What is the nature of a lens whose power is,- 4 D?

The power of a lens is, - 2 D. What is its focal length ?

What is the nature of a lens having a power of + 0.5 D ?

The optician's prescription for a spectacle lens is marked+ 0.5 D. What is the: (a) nature of spectacle lens?

(b) focal length of spectacle lens ?

A doctor has prescribed a corrective lens of power, -2 D. Find the focal length of the lens. Is the prescribed lens diverging or converging ?