Question

doctor prescribes spectacles to a patient with a combination of a convex lens of focal length 40 em, and concave lens of focal length 25 em then the power of spectacles will be.

• A. -6.5 D
• B. 1.5 D
• C. -1.5 D
• D. -8.5 D

Answer 1

The correct option is C -1.5 D

For combination of lenses, power
$P=P_1+P_2=\frac{100}{40}-\frac{100}{25}=-1.5D$