Read the information given below.

It is useful to design lenses of derived focal length using surface of suitable radii of curvature. Lens maker formula helps in finding various unknowns and is widely applied.

Find the power of a concave lens whose focal length is 40 cm

-2.5 D

The correct answer is option (1) : -2.5 D

$P=\frac{1}{f(m)}=\frac{1}{(-0.4)}$

$=-2.5 D$