First slide

The lens

Question

Two lenses are placed in contact with each other and the focal length of combination is 80 cm. If the focal length of one is 20 cm, then the power of the other will be

Moderate

A

1.66 D
B

4.00 D
C

–1.00 D
D

– 3.75 D

Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} \Rightarrow \frac{1}{80} = \frac{1}{20} + \frac{1}{f_{2}} \Rightarrow f_{2} = - \frac{80}{3} cm ∴ Power of second lens P_{2} = \frac{100}{f_{2}} = \frac{100}{- 80 / 3} = - 3 .75 D$

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