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Sunlight
*** Shopping-Tip: Sunlight
{{dablink|This article deals with light emitted by the sun; for other meanings see
sunlight (disambiguation).}}
Image:PrismAndLight.jpg thumb|300px|Prism splitting light
'''Sunlight''' in the broad sense is the total
spectroscopy spectrum of
electromagnetic radiation given off by the
Sun. On Earth, sunlight is filtered by the
Earth's atmosphere 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
geographical pole 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
watt W·
metre 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.
Life on Earth
The existence of nearly all
life on earth is fueled by light from the sun. Plants use the energy of sunlight to turn air into simple sugars—a process known as
photosynthesis. These sugars are then used as the building blocks which allow the plant to grow.
Animals use light from the sun indirectly, by either by eating plants or eating other animals that have eaten plants. The sugars produced by the plant are then broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as
glycolysis.
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. Both materials are the remnants of ancient plant matter, formed using energy from sunlight and then trapped within the earth for millions of years. Because the stored energy in these
fossil fuels was provided by the sun millions of years ago, they have allowed modern humans to use more solar energy each year than the sun provides. As the amount of fossil fuel is finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilisation (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.
Automobile 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,
Window shutter shutters or
curtains.
Sunbathing
Image:Girl-in-the-sun-1.jpg thumb|250px|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
sidewalk 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 color (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.
Sun tanning 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 excessive 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 a long time ago in races living in
tropical areas, such as
Africa.
Adverse effects on health
On this last point it is important to note that excessive sunlight exposure has been linked to all types of
skin cancer, which are caused by the
ultraviolet part of radiation contained in sunlight and 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 occurance 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.
See also
*
Light
*
Earth's atmosphere
*
Color temperature
*
Electromagnetic radiation hazard
*
Fraunhofer line
*
Effect of sun angle on climate
*
List of light sources
*
Moonlight
*
Ozone hole
*
Seasonal affective disorder
*
Solar radiation
*
Sunburn
*
Sunscreen
*
Season
*
Sunbeam
*
Skin color
Reference
#
Thom Hartmann Hartmann, Thom (1998). The Last Hours of Ancient Sunlight. London: Hodder and Stoughton. ISBN 0340822430.
Category:Natural resources
da:Sollys
de:Tageslicht
he:×?ור השמש
nl:Daglicht
ja:太陽光
zh:太阳光
*** Shopping-Tip: Sunlight
