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An object is placed 20 cm from a convex lens of focal length 10 cm. Using the lens formula, what is the image distance $v$?
A$-20\ \text{cm}$
B$+30\ \text{cm}$
C$+10\ \text{cm}$
D$+20\ \text{cm}$
Answer & Solution
Correct answer: D. $+20\ \text{cm}$
1. Lens formula: $\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$, with $f = +10$ cm and $u = -20$ cm.
2. Substitute: $\dfrac{1}{v} = \dfrac{1}{f} + \dfrac{1}{u} = \dfrac{1}{10} + \dfrac{1}{-20}$.
3. Compute: $\dfrac{1}{v} = \dfrac{2 - 1}{20} = \dfrac{1}{20}$.
4. So $v = +20$ cm, a real image on the far side.
5. The value $+10$ cm wrongly equals $f$ and ignores the object term, so it is a trap.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Lens Formula", p.29_
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