JEE Main & Advanced Physics Ray Optics Question Bank Photometry

  • question_answer
    A lamp is hanging along the axis of a circular table of radius r. At what height should the lamp be placed above the table, so that the illuminance at the edge of the table is \[\frac{1}{8}\] of that at its center                                              [MP PET 2005]

    A)            \[\frac{r}{2}\]                      

    B)            \[\frac{r}{\sqrt{2}}\]

    C)            \[\frac{r}{3}\]                      

    D)            \[\frac{r}{\sqrt{3}}\]

    Correct Answer: D

    Solution :

                       \[\frac{{{I}_{center}}}{{{I}_{edge}}}=\frac{{{({{r}^{2}}+{{h}^{2}})}^{3/2}}}{{{h}^{3}}}\]                    \[\Rightarrow 8=\frac{{{({{r}^{2}}+{{h}^{2}})}^{3/2}}}{{{h}^{3}}}\]\[\Rightarrow 2h={{({{r}^{2}}+{{h}^{2}})}^{1/2}}\]            \[\Rightarrow 4{{h}^{2}}={{r}^{2}}+{{h}^{2}}\]\[\Rightarrow 3{{h}^{2}}={{r}^{2}}\]Þ \[h=\frac{r}{\sqrt{3}}\]


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