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10th Class Science Light - Reflection and Refraction Question Bank Light

  • question_answer
    A lens forms a real image on a screen placed at a distance 100 cm from the object if the lens is moved by 20 cm towards the screen, another image is formed on the screen. The focal length of the lens is

    A)  6 cm                     

    B)         12 cm

    C)  18 cm                  

    D)         24 cm  

    Correct Answer: D

    Solution :

                    \[f=\frac{({{D}^{2}}-{{x}^{2}})}{4D}\] Here, \[D=100\,\,cm,\] and \[x=20\,\,cm\] \[\therefore \]  \[f=\frac{[{{(100)}^{2}}-{{(20)}^{2}}}{4\times 100}\]                   \[=\frac{10000-400}{400}\]                   \[\mathbf{=24}\,\,\mathbf{cm}\]


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