Albert O. Edwards, an assistant professor of ophthalmology at UT Southwestern Medical Center at Dallas and an expert on macular degeneration, provides this explanation:
Image: St. Luke's Cataract and Laser Institute
Among other functions, our eyes convert electromagnetic radiation in the form of light into electrical signals that can be interpreted by our brain. This process involves special cells called photoreceptors in the retina. These cells capture packets of light, called photons, and issue first a chemical and ultimately an electrical signal.
Photoreceptors come in two main types. Rod photoreceptors contain a pigment that absorbs dim light and leads to black-and-white images. Cone photoreceptors allow for color vision. They come in three types--each of which has a slightly different pigment that responds to a different wavelength of light, either red, blue or green. The colors we perceive depend on which photoreceptors are activated. People who are colorblind cannot detect some hues because they lack a certain type of retinal cone.
When the pigments absorb the light, proteins bound to them change their conformation. This change in the protein's shape prompts the chemical signal cascade, called visual transduction, that then triggers an electrical signal to the brain. The sensitivity of the part of the retina responsible for fine vision (the fovea) contains pigments that absorb light at a maximum wavelength of about 550 nanometers. Thus, 550 nanometers marks the upper limit for our spectral response.