If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined togethe

Question

If a convex lens of focal length 80 cm and a concave lens of focal length 50 cm are combined together, what will be their resulting power?\

Professor PC Thomas Classes · Answer

(b) : Focal length $f_{1}=80 \mathrm{~cm}$ and $f_{2}=-50 \mathrm{~cm}$ (Minus sign due to concave lens)\ Power of the combination $(P)=P_{1}+P_{2}$\ $=\frac{100}{f_{1}}+\frac{100}{f_{2}}=\frac{100}{80}-\frac{100}{50}=-0.75 \mathrm{D}$\