What is the nature of a lens whose power is, $-4 \mathrm{D}$?

The optician's prescription for a spectacle lens is marked $+0.5 \mathrm{D}$. What is the:

Nature of spectacles lens?

The optician's prescription for a spectacle lens is marked $+0.5 \mathrm{D}$. What is the:

Focal length of spectacle lens?

A convex lens of power $5 \mathrm{D}$ and a concave lens of power $7.5 \mathrm{D}$ are placed in contact with each other. What is the:

Power of this combination of lenses?