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  • The image obtained with a convex lens is erect arid. Its length is four times the length of the object. If the focal length of the lens is 20 cm, calculate the object and image distance respec

The image obtained with a convex lens is erect arid. Its length is four times the length of the object. If the focal length of the lens is 20 cm, calculate the object and image distance respectively

Option: 1

-20 cm, -60 cm


Option: 2

-15 cm, -60 cm


Option: 3

-20 cm, -50 cm


Option: 4

-20 cm,\infty


Answers (1)

best_answer

 

 

As we can see that the image formed is erect and hence virtual, so by sign convention we can write that the magnification produced by the lens is positive i.e., m=+4

\begin{array}{ll}{\text { Now, } m=\frac{f}{u+f}} \\ \\ {\therefore \quad 4=\frac{20}{u+20}} & {\text { or } u=-15 \mathrm{cm}} \\ \\ {\text { Again, }} & {m=\frac{f-v}{f}} \\ \\ {\therefore \quad} & {4=\frac{20-v}{20}} \\ \\ {\text { or }} & {v=20-80=-60 \mathrm{cm}}\end{array}

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seema garhwal

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