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


A.

E × B

According to electromagnetic wave property, the electric field is perpendicular to the magnetic field. The direction of electromagnetic wave propagation will be perpendicular to E and B then E × B = C.


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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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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


A.

Infrared radiations arise due to minor electron transitions in atoms

Infrared radiation arises due to major electron transitions in atoms, eg : 4 → 5, 5 → 6, 6 → 7 etc.


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.


D.

infrared, microwave, ultraviolet, gamma rays.

Decreasing order of the wavelength of various rays.

Microwave > infrared > Ultraviolet > Gamma

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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 


A.

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

It is an electromagnetic wave, therefore the correct choice is (a).

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