Test Index

ICSE Class 10 Physics 2020 Paper

Show Para  Hide Para 
A lens of focal length 20 cm20\,\text{cm} forms an inverted image at a distance 60 cm60\,\text{cm} from the lens.
© examsnet.com
Question : 39 of 67
Marks: +1, -0
How far is the lens present in front of the object?
Solution:  
Given : f=+20 cm,v=+60 cmf=+20\,\text{cm}, v=+60\,\text{cm}
We know,   1f=  1v−  1u\;\frac{1}{f}=\;\frac{1}{v}-\;\frac{1}{u}
⇒      120=  160−  1u\Rightarrow \;\; \;\frac{1}{20}=\;\frac{1}{60}-\;\frac{1}{u}
⇒      1u=  160−  120\Rightarrow \;\; \;\frac{1}{u}=\;\frac{1}{60}-\;\frac{1}{20}
⇒      1u=  1−360=  −260\Rightarrow \;\; \;\frac{1}{u}=\;\frac{1-3}{60}=\;\frac{-2}{60}
⇒      1u=  −130\Rightarrow \;\; \;\frac{1}{u}=\;\frac{-1}{30}
or     u=−30 cm\;\; u=-30\,\text{cm}.
∴\therefore The lens is present in front of the object at a distance of 30 cm30\,\text{cm} from it.
© examsnet.com
Go to Question:
Prev Question
Next Question