A 10cm tall candle when viewed using a convex lens appears to be 20cm tall as shown in the below figure. The focal length of the lens is 5cm. What is the object distance?
A
10cm
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B
1.0cm
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C
20cm
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D
2.5cm
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Solution
The correct option is D2.5cm Given:
Object height =h=10cm
Image height =h′=20cm
Focal length of the convex lens f=5cm
From magnification formula: m=h′h=2010=2
Also, m=vu=2
where u is the object distance and v is the image distance ⟹v=2u
From lens formula, 1v−1u=1f. 12u−1u=15 u=−2.5cm
Therefore, 2.5cm is the object distance from lens.