JEE Main & Advanced Physics Wave Optics / तरंग प्रकाशिकी Question Bank Self Evaluation Test - Wave Optics

  • question_answer
    From a medium of index of refraction\[{{n}_{1}}\], monochromatic light of wavelength \[\lambda \] is incident normally on a thin film of uniform thickness L \[(where\,\,L>0.1\lambda )\] and index of refraction \[{{n}_{2}}\]. The light transmitted by the film travels into a medium with refractive index \[{{n}_{3}}\].The value of minimum film thickness when maximum light is transmitted If \[({{n}_{1}}<{{n}_{2}}<{{n}_{3}})\] is

    A) \[\frac{{{n}_{1}}\lambda }{2{{n}_{2}}}\]

    B) \[\frac{{{n}_{1}}\lambda }{4{{n}_{2}}}\]

    C) \[\frac{\lambda }{4{{n}_{2}}}\]

    D) \[\frac{\lambda }{2{{n}_{2}}}\]

    Correct Answer: B

    Solution :

    [b] Equation of path difference form maxima in transmission (or weak reflection)    \[\Delta {{P}_{opt}}=2{{n}_{2}}L=\frac{{{\lambda }_{vacuum}}}{2},\frac{3{{\lambda }_{vacuum}}}{2}....\] \[\Rightarrow 2\left( \frac{{{n}_{2}}}{{{n}_{1}}} \right)\] \[L=\frac{\lambda }{2}\], \[\frac{3\lambda }{2}......\Rightarrow L=\frac{\lambda }{4{{n}_{2}}}\]  (notice that \[\lambda \]= wavelength in medium is related to \[{{\lambda }_{vacuum}}\]as, \[{{\lambda }_{vacuum}}={{n}_{1}}\lambda \])


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