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

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

Answer 1

The correct option is A $-1.5D$

Power = $\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}$

= $\frac{1}{+0.4m}+\frac{1}{-0.25m}$

$\frac{1}{F}=\frac{-0.25+0.4}{0.4\times (-0.25)}$

$\therefore P=\frac{1}{F}=\frac{0.15}{-0.1}=-1.5D$