Sensitivity

The human eye is sensitive to the electromagnetic spectrum between the wavelengths 400 nm (violet) and 700 nm (red), with maximal sensitivity at 550 nm (green). The range of light intensity to which we are exposed is huge, about 1011-fold. However the eye can respond to a single photon of light, 5–8 photons arriving within a short time are needed to give the experience of a flash of light in the dark-adapted state. As intensity is encoded by the visual system logarithmically, it is hard to distinguish the differences in intensity at high light levels