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Science · 2026 · 3 marks
CBSE 2026 · Region 5 · Set 3 · Q36
An object of height $\displaystyle 6$ cm is placed at a distance of $\displaystyle 30$ cm from the optical centre of a concave lens of focal length $\displaystyle 15$ cm. Use lens formula to determine :(a)The distance of image from the optical centre.(b)The height of the image formed.
An object of height $\displaystyle 6$ cm is placed at a distance of $\displaystyle 30$ cm from the optical centre of a concave lens of focal length $\displaystyle 15$ cm. Use lens formula to determine :
(a)
The distance of image from the optical centre.
(b)
The height of the image formed.
Marking-scheme solution
(a)
Height of object \(\displaystyle =6.0 \mathrm{~cm}\)
\[\begin{aligned}
& u=-30 \mathrm{~cm} \\
& f=-15 \mathrm{~cm} \\
& \frac{1}{\mathrm{v}}-\frac{1}{u}=\frac{1}{f} \\
& \frac{1}{\mathrm{v}}=\frac{1}{f}+\frac{1}{u} \\
& \frac{1}{\mathrm{v}}=\frac{1}{-15}+\frac{1}{-30}
\end{aligned}
\]
\[\mathrm{v}=-10 \mathrm{~cm}
\]
(b) \[\begin{aligned}
m & =\frac{\mathrm{v}}{u} \\
m & =\frac{-10}{-30} \\
m & =\frac{1}{3}
\end{aligned}
\]
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