ICSE Class 10 Physics 2023 Solved Paper

Question : 44 of 76

Marks: +1, -0

A convex lens of focal length

10 {\cm}

is placed at a distance of

60 {\cm}

from a screen. How far from the lens should an object be placed so as to obtain a real image on the screen?

Solution:

Given:

f=+10 {\cm}, v=+60 {\cm}, u=

?
We know,

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

∴ \;\;{1}/{v}\;=\;{1}/{f}+\;{1}/{u}

⇒\;\;{1}/{+60}\;=\;{1}/{+10}+\;{1}/{u}

⇒\;\;{1}/{u}\;=\;{1}/{60}-\;{1}/{10}=\;{1-6}/{60}

⇒\;\;{1}/{u}\;=\;{-5}/{60}

\;\text " or "\;\;u\;=\;{60}/{-5}=-12 {\cm} .

Thus, the object must be placed at a distance of

12 {\cm}

in front of the lens.

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