12th Class Physics EM Waves Question Bank MCQs - Electromagnetic Waves

  • question_answer
    The ratio of contributions made by the electric field and magnetic field components to the intensity of an EM wave is

    A) \[c:1\]

    B) \[{{c}^{2}}:1\]

    C) \[1:1\]

    D) \[\sqrt{c}:1\]

    Correct Answer: C

    Solution :

    Option [c] is correct.
    Explanation: Average energy by electric field
    \[{{\operatorname{E}}_{0}}\] is \[{{\operatorname{U}}_{av}}\]
    \[{{\operatorname{U}}_{av}}=\frac{1}{2}{{\varepsilon }_{0}}E_{0}^{2}\]
    But   \[{{\operatorname{E}}_{0}}={{\operatorname{cB}}_{0}}\]
    \[{{({{\operatorname{U}}_{av}})}_{\operatorname{electric}\ field}}=\frac{1}{2}{{\varepsilon }_{0}}{{(c{{B}_{0}})}^{2}}=\frac{1}{2}{{\varepsilon }_{0}}{{c}^{2}}\operatorname{B}_{0}^{2}\]
    \[=\frac{1}{2}{{\varepsilon }_{0}}.\frac{1}{{{\mu }_{0}}{{\varepsilon }_{0}}}{{({{B}_{0}})}^{2}}\because {{c}^{2}}=\frac{1}{{{\mu }_{0}}{{\varepsilon }_{0}}}\]
    \[{{({{u}_{av}})}_{\operatorname{electric}\ field}}=\frac{1}{2{{\mu }_{0}}}B_{0}^{2}{{({{\operatorname{U}}_{av}})}_{(magnetic\ field)}}\]
        \[\operatorname{Ratio}=\frac{({{\operatorname{U}}_{av}})\operatorname{electric}\ field}{({{\operatorname{U}}_{av}})(magnetic\ field)}\]
                \[=\frac{1}{1},i.e.,1\ :1\]


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