JEE Main & Advanced Physics Photo Electric Effect, X- Rays & Matter Waves Question Bank Critical Thinking

  • question_answer
    The eye can detect 5 ´ 104 photons per square metre per sec of green light (l = 5000 Å) while the ear can detect \[{{10}^{-13}}\,(W/{{m}^{2}})\]. The factor by which the eye is more sensitive as a power detector than the ear is close to

    A)            5    

    B)            10

    C)            106

    D)            15

    Correct Answer: A

    Solution :

                       \[E=\frac{12375}{5000}=2.475\,eV\,\approx 4\times {{10}^{-19}}J\] So the minimum intensity to which the eye can respond \[{{I}_{Eye}}=\] (Photon flux) ´ (Energy of a photon) Þ \[{{I}_{Eye}}=(5\times {{10}^{4}})\times \,(4\times {{10}^{-19}})\tilde{}2\times {{10}^{-14}}\,(W/{{m}^{2}})\] Now as lesser the intensity required by a detector for detection, more sensitive it will be  \[\frac{{{S}_{Eye}}}{{{S}_{Ear}}}=\frac{{{I}_{Ear}}}{{{I}_{Eye}}}\]\[=\frac{{{10}^{-13}}}{2\times {{10}^{-14}}}=5\] i.e. as intensity (power) detector, the eye is five times more sensitive than ear.


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