with an electromagnetic wave in vacuum is given by E=i 40 cos (kz-6x108t), where E, z= and t are in volt/m, metre and second respectively. The value of wave vector k is

  • 2 m-1

  • 0.5 m-1

  • 6 m-1

  • 6 m-1


A.

2 m-1

Electromagnetic wave equation 
straight E space equals space straight E subscript straight o space cos space left parenthesis kz minus ωt right parenthesis
Speed space of space electromagnetic space wave space straight v space equals space straight omega over straight k
space Given space equation
straight E space equals space straight i 40 space cos space left parenthesis kz minus 6 space straight x space 10 to the power of 8 space straight t right parenthesis
comparing space Eqs. space left parenthesis straight i right parenthesis space and space left parenthesis ii right parenthesis comma space we space get
straight omega space equals space 6 space straight x space 10 to the power of 8
and
straight E subscript straight o space equals 40 space straight i
here comma space wave space factor space straight k space equals space straight omega over straight v space equals space fraction numerator 6 space straight x space 10 to the power of 8 over denominator 3 straight x space 10 to the power of 8 end fraction space equals space 2 space straight m to the power of negative 1 end exponent

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44.

The condition under which a microwave oven heats up a food item containing water molecules most efficiently is

  • the frequency of the microwave must match the resonant frequency of the water molecules

  • the frequency of the microwave has no relation  with natural frequency of water molecules

  • microwave are heat waves, so always produce heating 

  • microwave are heat waves, so always produce heating 

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45.

The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is

  • gamma rays, ultraviolet, infrared, microwaves

  • microwaves, gamma rays, infrared, ultraviolet

  • infrared, microwave, ultraviolet, gamma rays.

  • infrared, microwave, ultraviolet, gamma rays.

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46.

The energy of the EM waves is of the order of 15 keV. To which part of the spectrum does it belong?

  • X - rays

  • infrared rays

  • Ultraviolet rays

  • Ultraviolet rays

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47.

The velocity of electromagnetic radiation in a medium of permittivity straight epsilon subscript straight o and permeability straight mu subscript straight o is given by

  • square root of epsilon subscript o over mu subscript o end root
  • square root of straight mu subscript straight o straight epsilon subscript straight o end root
  • fraction numerator 1 over denominator square root of straight mu subscript straight o straight epsilon subscript straight o end root end fraction
  • fraction numerator 1 over denominator square root of straight mu subscript straight o straight epsilon subscript straight o end root end fraction
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48.

The electric and magnetic field of an electromagnetic wave are:

  • in phase and parallel to each other

  • in opposite phase and perpendicular to each other

  • in opposite phase and parallel to each other

  • in opposite phase and parallel to each other

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49.

X-rays are

  • electromagnetic radiation

  • stream of electron

  • stream of proton

  • stream of uncharged particle


50.

Who discovered X-rays?

  • Roentgen

  • Madam Curie

  • Rutherford

  •  All of these


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