The visual sense depends entirely on reflected light. Objects in our environment reflect light that enters our eye, forms an image and transmits information to our brain for processing. This being said is important to recognise the difference between photoreception and vision and thus a difference between photoreceptors and eyes. Photoreception and phototaxis (reaction to light) is observed in single cell animals, microorganisms and lower invertebrates, who neither sense nor are conscious of the light. The term "eye" is generally reserved for organs with a photoreceptory epithelium (the retina) and a lens to focus the light into an image. Vision occurs when this image is electrically transmitted to the brain for conscious analysis and response.

Thus, our own advanced human vision is due to a highly complex brain working with a simple lens eye. Other vertebrates who lack our cerebral development depend on a more elaborate eye. Nonetheless, most lens eyes function under the same optical principles.

When light rays hit a medium with a higher optical density, they are slowed and refracted (or bent with a slight change in direction). In the case of a convex surface such as a lens, the angle of refraction depends on the degree of curve. Thus, referring to the figure to the right, each parallel light ray hits the lens at a different angle depending on its distance from the center (marked as •). The rays entering the curved lens further from the center are bent at a sharper angle and those nearer the center are bent less. At a fixed distance (marked by the red x) beyond the lens, all the rays converge on a single point called the focus . This optical principle is used by the eye to focus images on the retina.

How is an image formed?

It is useful to think of an image as a collection of points all corresponding to a point on the object. Light reflecting off the object moves in straight lines in all directions away from the object. The points of light from the object that hit the lens of an eye are bent (or refracted) and brought into focus as a point of light on the retinal image. Thus, the image is a mosaic of light points that are brought to focus by the refraction of the lens. An image is projected at a fixed focal distance from the lens and appears small and upside-down (see the figure below). The projection can be captured on a screen, the retina. If the object moves closer or farther from the lens the image will blur unless the curvature of the lens is changed. Luckily for us, small muscles attached to the eye lens are constantly changing its curvature (a process called accommodation), and our world appears as a sharply focussed image. The points of light of the image then stimulate the retinal cells to induce information transferal to the brain.

The Retina

What is the retina made up of?

The retina is made up of 4 layers of cells. The first is the pigment epithelium which has little to do with light reception and more to do with protection. The second is the visual cell layer, home to the rods and cones that absorb the light points. The third is the bipolar layer. The bipolar cells are basically conductor cells, transferring the image "message" to the nerve cells of the ganglion layer (the final retinal layer). The ganglion cells are nerve cells with narrow axons that run along the inferior surface of the retina and join in a bundle at the base to form the optic nerve fibers.

How does the retina work?

We have yet to deal with the real issue of how light energy is converted into electric energy to be sent to the brains visual center. This phenomenon is based on basic biochemistry.

The outer section of rods and cones are highly folded membranes storing light-sensitive molecules. In the rod, the light is trapped by the trans-membrane protein called rhodopsin. Rhodopsin is made up of an amino acid sequence (opsin) and a chromatophore (retinal). When light hits a rod, the retinal molecule is "photo-excited" and the light induces a chemical phenomenon known as isomerization. In simple terms, this means the molecule changes shape. The bonds between the retinal and the opsin are twisted and they separate. When chemical bonds are broken energy is released. In this case, energy is released in the form of an electrical impulse that is passed to the bipolar cell and on to the ganglion cell to gradually make its way to the optic nerve and on to the brain. The process for color vision in cones in similar but works with different photochemicals. There are three different general cone cell types each with a different photo-sensitive protein. These proteins resemble rhodopsin in structure but are specialised to react to either red, green or blue light. Our brain mixes these primary colours, so we can distinguish innumerable colours.

The light points making up the image create a pattern of differentially stimulated cells on the retina. And this pattern is reflected the electrical waves that arrive at the brain. The brain translates this information and allows us to "see" to world around us. Thus, in this way, our eyes do not see, but we see with our eyes.