Refraction is the bending of light rays at the junction of two transparent media of different density; the eye's total refractive power is 60 D (cornea 43 D, lens 17 D).
Emmetropia: the normal refractive state, in which parallel rays from a distant object (more than 6 m) focus on the retina without accommodation.
Physical Principles of Refraction
Refractive power is measured in diopters (D), the reciprocal of the focal distance in meters; a lens of focal length 1 m has a power of 1 D.
Degree of bending depends on three factors:
Angle of incidence (oblique rays bend more).
Density difference between media (bending is toward the midline, denser medium).