NEET Physics EM Waves NEET PYQ-EM Waves

  • question_answer
    The electric field of an electromagnetic wave in free space is given by
    \[\overset{\to }{\mathop{\mathbf{E}}}\,=10\cos ({{10}^{7}}t+kx)\mathbf{\hat{j}}v/m,\] where t and x are in seconds and metres respectively. It can be inferred that                                                                        [AIPMT (M) 2010]
    (1) The wavelength \[\lambda \] is 188.4 m.
    (2) The wave number k is 0.33 rad/m.
    (3) The wave amplitude is 10 V/m.
    (4) The wave is propagating along \[+x\] direction Which one of the following pairs of statements is correct?

    A)  (3) and (4)       

    B)  (1) and (2)       

    C)  (2) and (3) 

    D)  (1) and (3)

    Correct Answer: D

    Solution :

    The electric field of electromagnetic wave
    \[\overset{\to }{\mathop{\mathbf{E}}}\,=10\cos ({{10}^{7}}t\pm kx)\,\mathbf{j}\]
    Amplitude \[=10\text{ }V/m\]
    \[\because \]       \[c=\frac{\omega }{k}\]
    \[\therefore \]      \[3\times {{10}^{8}}=\frac{{{10}^{7}}}{k}\]
    or         \[k=\frac{1}{30}\]
    or         \[\frac{2\pi }{\lambda }=\frac{1}{30}\]
    or         \[\lambda =188.4\,\text{m}\]
    So, (1) and (3) option are correct.

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