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JEE Main & Advanced Physics Ray Optics Question Bank Refraction at Curved Surface

  • question_answer
    An object is placed first at infinity and then at 20 cm from the object side focal plane of the convex lens. The two images thus formed are 5 cm apart. The focal length of the lens is                                            [SCRA 1994]

    A)            5 cm                                         

    B)            10 cm

    C)            15 cm                                      

    D)            20 cm

    Correct Answer: B

    Solution :

                       For first case : \[\frac{1}{f}=\frac{1}{v}-\frac{1}{\infty }\Rightarrow f=v\]                    For second case \[\frac{1}{f}=\frac{1}{(f+5)}-\frac{1}{-(f+20)}\Rightarrow f=10\,\,cm\]                                  Alternative sol. ? \[{{f}^{2}}={{x}_{1}}{{x}_{2}}\] Þ \[f=10cm.\]


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