<P> 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, a 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 "164 Watts per square meter over a 24 hour day". </P> <P> The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D and a mutagen . </P>

Where does the sun’s visible light come from