JEE Main & Advanced Physics Ray Optics Question Bank Critical Thinking

  • question_answer
    The diameter of moon is \[3.5\times {{10}^{3}}\]km and its distance from the earth is \[3.8\times {{10}^{5}}\]km. If it is seen through a telescope whose focal length for objective and eye lens are 4 m and 10 cm respectively, then the angle subtended by the moon on the eye will be approximately    [NCERT 1982; CPMT 1991]

    A)            15o

    B)    20o  

    C)            30o                                            

    D)    35o

    Correct Answer: B

    Solution :

                       \[|m|\ =\frac{{{f}_{o}}}{{{f}_{e}}}=\frac{400}{10}=40\] Angle subtented by moon on the objective of telescope \[\propto \ =\frac{3.5\times {{10}^{3}}}{3.8\times {{10}^{3}}}=\frac{3.5}{3.8}\times {{10}^{-2}}rad\] Also \[|m|\ =\frac{\beta }{\alpha }\Rightarrow \]Angular size of final image \[\beta =|m|\ \times \alpha \]\[=40\times \frac{3.5}{3.8}\times {{10}^{-2}}\] = 0.36 rad \[=0.3\times \frac{180}{\pi }\approx {{21}^{o}}\]


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