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Electromagnetic Waves Test - 5
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Electromagnetic Waves Test - 5
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  • Question 1/10
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    A laser emits a beam of light of 2 mm diameter. If the power of the beam is 10 mW, then find its intensity.

    Solutions

     

  • Question 2/10
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    If ue and um respectively are the electric and magnetic field densities in an electromagnetic (EM) wave and 'c' is the speed of light, then

    Solutions

     

  • Question 3/10
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    In the absence of atmosphere around Earth, the average surface temperature of Earth

    Solutions

    In the presence of the atmosphere, heat radiations reflected by the earth are further reflected back to the earth by the atmosphere, which increases the temperature. In the absence of the atmosphere, no such reflection back to earth will take place. Hence the heat radiations will be reflected back and temperature of the earth will decrease.

     

  • Question 4/10
    1 / -0

    The Poynting vector  gives the

    Solutions

    The Poynting vector  gives the magnitude and direction of the energy flow in an EM wave. In physics, the Poynting vector represents the directional energy flux (the energy transfer per unit area per unit time) of an electromagnetic field. The SI unit of the Poynting vector is the watt per square metre (W/m2). It is named after its discoverer, John Henry Poynting, who first derived it in 1884.

     

  • Question 5/10
    1 / -0

    The dielectric constant of air is 1.006. The speed of an electromagnetic wave travelling in air is a × 108ms–1, where a is about

    Solutions

    For an electromagnetic wave (in vacuum),

    Air acts almost as vacuum, hence
    a = 3 (approx.)

     

  • Question 6/10
    1 / -0

    Which of the following electromagnetic waves emitted by the sun are responsible for heating Earth's atmosphere due to greenhouse effect?

    Solutions

    Infrared radiations emitted by the sun are responsible for heating Earth's atmosphere due to greenhouse effect. 

     

  • Question 7/10
    1 / -0

    The pressure exerted by an electromagnetic wave of intensity I (W/m2) on a non-reflecting surface is
    (where c is the velocity of light)

    Solutions

    As we know,
    Pressure = Force/Area
    Energy of the incident beam is given by E = pc
    For a non-reflecting surface, final momentum of the incident beam = 0
    Hence, change in momentum of the beam = E/c

    Here, I is the intensity of the light, which is defined as energy per unit area per second.

     

  • Question 8/10
    1 / -0

    If Earth receives 20 cal min-1 cm-2 of solar energy, then what are the respective amplitudes of electric and magnetic fields of radiations? (Use SI units)

    Solutions

     

  • Question 9/10
    1 / -0

    At what speed (m/s) does an electron have a de Broglie wavelength of 1.0 nm?

    Solutions

     

  • Question 10/10
    1 / -0

    Which of the following electromagnetic waves are used in telecommunication?

    Solutions

    Microwaves are used in telecommunication.
    So, option 4 is correct.
    Radio waves - These are used for television and radio programmes.
    Visible radiation - These are used in photography.
    Ultraviolet rays - These can be used to kill microbes. Hospitals use UV lamps to sterilise surgical equipment and the air in operating theatres.

     

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