Question

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

• A. $+6.67D$
• B. $-6.5D$
• C. $-1.5D$
• D. $+6.5D$

Answer 1

The correct option is C $-1.5D$

Given,

$f_1=+40cm$

$f_2=-25cm$

Power, $P=\frac{1}{f}$

The power of combination,

$P=\frac{1}{f_1}+\frac{1}{f_2}$

$P=\frac{100}{+40cm}+\frac{100}{-25cm}$

$P=-1.5D$

The correct option is C.