Prisms can change the direction of parallel light rays. The optical strength of a prism is specified in prism diopters. Prism lenses can be combined with spherical and toric lenses. When prescribing eyeglasses, the ophthalmologist specifies the strength and the position of the base of the prism. Prism lenses are used to correct heterophoria (latent strabismus) and ocular muscle pal-sies, and in preparation for surgery to correct strabismus.
A 1 diopter prism deflects a ray of light 1 cm at a distance of 1 m from the base of the prism.