The human eye is remarkably sensitive. The first stage in the process of seeing is the absorption of light in cells called rods and cones, which lie in the retina, a thin sheet of tissue at the back of the eye. The retina records the image of the world around you somewhat like film does in a camera. Light is composed of small packets of energy called photons, and the eye is so sensitive that the absorption of a single photon is all that is required to stimulate a single rod photoreceptor.
by David R. Williams