The focal length of the lens is 40 cm. In the middle, the lens is less than at the edges. What kind of lens is it?

The focal length of the lens is 40 cm. In the middle, the lens is less than at the edges. What kind of lens is it? Find its optical power.

From the assignment, we can conclude that you are not given a spacious lens, but a concave one. Therefore, finding the optical power is now easier:
Focal length (F) = 40;
Optical power (D) -?;
Let’s write the formula:
D = 1 / F;
D = 1 / 0.4;
D = 2.5;
Answer: optical power 2.5.