A dioptre is a unit of power in a lens, and its positive or negative value expresses the refractive power of the lens. The positive sign corresponds to the converging lens and the negative sign corresponds to the diverging lens.
Where the converging lens is concerned, the rays that pass through it meet at a point to “converge”. Converging lenses have become the perfect way to correct hypermetropia, presbyopia and some types of astigmatism.
Diverging lenses are demarcated by two surfaces, one of which is concave. They are able to separate the rays that cross them and make them “diverge”. They are used in refraction to make up for myopia errors and some types of astigmatism.
Mr. José Álvarez