The colour of the eye is a polygenic phenotypic character determined by two different factors: pigmentation of the iris of the eye and dependence on the dispersion of light by the cloudy medium in the iris stroma. In humans, iris pigmentation varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content in the iris stroma on the front of the iris) and the cellular density of the stroma. The appearance of blue and green, as well as hazel eyes, results from the scattering of Tyndall's light in the stroma, a phenomenon similar to that explained by the blue sky called Rayleigh Scattering. Neither the blue pigment nor the green pigment are present in the human iris or in the ocular fluid. The colour of the eye is, therefore, an example of structural colour and varies according to the lighting conditions, especially for lighter colored eyes.