University of Edinburgh
 

BSL Physics Glossary - spectacles/focus - definition

Converging lens or diverging lens - produce focused images. In long sight the corneal curvature is not strong enough to produce a focused image on the retina, so an additional converging lens is used. In short sight the converging effect is too strong for the eyeball, so this is corrected with diverging lens. Converging lens are thicker in the centre and they make the parallel rays converge. Diverging lens are thinner in the middle; they make the parallel rays diverge.