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CBSE Class 10 Science 2025 Solved Paper All Sets

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Question : 10 of 20
Marks: +1, -0
To get an image of magnification –1 on a screen using a lens of focal length 20 cm, the object distance must be :
Solution:  
m=ff+um=\frac{f}{f+u}
−1=2020+u-1=\frac{20}{20+u}
−20−u=20-20-u=20
u=−40 cmu=-40\,\text{cm}
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