Background
The retina is the region of the eye that allows that receives and is changed by electromagnetic radiation. Only electromagnetic radiation of wavelengths that can change retinal cells can be perceived by humans as light. These wavelengths are called the visible spectrum. The range of the visible spectrum is usually between 400 and 700 nm. Electromagnetic radiation that has a wavelength outside of this window is does not affect retinal cells and therefore cannot be perceived. The figure below shows the relationship between the wavelength of light and the perceived color of the light.
http://bmrc.berkeley.edu/courseware/cs160/spring99/Lectures/08-Perception/sld006.htm
Details of Retinal Anatomy
The retina is found on the inside back of the eyeball. It is a thin layer of nervous tissue consisting that exists in seven distinct regions that can be seen below.
http://thalamus.wustl.edu/course/eyeret.html
In the above image, the light is entering the retina from the bottom of the image. This means that light has to travel through a series of cells until it makes it to the photoreceptor, the region of the eye that is actually activated by electromagnetic radiation. While the retina is often thought of as just an area that receives information, the early stages of information processing also occur in the retina. The horizontal, bipolar, and amacrine cells seen above all begin processing visual stimuli while the ganglion cells relay visual information back to the brain for further processing.
Parts of the Retina
The retina is not uniform throughout. The area of the retina in the periphery is very similar to the image seen above. However, the region that is in the center of focus is different. It is thinner and looks like the region below. It is called the fovea (see below). In another region of the retina, there are no photoreceptors present meaning that this region of the retina cannot receive visual information forming a blind spot. This is where the optic nerve meets the retina. More specifically it is where ganglion cells send information to the brain. This is spot of the retina is called the optic disc. To see an interesting site about the blind spot, click here.
http://thalamus.wustl.edu/course/eyeret.html
Photoreceptors
The cells in the retina that receive electromagnetic radiation are called photoreceptors. These cells come in two basic forms, rod cells and cone cells.
Rod cells are larger in size than cone cells and much more common in the retina than cone cells, especially outside of the fovea. Rod cells contain the pigment rhodopsin whose conformation changes when it is exposed to many wavelengths of visible They are very sensitive to light and can be activated by a single photon. The visual processing done by
Cone cells are smaller in size than rod cells. Each cone cell contains one of three photopigments. They are not as sensitive to light as rod cells are requiring about 100 photons in a short period of time to cause a change in the cone cell's behavior. While cone cells are less common than rods, they are the only kind of photoreceptor found in the fovea.