**3. Solar radiation fundamentals**

#### **3.1 Electromagnetic spectrum of the sun**

The sun emits energy in form of electromagnetic waves which are propagated in space without any need of a material medium and with a speed, c = 3 x 108 ms-1. Electromagnetic radiation emitted by the Sun reaching out in waves extends from fractions of an Angstrom to hundreds of meters, from x – ray to radio waves.

An angstrom is a unit of length given by 1A = 10-8 cm = 10-4 μm.

Electromagnetic radiations are usually divided into groups of wavelengths. The wavelength regions of principal importance to the earth and its atmosphere are the;

Ultraviolet ( UV ) – ( 0.3 – 0.4 μm ) representing 1.2% Visible (VIS) - ( 0.4 - 0.74μm ) representing 49% Infrared ( IR ) - ( 0.74 – 4. 0 μm ) representing 49%

It was discovered that 99% of the Sun's radiant energy to the earth is contained in these wavelength regions, that is, between 0.3 and 4μm and comes mostly from the photosphere part of the sun.
