The human eye resembles a camera. It has an iris and a lens, and the retina-a membrane deep inside the eye-functions much like film. Light passes through the lens of the eye and strikes the retina, thereby stimulating its photoreceptor cells, which in turn emit a signal. This signal travels over the optic nerve to the brain, where color is perceived. There are two types of photoreceptor cells known as "cones" and "rods." Rods detect brightness and darkness, while cones detect color. Between the two, there are some 200 million photoreceptor cells. Cones are further classified into L, M, and S cones, each of which senses a different color wavelength, allowing us to perceive color.