What is 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, a combination of bright light and radiant heat. When it is blocked by the 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 persquare meter.

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D3 and amutagen. 

Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. A photon starting at the centre 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 autotrophic organisms to convert light energy, normally from the Sun, into chemical energy that can be used to fuel the organisms' activities .

Although the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun. The spectrum of nearly all solar electromagnetic radiation striking the Earth's atmosphere spans a range of 100 nm to about 1 mm  (1,000,000 nm). This band of significant radiation power can be divided into five regions in increasing order of wavelengths:

• Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the human eye). Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiation has germicidal properties, and is used in germicidal lamps.

• Ultraviolet B or (UVB) range spans 280 to 314 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for thephotochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes Sunburn, but is also required for vitamin D synthesis in the skin and fur of mammals. 

• Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths andtanning beds) and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and is able to cause cancer.

• Visible range or light spans 380 to 780 nm. As the name suggests, it is this range that is visible to the naked eye. It is also the strongest output range of the Sun's total irradiance spectrum.

• Infrared range that spans 700 nm to 1,000,000 nm (1 mm) . It is responsible for an important part of the electromagnetic radiation that reaches Earth. It is also divided into three types on the basis of wavelength:

• Infrared-A: 700 nm to 1,400nm
• Infrared-B: 1,400 nm to 3,000 nm
• Infrared-C: 3,000 nm to 1 mn.

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