   #copyright

Sunlight

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather

   Sunlight shining through sequoia trees in Muir Woods
   Sunlight shining through sequoia trees in Muir Woods
   Prism splitting light
   Prism splitting light

   Sunlight in the broad sense is the total spectrum of the
   electromagnetic radiation given off by the Sun. On Earth, sunlight is
   filtered through the atmosphere, and the solar radiation is obvious as
   daylight when the Sun is above the horizon. This is usually during the
   hours known as day. Near the poles in summer, sunlight also occurs
   during the hours known as night and in the winter at the poles sunlight
   may not occur at any time. When the direct radiation is not blocked by
   clouds, it is experienced as sunshine, a combination of bright light
   and heat. Radiant heat directly produced by the radiation of the sun is
   different from the increase in atmospheric temperature due to the
   radiative heating of the atmosphere by the sun's radiation.

   The World Meteorological Organization defines sunshine as direct
   irradiance from the Sun measured on the ground of at least 120 W· m^−2.

   Direct sunlight gives about 93 lumens of illumination per watt of
   electromagnetic power, including infrared, visible, and ultra-violet.
   This compares with the best fluorescent lights.

   The sun's nuclear energy source was discovered by Hans Bethe.

   Sunlight is a key factor in the process of photosynthesis

Life on Earth

   Spectrum of blue sky clearly showing solar Fraunhofer lines and
   atmospheric water absorption band.
   Spectrum of blue sky clearly showing solar Fraunhofer lines and
   atmospheric water absorption band.

   The existence of nearly all life on earth is fueled by light from the
   sun. Most autotrophs, such as plants, use the energy of sunlight to
   turn air into simple sugars—a process known as photosynthesis. These
   sugars are then used as building blocks and in other synthetic pathways
   which allow the organism to grow.

   Heterotrophs, such as animals, use light from the sun indirectly by
   consuming the products of autotrophs, either directly or by consuming
   other heterotrophs. The sugars and other molecular components produced
   by the autotrophs are then broken down, releasing stored solar energy,
   and giving the heterotroph the energy required for survival. This
   process is known as respiration.

   In prehistory, humans began to further extend this process by putting
   plant and animal materials to other uses. They used animal skins for
   warmth, for example, or wooden weapons to hunt. These skills allowed
   humans to harvest more of the sunlight than was possible through
   glycolysis alone, and human population began to grow.

   During the Neolithic Revolution, the domestication of plants and
   animals further increased human access to solar energy. Fields devoted
   to crops were enriched by inedible plant matter, providing sugars and
   nutrients for future harvests. Animals which had previously only
   provided humans with meat and tools once they were killed were now used
   for labour throughout their lives, fueled by grasses inedible to
   humans.

   The more recent discoveries of coal, petroleum and natural gas are
   modern extensions of this trend. These fossil fuels are the remnants of
   ancient plant and animal matter, formed using energy from sunlight and
   then trapped within the earth for millions of years. Because the stored
   energy in these fossil fuels has accumulated over many millions of
   years, they have allowed modern humans to massively increase the
   production and consumption of primary energy. As the amount of fossil
   fuel is large but finite, this cannot continue indefinitely, and
   various theories exist as to what will follow this stage of human
   civilization (e.g. alternative fuels, Malthusian catastrophe, new
   urbanism, peak oil).

Cultural aspects

   Many people find direct sunlight to be too bright for comfort,
   especially when reading from white paper upon which the sun is directly
   shining. Indeed, looking directly at the sun can cause permanent vision
   damage. To compensate for the brightness of sunlight, many people wear
   sunglasses. Cars, many helmets and caps are equipped with visors to
   block the sun from direct vision when the sun is at a low angle.

   In colder countries many people prefer sunnier days and often avoid the
   shade. In hotter countries the converse is true; during the midday
   hours many people prefer to stay inside to remain cool. If they do go
   outside, they seek shade which may be provided by trees, parasols, and
   so on.

   Sunshine is often blocked from entering buildings through the use of
   window blinds, awnings, shutters or curtains.

Sunbathing

   Sunbathing is popular for the cosmetic benefits of a sun tan, although
   there are also risks of cellular damage to the skin. Sunbathing is a
   popular leisure activity in which a person sits or lies in direct
   sunshine. People often sunbathe in comfortable places where there is
   ample sunlight. Some common places for sunbathing include the beach,
   open air swimming pools, the park, the garden, and pavement (sidewalk)
   cafés. Sunbathers typically wear limited amounts of clothing (such as
   swimsuits), or go topfree or simply go nude.

   An alternative some use to sunbathing is to use a sunbed that generates
   ultraviolet light and can be used indoors regardless of outdoor weather
   conditions and amount of sun light.

   For many people with pale or brownish skin, an additional or primary
   purpose for sunbathing is to darken one's skin colour (get a sun tan)
   as this is considered in some cultures to be beautiful, associated with
   outdoor activity, vacations or holidays, and health. Indeed, the body
   produces vitamin D from sunlight (specifically from the UVB band of
   ultraviolet light), and excessive seclusion from the sun can lead to
   deficiency. An additional reason that some people prefer nude
   sunbathing is that an "all-over" or "even" tan can be obtained.

   Skin tanning is achieved by an increase in the dark pigment inside skin
   cells called melanocytes and it is actually an automatic response
   mechanism of the body to sufficient exposure to ultraviolet radiation
   from the sun or from artificial sunlamps. Thus, the tan gradually
   disappears with time, when one is no longer exposed to these sources.
   The skin of darker-skinned people may represent an evolutionary
   advantage developed time ago in races living in tropical areas, such as
   Africa.

Effects on health

   High Resolution Solar Spectrum
   High Resolution Solar Spectrum

   While the production of vitamin D is good, it is important to note that
   excessive sunlight exposure has been linked to all types of skin cancer
   caused by the ultraviolet part of radiation in sunlight and from
   sunlamps. Sunburns are mild to severe inflammation effects to the skin
   and can be avoided by using a proper sunscreen cream or lotion or by
   gradually building up melanocytes over days and weeks of increasing
   exposure. Another detrimental effect of UV exposure is accelerated skin
   aging (also called skin photodamage), which produces a rather ugly and
   difficult to treat cosmetic effect. The decrease in the atmosphere's
   ozone layer in the last decades is increasing the incidence of such
   health hazards and extra precautions should be taken by people who are
   exposed daily to strong sunlight.

   A lack of sunlight, on the other hand, is considered one of the primary
   causes of seasonal affective disorder, a serious form of the "winter
   blues". SAD occurrence is noticed more prevalently the further away
   from the tropics the sample is taken, and most of the treatments (other
   than prescription drugs) involve replicating sunlight. This replication
   is done using lamps tuned to specific wavelengths of light or
   full-spectrum bulbs.

   There are two further beneficial effects of sunlight. Firstly it
   upregulates the manufacture of vitamin D in humans when sunlight is
   incident upon skin surfaces. Secondly, the use of sunlight in lieu of
   artificial light to illuminate building interiors avoids certain
   adverse health effects of over-illumination by electric lights as well
   as promoting energy conservation.

   Retrieved from " http://en.wikipedia.org/wiki/Sunlight"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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