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

  • question_answer
    A convex lens is in contact with concave lens. The magnitude of the ratio of their focal length is 2/3. Their equivalent focal length is 30 cm. What are their individual focal lengths?

    A) -15, 10             

    B) -10, 15     

    C) 75, 50              

    D) -75, 50     

    Correct Answer: A

    Solution :

    [a] \[\frac{|{{f}_{1}}|}{|{{f}_{1}}|}=\frac{2}{3}\] \[{{f}_{1}}:\]focal length of convex lens. \[\frac{1}{f}=\frac{1}{{{f}_{1}}}-\frac{1}{{{f}_{2}}}\] \[\Rightarrow \frac{1}{30}=\frac{1}{{{f}_{1}}}-\frac{1}{3{{f}_{1}}}\] \[{{f}_{1}}=10cm,{{f}_{2}}=-15cm\]


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