The Human Eye

The human eye is, structurally, a natural optical instrument built almost entirely from refracting elements, that forms a real, inverted image of the outside world on a light-sensitive layer at its back. Light entering the eye first passes through the cornea, a transparent curved membrane at the very front, which does most (roughly two-thirds) of the eye's total converging (refracting) power because of the comparatively large jump in refractive index between air and the corneal tissue. Behind the cornea lies the iris, a coloured, muscular diaphragm with a central circular opening, the pupil, whose diameter the iris muscles automatically widen in dim light and narrow in bright light, controlling how much light enters. Behind the pupil sits the eye lens, a flexible, transparent, convex crystalline lens held in place by the ciliary muscles, which provides the remaining, finer part of the eye's total converging power and, crucially, is the one element of the eye whose own focal length can be actively adjusted (see accommodation, next sub-section). The combined refraction at the cornea and the eye lens brings light to a focus on the retina, the light-sensitive layer lining the inside of the back of the eyeball, which contains millions of photoreceptor cells (rods, sensitive to dim light and to shades of brightness, and cones, sensitive to colour), and which converts the real, inverted optical image focused on it into electrical nerve signals, carried to the brain along the optic nerve for interpretation (it is the brain, not the eye, that then constructs the perceived upright sense of the scene from this inverted retinal image). The overall optical action of the eye -- forming a real, inverted, diminished image o …