First slide

The lens

Question

If convex lens of focal length 80cm and a concave lens of focal length 50 cm are combined together, what will be their resulting power

Moderate

A

+ 6.5 D
B

-6.5 D
C

+7.5 D
D

-0.75 D

Solution

$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} \Rightarrow \frac{1}{F} = \frac{1}{+ 80} + \frac{1}{- 50} \Rightarrow F = - \frac{400}{3} \Rightarrow P = \frac{- 3}{4} D$

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