A dioptre is a unit of lens power and its positive or negative power expresses the refractive power of a lens. The positive sign corresponds to the converging lens and the negative power corresponds to the diverging power. Converging lenses are perfect way of correcting hypermetropia, prebyopia and other astigmatisms.
Diverging lenses are demarcated by two surfaces, one of them being concave. They have the ability to separate the rays that pass through them and make them 'diverge'. They are used in refraction to make up for the myopia errors and some types of astigmatism.