JEE Main & Advanced Sample Paper JEE Main Sample Paper-18

  • question_answer
    In an astronomical telescope in normal adjustment, a straight black line of length L is drawn on the objective lens. The eyepiece forms a real image of this line. The length of this image is\[l\]. The magnification of the telescope is

    A)  \[L\text{/}l\]                                    

    B)  \[\frac{L}{l}+1\]

    C)  \[\frac{L}{l}-1\]               

    D)  \[\frac{L+1}{L-1}\]

    Correct Answer: A

    Solution :

     Treat the line on the objective lens as object and eyepiece as the lens. \[u=-\,({{f}_{o}}+{{f}_{e}}),\,f={{f}_{e}}\] \[\frac{1}{V}-\frac{1}{-\,({{f}_{o}}+{{f}_{e}})}=\frac{1}{{{f}_{e}}}\] \[v=\frac{({{f}_{o}}+{{f}_{e}}){{f}_{e}}}{{{f}_{o}}}\] \[m=\left| \frac{v}{u} \right|=\frac{{{f}_{e}}}{{{f}_{o}}}=\frac{l}{L}\] Magnification of telescope\[=\frac{{{f}_{o}}}{{{f}_{e}}}=\frac{L}{l}\]


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