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Agriculture is the exploitation of solar energy under adequate supply of nutrients and water by maintaining plant growth.  So it is but natural that any efforts of thoroughly understanding of solar radiation will be immense use for its fullest exploitation by the crop plants in terms of their growth and yield.

The sun is the primary source of energy. Supplying about 99.9% out of total energy available at the earth surface.  The temperature of the Sun is 6000 K and gives out energy about 5.6 x10 27 cal per minute.  The Sun radiates its energy in the form of wave lengths from 0.15 to 4.0 u and are generally called as short wave lengths.  On contrary after absorption of solar energy, earth emits its energy between 4 to 100 u and is categorized as long wave length.

There are three methods of transfer of heat or energy that means there are three different ways by which heat can flow from one point to another are:
1. Conduction
2. Convection

For conduction and convection of heat, material medium is necessary.  But for radiation material medium is not necessary, because radiation takes place in the form of Electro magnetic waves.

The ultimate source of all the energy for physical and biological processes occurring on the earth is radiation received from the sun that is why it is commonly called solar radiation.

Some Terms (Definitions):

The transfer of heat energy in the form of electro magnetic waves with the speed of light is known as radiation (Light speed is 3×105 km/second).

2. Solar insulation:
The heat energy received from the Sun is known as solar insulation.

It is defined as the amount of energy received on a unit surface in a unit time.  ( In Meteorology we commonly use cal cm -2 min or largely min -1 as the unit of radiant flux density)

4. Emissive:

It is defined as the ratio of the emittance of a given surface at a specified wave length and temperature to the emittance of an ideal black body at the same wave length and temperature.

5. Absorptive:
It is defined as the ratio of the amount of radiant energy absorbed to the total amount incident upon the substance.

6. Reflectivity:
Is defined as the ratio of the radiant energy reflected to the total that is incident upon the surface.

7. Transmmissivity:
Is defined as the ratio of the transmitted radiation to the total radiation incident upon the medium.