First slide

The lens

Question

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of combination is

Moderate

A

-1.5 D
B

-6.5 D
C

+6.5 D
D

+6.67 D

Solution

Focal length of the combination can be calculated as
$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} \Rightarrow \frac{1}{F} = \frac{1}{(+ 40)} + \frac{1}{(- 25)} \Rightarrow F = - \frac{200}{3} cm ∴ P = \frac{100}{F} = \frac{100}{- 200 / 3} = - 1 .5 D$

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