How does eyesight and seeing happen?” This is a particular question which has been asking by many of philosophers and scientists during the history. In the ancient Greek, it was believed that light is radiated from the eye toward the objects and its reflection makes the objects visible. In the ?th and ?th centuries BC, Aristotle, and Euclid tried to refute this theory by the use of dark room. They placed an opaque plate at the behind of dark room in order to see the reflected picture by their eyes. After obtaining more knowledge from the biology, interior and neural tissue of the eye, with advancements in physics and optics, the mentioned question changed to: “How many photons will start the threshold sensitivity of the eye?” Answering the question is the aim of this thesis. The retina is what reacts to the light inside of eye, and there are millions of cone and rod photoreceptor cells on the retina which reacts to the light and photon that entered to the eye. Thus this question could be investigated in these two cases: ?) In vivo (for human eye under a psychophysics experiment) and ?) In vitro (for a photoreceptor cell which is separated from the retina, in an isolated state and laboratory conditions). So, we will explain the evolutionary process and eye biology at first, then we will investigate optics and photon probability distribution functions of light sources. Some parameters of light sources are effective in measuring threshold sensitivity of the eye and in measuring photoreceptor cells in the cases of in vivo and in vitro. These parameters are analyzed for photoreceptor cells and eye. In addition to study threshold sensitivity of the photoreceptor cell relative to the photon, the sensitivity of the photoreceptor cells to the light wavelength is investigated. In order to measure the threshold sensitivity, noise effect on photoreceptor cell and eye is very significant. This Effect is considered and the results are presented that photoreceptors are like quantum detectors that can detect single photons. Also, a phsychophysics experiment shows that human eye can detect a single photon by a probability above chance
