Dioptrics, (Greek dioptrika, from dia — through and opteuo — I see), the traditional (and gradually falling out of use) name of the branch of geometrical optics that deals with the refraction of light as it passes through individual refracting surfaces and systems of such surfaces. The term “Dioptrics” is often applied to the eye: the dioptrics of the eye is a description of the properties of the eye as an optical instrument.