CBSE Class 10 Science 2021 Term 1 Solved Paper

Question : 43 of 60

Marks: +1, -0

3. A converging lens forms a three times magnified image of an object, which can be take on a screen. If the focal length of the lens is

30 {\cm}

, then the distance of the object from the lens is:

Solution:

Explanation: We know

f=R \/ 2

\; {\m}= {\v} \/ {\u} \;\; ( {\m}=\;\text " magnification, "\;

\; {\v}=\;\text " image distance, "\; {\u}=\;\text " object distance) "\;

\;\; -3 {\u}= {\v}

\;1 \/ {\v}-1 \/ {\u}=1 \/ {\f}

\;\; -1 \/ 3 {\u}-1 \/ {\u}=1 \/ 30

\;\; - {\u}-3 {\u} \/(3 {\u})=1 \/ 30

\;\; -4 u \/ 3 {\u}^2=1 \/ 30

\;\; -3 {\u} \/ 4=30

\;u=-40 {\cm}

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