This highly complex system of vision is designed to detect light energy and transduce it into electrical information that is sent to the visual cortex of the brain for perception. Interestingly, light input in the right eye is perceived in the left part of the bran and vice versa. This crossing of the information and the slightly offset position of the eyes allow the brain to perceive different dimensions, including depth perception.
The eyes are organs that capture and focus light energy on the back portion of the eyeball where the retina is. The retina contains the photoreceptor cells. The sclera forms the outer covering of the eye and can be found in the front as the cornea (clear portion of the eye), which allows photons of light to pass into the eye. Under the cornea is a spherical diaphragm that can open or close to regulate the amount of light that will pass into the next compartment. This colored portion of the eye is called the iris, and its opening, which is black because it is dark in the back of the eye, is the pupil.
Suspended behind the irish and positioned immediately behind the pupil, the lens is kept in place by the ciliary muscle, which can apply tension to the lens to stretch it or allow it to contract in order to change the focal length of the light entering the eye (i.e., permit focal accommodation to see objects up close and then far away as the focus of the eye automatically adjusts). After light enters the eye and is focused by the lens, it passes through the large chamber of the eyeball that is filled with a gelatinous material called vitreous humor before passing into the photoreceptor layer: the retina.
The back of the eyeball has a layer of photoreceptor cells called the retina. This layer is actually designed upside down from what one may imagine. Light first must pass through several layers of neurons and interconnecting cells before the photon will impact and excite a photoreceptor cell buried in the deeper parts of the retina. The only part of the retina deeper than the photoreceptors is a layer of pigmented epithelial cells that will absorb light and prevent its reflection to reduce extraneous light and improve fine detail.
These cells can be found in the deep retina and contain a photo pigment that, when activated by a photon of light, will change shape and lead to a signal transduction cascade, ultimately generating an electrical signal in a neuron that will signal the visual cortexof the brain.
One type of photoreceptor is the rod cells. Shaped much like a comb for your hair, these rows of folded membranes that resemble the teeth of a comb contain the photo pigment rhodopsin, which, when activated, will lead to change in the shape of the larger molecule where it is attached (opsin). An essential component of this photo pigment complex is a molecule called retinol, which is related to vitamin A. Rods are the more sensitive of the 2 types of photoreceptors and are responsible for vision in low light, which many have referred to as black and white vision. Also concentrated at the edge of the retina you will find the cones. Cones are essential for peripheral vision.
Cone cells are the second type of photoreceptor and are responsible for color vision in the retina. These cells also have folds of photo pigment, but are shorter than the rods and have a tapering or “cone” shape, hence the name. Three different cones are based on the photopsin (pigment) and the wavelength of light they detect. They are the red, green, and blue cones. Which cones are simulated and to what degree determines the color of light that is signaled and perceived. This is not unlike your television screen or computer monitor where the same 3 colors are used to blend into any visual color in the spectrum. While cones are much less abundant than rods in the retina in general, in the fovea of the retina (the central part of the retina responsible for sharp central vision), cones greatly outnumber rods.
Source: Langford, K. (2015). Sensory System. In The everything guide to anatomy and physiology: All you need to know about how the human body works. F W Media.