A convex lens and a concave lens, each having same focal length of $25 \mathrm{~cm}$, are put in contact to…
A convex lens and a concave lens, each having same focal length of $25 \mathrm{~cm}$, are put in contact to form a combination of lenses. The power in diopters of the combination is:
zero
25
50
infinite
Solution
Focal length of lens
$f_1=25 \mathrm{~cm}$
focal length of concave lens
$f_2=25 \mathrm{~cm}$
Power of combination in dioptres
$\begin{aligned}
P=P_1+P_2 & =\frac{100}{f_1}+\frac{100}{f_2} \\
& =\frac{100}{25}+\frac{100}{-25}=0 .
\end{aligned}$