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A 5 cm tall object is placed 20 cm in front of a convex lens of focal length 10 cm. Using the lens formula, what is the image distance v?
A−20 cm
B+6.7 cm
C+10 cm
D+20 cm
Answer & Solution
Correct answer: D. +20 cm
1. Apply the sign convention: u = −20 cm, f = +10 cm.
2. Lens formula: $\frac{1}{v} = \frac{1}{u} + \frac{1}{f} = \frac{1}{-20} + \frac{1}{10}$.
3. $\frac{1}{v} = \frac{-1 + 2}{20} = \frac{1}{20}$.
4. So v = +20 cm; the positive sign means a real image on the opposite side. Dropping the sign on u would give the wrong −20 cm.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 7 "Lenses", p.95_
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