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JEE Main & Advanced Physics Ray Optics Question Bank Self Evaluation Test - Ray Optics and Optical Instruments

  • question_answer
    A telescope has an objective lens of focal length 150 cm and an eyepiece of focal length 5 cm. If a 50 m tall tower at a distance of 1 km is observed through this telescope in normal setting, the angle  formed by the image of the tower is \[\theta \], then \[\theta \] is close to :

    A) 6.1 rad

    B) 3.2 rad

    C) 1.5 rad

    D) 0.2 rad

    Correct Answer: C

    Solution :

    [c] Magnifying power of telescope, \[MP=\frac{\beta \left( \text{angle subtended by image at eye piece} \right)}{\alpha \left( \text{angle subtended by object on objective} \right)}\]Also, \[MP=\frac{{{f}_{0}}}{{{f}_{e}}}=\frac{150}{5}=30\text{     }\alpha =\frac{50}{1000}=\frac{1}{20}rad\] \[\therefore \,\,\,\,\,\beta =\theta =MP\times \alpha =30\times \frac{1}{20}=\frac{3}{2}=1.5rad\]


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