Question

An eye specialist prescribes spectacles having a combination of a convex lens of focal length $40 \,cm$ in contact with a concave lens of focal length $25\, cm$. The power of this lens combination will be

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

Answer 1

Answer: (B)

- 1.5 D

Answer 2

Power $=\frac{1}{\text { focal length }}$ Focal length of combination of convex and concave lens is given by $\frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2}}$ where $f_{1}$ and $f_{2}$ be the focal lengths of convex and concave lenses respectively. Now, $\frac{1}{F}=\frac{1}{0.4}+\frac{1}{(-0.25)}$ $\frac{1}{F}=-1.5$ $\therefore$ Power $P=-1.5 \,D$