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

  • question_answer
    The image of an illuminated square is obtained on a screen with the help of a converging lens. The distance of the square from the lens is 40 cm. The area of the image is 9 times that of the square. The focal length of the lens is :

    A) 36 cm

    B) 27 cm

    C) 60 cm

    D) 30 cm

    Correct Answer: D

    Solution :

    [d] If side of object square = \[\ell \] and side of image square = \[\ell \]? From question, \[\frac{\ell {{'}^{2}}}{\ell }=9\text{ or }\frac{\ell '}{\ell }=3\] i.e., magnification \[m=3~\text{          }u=-40cm\] \[v=3\times 40=120cm\text{   f=?}\] From formula, \[\frac{1}{v}-\frac{1}{u}=\frac{1}{f}\] \[\frac{1}{120}-\frac{1}{-40}=\frac{1}{f}\text{ or }\frac{1}{f}=\frac{1}{120}+\frac{1}{40}=\frac{1+3}{120}\] \[\therefore 30cm\]


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