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

  • question_answer
                            The image of an extended object placed in front of a concave mirror is formed at a distance of 40 cm from the object. If the image is 3 times bigger than the object, the magnitude of focal length of the mirror is

    A) 15 cm                          

    B) 10 cm  

    C) 20 cm                

    D) 5 cm                

    Correct Answer: A

    Solution :

    We know, \[m=\frac{{{h}_{i}}}{{{h}_{0}}}=\frac{-v}{u}\] Here, \[m=3,v-u=40\,cm\] So, \[m=\frac{-v}{u}\]or, \[-m=\frac{v}{u}\] or, \[-m-1=\frac{v}{u}-1\] or, \[-3-1=\frac{40}{u}\] or, \[-4=\frac{40}{u}\] or, \[u=-10\,cm\] \[\therefore \]\[v-u=40\,cm\] or \[v+10=40\,cm\] or, \[v=30\,cm\] We know, \[\frac{1}{f}=\frac{1}{u}+\frac{1}{v}\] So \[f=\frac{uv}{u+v}\] or \[f=\frac{-300}{20}=-15\,cm\]


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