ENCYCLOPEDIA 4U .com

 Web Encyclopedia4u.com

Solar radiation is radiant energy emitted by the sun, particularly electromagnetic energy. About half of the radiation is in the visible short-wave part of the electromagnetic spectrum. The other half is mostly in the near-infrared part, with some in the ultraviolet part of the spectrum [1].

Solar radiation is due to nuclear fusion reactions. Fusion is the process whereby stars produce huge quantities of energy from the fusion of hydrogen or helium, in one of the most efficient processes of energy generation. Some stars are known to emit radiation of other wavelengths.

Solar neutrinos are a different type of radiation emitted by the nuclear reactions in stars. Electrons and positrons (anti-electrons) are delocalised because the matter in stars is a plasma. These leptons may also be considered a form of solar radiation, but they do not travel far from the solar body. Fusion begins with the combination of four hydrogen-1 nuclei to create two hydrogen-2 nuclei. As a result, two positrons (positive electrons) and two neutrinos are released. These two hydrogen-2 nuclei, together with another two hydrogen-1 nuclei, form two helium-3 nuclei and release gamma radiation. These two unstable isotopes of helium fuse to form helium-4 and two particles of hydrogen-1.

The whole process can be summed up by saying that four protons undergo fusion to produce a helium nucleus and energy. The energy radiated away in the form of gamma radiation, as well as the positrons and neutrinos, is solar radiation. The hydrogen-1 nuclei are not radiation, by its strict definition, as they are usually used again as an input in the fusion chain-reaction.

Also note that energy is conserved, so by calculating the mass of the four protons, and the mass of the helium nucleus, and subtracting you can calculate the mass of energy (energy and mass are interchangeable) emitted in gamma and positron radiation. The equation E = mc2 can be used to convert between mass and energy in joules. Since these values are very small, it is useful to convert joules into electron volts. An eV (electron volt) is equal to 1.6 × 10-19 joules, but in most cases the energy released in reactions is measured in MeV or mega-electron volts, or larger quantities.

## Climate effect of solar radiation

On earth solar radiation is obvious as daylight when the sun is above the horizon. This is during daytime, and also in summer near the poles at night, but not at all in winter near the poles. When the direct radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright yellow light (sunlight in the strict sense) and heat. The heat on the body, on objects, etc., that is directly produced by the radiation should be distinguished from the increase in air temperature.