JEE Main & Advanced Sample Paper JEE Main - Mock Test - 29

  • question_answer
    An electromagnetic wave of frequency 3 MHz passes from vacuum into a dielectric medium with permittivity \[{{\varepsilon }_{r}}=4\]. Then

    A) the wavelength and frequency both  remain unchanged

    B) the wavelength is doubled and the frequency remains unchanged

    C) the wavelength is doubled and the frequency becomes half

    D) the wavelength is halved and the frequency remains unchanged.

    Correct Answer: D

    Solution :

    [d]: Frequency remains unchanged with change of medium.      Refractive index, \[n=\frac{c}{v}=\frac{1/\sqrt{{{\varepsilon }_{0}}{{\mu }_{0}}}}{1/\sqrt{\varepsilon \mu }}=\sqrt{{{\varepsilon }_{r}}{{\mu }_{r}}}\] Since \[{{\mu }_{r}}\] is very close to \[1,n=\sqrt{{{\varepsilon }_{r}}}=\sqrt{4}=2\] Thus, tedium \[{{\lambda }_{medium}}=\frac{\lambda }{n}=\frac{\lambda }{2}\]


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