Sight and seeing is the ability to transform waves of light energy from outside sources into images in the brain giving useful information about an organism's surroundings. Essentially all members of the animal kingdom possess eyes, 96% in fact! Vertebrates (animals with spines) and mollusks like octopuses and squids have camera-type eyes, while insects and other animals have compound eyes.  The process of sight can broadly be split into two categories: what happens when light meets the eyes and how the eyes work, and what happens when this information reaches the brain to be processed. 

Light takes a path according to the National Eye Institute and NIH that first starts with passing through the cornea, which is the clear protective dome layer on the surface of the eye. The cornea bends light to focus it better into the eye. This light then enters the eye through a small opening in the iris called the pupil. The iris is the colored part at the center of the eye, which is actually made of a smooth muscle that changes the size of the pupil to further control the light coming into the eye. The pupil is in fact just a hole in the iris, not a structure in it of itself. 

After light passes through the pupil, it reaches the lens, which is a clear circular dome that acts just like a camera lens. The lens and cornea work in tandem to focus light correctly onto the retina inside the eye. Filling the empty space inside the eye is a jelly-like substance called vitreous fluid, which is firm gel that holds the eye's shape. When light hits the special photoreceptor cells in the retina, they turn the photon particles from the light waves into electrical signals for the brain to interpret. The brain receives these electric signals via the optic nerve extending from the back of the eye. 

The retina is arguably the most interesting part of the inner eye because it's where all the light is turned into electrical signals for the brain to use. In humans and most animals, it contains two types of cells, dubbed rods and cones. Cones detect colors in red green and blue as well as detailed images. They are fewer in number compared to rods at around 6 million and don't work well in low light. All red and green detecting cones are found around the center of the retina in the middle of an area called the macula, the fovea. 64% of cones detect red, 34% detect green, and only 2% detect blue. All the blue-detecting cones are found outside the fovea and separate from "red" and "green" cells. 

Rods are more numerous on the retina at around 100 million cells and perform well in low light and at night. However, unlike cones, rods are not good at detecting color or detail and mostly detect changes in light and movement. 

Visual information from the retina leaves the retina through the optic nerve located at the back of the eye. This interestingly contributes to everyone having small blind spots, due to there being no photoreceptor cells where the optic nerve meets the retina. Rod and cone cells lining the retina turn photons from light entering the eye into an electrical signal the brain can interpret. The nervous tissue of the optic nerve relays the electrical signals into the brain until some signals and tissue cross at the optic chiasm, taking image information from the left eye to the right side of the brain and vice-versa. Next, the electric signal enters an area called the lateral geniculate nucleus (LGN) of the thalamus, which is a cluster of neuron cell bodies. This is where information for sight is "sent" to be "distributed" to the primary visual cortex, which is a thin sheet of brain matter in either hemisphere of the occipital lobe, a section at the back of the brain. 

Although visual processing mechanisms are not yet fully understood, recent anatomical and physiological findings from primate studies suggest that visual signals are categorized by the brain into at least three separate processing systems. One for processing information based on shape and size, another for processing and integrating information about colors, and a third that processes perceived movement and spatial relationships.