A convex lens has focal length $+10$ cm. A pencil placed $30$ cm from the lens forms an image at:
A$10$ cm, equal to the focal length of the lens
B$30$ cm, equal to the object distance behind the lens
C$+15$ cm, by $1/v - 1/u = 1/f$ with $u = -30$
D$-5$ cm, a virtual image on the same side as the pencil
Answer & Solution
Correct answer: C. $+15$ cm, by $1/v - 1/u = 1/f$ with $u = -30$
$1/v - 1/(-30) = 1/10 \Rightarrow 1/v = 1/10 - 1/30 = 1/15$, $v = 15$ cm.
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