Reduced Eye

REDUCED EYE, a conditional optical system that possesses the same properties in terms of light refraction as the actual eye, but is significantly more convenient for all kinds of calculations. As is known, any system of spherical optical surfaces, through the centers of which a straight line can be drawn, has six cardinal points: a) two principal points, b) two nodal points, and c) two focal points. If an object is placed at the first principal point, its image, equal in size to the object, will appear to be located at the second. A ray passing through the first nodal point appears, after refraction, to emerge from the second nodal point. In the human eye, the principal and nodal points are extremely close to each other, and therefore it is possible without significant error to replace all the refracting surfaces of the eye with a single optical surface. Such an imaginary conditional eye bears the name R. g. It has a refractive index_n equal to 1.33, a radius of the optical surface of 5 mm, a focal length of 15.5 mm, and a distance to the "retina" of 22.6 mm (see also Eye).