First slide

The lens

Question

A convex lens of focal length 84 cm is in contact with a concave lens of focal length 12 cm. The power of combination (in diopters) is

Moderate

A

$\frac{25}{24}$
B

$\frac{25}{18}$
C

$\frac{- 50}{7}$
D

$\frac{+ 50}{7}$

Solution

Focal length of the combination
$\frac{1}{F} = \frac{1}{f_{1}} + \frac{1}{f_{2}} = \frac{1}{(+ 84)} + \frac{1}{(- 12)} \Rightarrow F = - 14 cm ∴ P = \frac{100}{F} = \frac{100}{- 14} = - \frac{50}{7} D$

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