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Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum light perceptible to the human eye as well as invisible infrared (typically perceived by humans as warmth) and ultraviolet (which can have physiological effects such as sunburn) lights. However, according to the American Meteorological Society, there are "conflicting conventions as to whether all three ... are referred to as light, or whether that term should only be applied to the visible portion of the spectrum". Upon reaching the Earth, sunlight is scattered and attenuation through the Earth's atmosphere as daylight when the Sun is above the horizon. When direct solar radiation is not blocked by , it is experienced as sunshine, a combination of bright light and radiant heat (atmospheric). When cloud cover or reflected off other objects, sunlight is diffused. Sources estimate a global average of between 164 watts to 340 watts per square meter over a 24-hour day; this figure is estimated by NASA to be about a quarter of Earth's average total solar irradiance.
The ultraviolet radiation in sunlight has both positive and negative health effects, as 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. Extract of page 56 A 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.
Sunlight is a key factor in photosynthesis, the process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be used to synthesize carbohydrates and fuel the organisms' activities.
Daylighting is the natural lighting of interior spaces by admitting sunlight. Solar irradiance is the rate of solar energy received by a unit area from sunlight.
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