 Sunlight, sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through Earth's atmosphere, and is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, the combination of bright light and radiant heat. When it is blocked by clouds or reflects off other objects, it is experienced as diffused light. The World Meteorological Organization uses the term sunshine duration to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per square meter. Other sources indicate an average over the entire Earth of 160 watts per square meter over a 24-hour day. The ultraviolet radiation in sunlight has both positive and negative health effects. It is both a requisite for vitamin D3 synthesis and a mutagen. Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. The photon starting at the center of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface. Light is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy, normally from the Sun, into chemical energy that can be used to synthesize carbon hydrates and to fuel the organisms' activities.