It is approximately spherical in shape, with its outer layers, such as the outermost, white part of the eye (the sclera) and one of its inner layers (the pigmented choroid) keeping the eye essentially light tight except on the eye's optic axis. The eye can be considered as a living optical device. 65(3): 281–322.The human eye is a sensory organ, part of the sensory nervous system, that reacts to visible light and allows us to use visual information for various purposes including seeing things, keeping our balance, and maintaining circadian rhythm. 'Optimization, Constraint, and History in the Evolution of Eyes'.

'Eye Color Changes Past Early Childhood'.

Bito, LZ Matheny, A Cruickshanks, KJ Nondahl, DM Carino, OB (1997).

Eye color can change over time, mainly due to hormonal changes or chemical reactions in the body.

The color is a result of Rayleigh scattering, which is also responsible for the blue color of the sky. Human eyes stay about the same size from birth until death. The reason is that it's too hard to reconnect the million-plus nerve fibers of the optic nerve.

Doctors are unable to transplant an entire eye.The hole in vision isn't noticeable because each eye fills in the other's blind spot. This is the point where the optic nerve attaches to the eyeball. There are special cells in the eyes that detect light but aren't involved in forming images. Blind people who still have eyes can sense the difference between light and dark.Insects do perceive ultraviolet light, but their compound eyes don't focus as sharply as human eyes, so the energy is spread out over a larger area. The reason humans evolved to not see UV light is because the light has enough energy to damage the rods and cones. The lens absorbs it before it can reach the retina. People don't see ultraviolet light, but the human retina can detect it.If you wear special goggles that make you view everything upside down, after a few days your brain will adapt, again showing you the 'correct' view. When the brain translates the image, it automatically flips it. The eye acts exactly like a camera in the sense that the image formed on the retina is inverted (upside down).

The functioning of the eye is fairly simple, but there are some details you might not know: The vitreous humor is a transparent watery gel that supports the eye and allows for this distance. Vitreous Humor: A certain distance is required to focus light.Ciliary muscles surround the lens, relaxing to flatten it to image distant objects and contracting to thicken the lens to image close-up objects. Lens: While most of the focusing of light is done by the cornea, the lens allows the eye to focus on either near or distant objects.As the pupil dilates (gets bigger), more light enters the eye. The size of the pupil is determined by the iris, the contractile ring that is associated with eye color. Iris and Pupil: Light passes through the cornea and aqueous humor through an opening called the pupil.The aqueous humor helps to shape the cornea and provides nourishment to the eye. Aqueous Humor: The fluid beneath the cornea has a composition similar to that of blood plasma.The eyeball is rounded, so the cornea acts as a lens. Cornea: Light enters through the cornea, the transparent outer covering of the eye.To understand how the eye sees, it helps to know the eye structures and functions:

Human eyeballs