Thank you for reporting, we will resolve it shortly
Q.
If a convex lens of focal length $ 75\, cm $ and a concave lens of focal length $ 50\, cm $ are combined together, what will be their resulting power ?
Convex lens is a converging lens while concave lens is a diverging one.
Power of a lens is defined as reciprocal of focal length measured in metres.
$\therefore \,P = \frac{1}{f\,\left(in\,m\right)}D$ or $P = \frac{100}{f\,\left(cm\right)}D$
Given, $f _{1} = + 75\, cm$, $f_{2} = − 50 \,cm$
$\therefore \, \frac{1}{f} = \frac{1}{f_{1}}+\frac{1}{f_{2}}$
$= \frac{1}{75} - \frac{1}{50}$
$= \frac{2-3}{150} = -\frac{1}{50}$
$\Rightarrow f = - 150\,cm$
$\therefore $ Power $= \frac{100}{f} = - \frac{100}{150}$
$= - 0.66\,D$
Since, focal length of combined lens is negative, it behaves as a diverging lens.