How do I know if my glasses have a convex or concave lens?

Answer

Convex lenses are converging and given a positive sign. Hollow lenses are negative and are given a negative sign. This is so by agreement.

For example, if you have glasses with +2 lenses, this means that the glasses have a convergence of +2 dioptres. The “convergence” of a lens is the inverse of the focal length f, so the convergence is 1/f. The focal length of the lens is smaller if the lens is stronger. That is, the convergence of the lens is greater if the lens is stronger.

You need a convex lens in your glasses if your eyes are “not convex enough”. Then you are farsighted. You need a concave lens if your eyes are “too convex”, then you are nearsighted.