JEE Main & Advanced JEE Main Paper (Held on 12-4-2019 Morning)

  • question_answer
    An electromagnetic wave is represented by the electric field \[\vec{E}={{E}_{0}}\hat{n}\sin [\omega t+(6y-8z)]\] . Taking unit vectors in x, y and z directions to be \[\hat{i}.\hat{j},\hat{k},\]the direction of propagation \[\hat{s},\] is :                                                                                                 [JEE Main Held on 12-4-2019 Morning]

    A) \[\hat{s}=\frac{4\hat{j}-3\hat{k}}{5}\]                   

    B) \[\hat{s}=\frac{3\hat{j}-4\hat{j}}{5}\]

    C) \[\hat{s}=\left( \frac{-3\hat{j}+4\hat{k}}{5} \right)\]

    D) \[\hat{s}=\frac{-4\hat{j}+3\hat{j}}{5}\]

    Correct Answer: C

    Solution :

    \[\vec{E}={{E}_{0}}\hat{n}\sin (\omega t+(6y-8z))\] \[={{E}_{0}}\hat{n}\sin (\omega t+\vec{k}.\vec{r})\]where \[\vec{r}=x\hat{i}+y\hat{j}+z\hat{k}\]and \[\hat{k}.\vec{r}=6y-8z\]\[\Rightarrow \vec{k}=6\hat{j}-8\hat{k}\] direction of propagation\[\hat{s}=-\hat{k}\]\[=\left( \frac{-3\hat{j}+4\hat{k}}{5} \right)\]           


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