Question

Two thin lenses having optical powers of $-10D$ and $+6D$ are placed in contact with each other. The focal length of the combination is:

• A. $+0.25cm$
• B. $-0.25cm$
• C. $+0.25m$
• D. $-0.25m$

Answer 1

The correct option is D $-0.25m$

Given, optical powers of lens
$P_1=-10D$
and $P_2=+6D$
When these are kept in contact to each other,
Power of combination $P=P_1+P_2=-10+6$
$P=-4D$
Focal length (in metre)$=\frac{1}{P\left(indiopter\right)}$
$=-\frac{1}{4}$
$=-0.25m$