First slide

The lens

Question

A magnifying glass is to be used at the fixed object distance of 1 inch. If it is to produce an erect image magnified 5 times its focal length should be

Moderate

A

0.2 inch
B

0.8 inch
C

1.25 inch
D

5 inch

Solution

$m = \frac{v}{u} = 5 \Rightarrow v = 5 inch (Given u = 1 inch) Using sign convention u = - 1 inch, v = - 5 inch ∴ \frac{1}{f} = \frac{1}{v} - \frac{1}{u} = \frac{1}{- 5} - \frac{1}{- 1} \Rightarrow f = 1 .25 inch$

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