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Electromagnetic Waves

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Physics Part I

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Physics

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Class 10 Class 12

Welders wear special goggles or face masks with glass windows to protect their eyes from electromagnetic radiations. Name the radiations and write the range of their frequency. 


The radiations are ultraviolet radiations.

The frequency range of UV rays is 1015 –1017Hz.

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During the propagation of electromagnetic waves in a medium:

  • Electric energy density is equal to the magnetic energy density.

  • Both electric and magnetic energy densities are zero.

  • Electric energy density is double of the magnetic energy density.

  • Electric energy density is half of the magnetic energy density.


C.

Electric energy density is double of the magnetic energy density.

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An electromagnetic wave in vaccum has the electric and magnetic fields straight E with rightwards arrow on top and straight B with rightwards arrow on topwhich are always perpendicular to each other. the direction of polarisation is given by straight x with rightwards arrow on top and that of wave propagation by straight k with rightwards arrow on top then

  • straight X with rightwards arrow on top vertical line vertical line straight B with rightwards arrow on top space and space straight X with rightwards arrow on top vertical line vertical line straight B with rightwards arrow on top space straight x space straight E with rightwards arrow on top
  • straight X with rightwards arrow on top vertical line vertical line straight E with rightwards arrow on top space and straight k with rightwards arrow on top vertical line vertical line space straight E with rightwards arrow on top straight x straight B with rightwards arrow on top
  • straight X with rightwards arrow on top vertical line vertical line straight B with rightwards arrow on top space and space straight k with rightwards arrow on top vertical line vertical line straight E with rightwards arrow on top space straight x straight B with rightwards arrow on top
  • straight X with rightwards arrow on top vertical line vertical line straight E with rightwards arrow on top space and space straight k with rightwards arrow on top vertical line vertical line space straight B with rightwards arrow on top space straight X space straight E with rightwards arrow on top

C.

straight X with rightwards arrow on top vertical line vertical line straight B with rightwards arrow on top space and space straight k with rightwards arrow on top vertical line vertical line straight E with rightwards arrow on top space straight x straight B with rightwards arrow on top
203 Views

The threshold frequency for a metallic surface corresponds to an energy of 6.2 eV, and the stopping potential for a radiation incident on this surface 5 V. The incident radiation lies in

  • X-ray region

  • ultra-violet region

  • infra-red region

  • visible region


B.

ultra-violet region

straight lambda space equals space fraction numerator 1242 space eVnm over denominator 11.2 end fraction space almost equal to space 1100 space straight A
straight lambda space equals space fraction numerator 1242 space eVnm over denominator 11.2 end fraction space almost equal to space 1100 space straight A
457 Views

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

  • 3 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

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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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 

  • infrared waves produce heating in a microwave oven 


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).

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

673 Views