JEE Main & Advanced Physics Ray Optics Question Bank Critical Thinking

  • question_answer
    The apparent depth of water in cylindrical water tank of diameter 2R cm is reducing at the rate of x cm/minute when water is being drained out at a constant rate. The amount of water drained in c.c. per minute is (n1 = refractive index of air, n2 = refractive index of water)          [AIIMS 2005]

    A)            x p R2 n1/n2                           

    B)            x p R2 n2/n1

    C)            2 p R n1/n2                             

    D)            p R2 x

    Correct Answer: B

    Solution :

               Apparent depth \[h'=\frac{h}{_{air}{{\mu }_{liquid}}}\] Þ \[\frac{dh'}{dt}=\frac{1}{_{a}{{\mu }_{w}}}=\frac{1}{_{a}{{\mu }_{w}}}\frac{dh}{dt}\]Þ\[x=\frac{1}{_{a}{{\mu }_{w}}}\frac{dh}{dt}\]Þ \[\frac{dh}{dt}={{\,}_{a}}{{\mu }_{w}}\,x\] Now volume of water \[V=\pi {{R}^{2}}h\] Þ \[\frac{dV}{dt}\] \[=\pi {{R}^{2}}\frac{dh}{dt}\] \[=\pi {{R}^{2}}.{{\,}_{a}}{{\mu }_{w}}\,x\]             \[={{\,}_{a}}{{\mu }_{w}}\pi {{R}^{2}}x\] \[=\frac{{{\mu }_{w}}}{{{\mu }_{a}}}\pi {{R}^{2}}x\]\[=\left( \frac{{{n}_{2}}}{{{n}_{1}}} \right)\,\pi {{R}^{2}}x\]


You need to login to perform this action.
You will be redirected in 3 sec spinner