Question

If a lens has power -2.5D, then it is a :

• A. convex lens with a focal length of $40cm$
• B. concave lens with a focal length of $40cm$
• C. convex lens with a focal length of $0.4cm$
• D. concave lens with a focal length of $0.4cm$

Answer 1

The correct option is B concave lens with a focal length of $40cm$

$P=\frac{1}{f\left(inmeter\right)}$

$f=\frac{1}{P}=\frac{1}{2.5}=\frac{1}{-25}\Rightarrow f=\frac{-1000}{25}cm=-40cm$

Focal length (f) of a concave lens is always negative.