The Radiation Laws

 The Radiation Laws

    The Earth atmosphere system is sustained by the supply of energy from the sun. solar radiation is the ultimate source of energy that results in the varied atmospheric condition experienced on Earth. To comprehend fully the role of solar energy in the function of atmosphere requires, actual understanding of the radiation laws. The following are the laws that governs the way in which the incoming solar radiation will react with the atmosphere as well as the surface of the Earth.

The first law governs the relation between frequency, wave length and velocity. According to which the product of wave length and the frequency of the wave is a constant, which will be equal to speed of light.

c = λ f   

Where λ is the wave length, f is the frequency, and c is the speed of light (3 X 1010 cm / S). This law indicates wave length (λ) increases  frequency (f) decreases. That is short waver are characterized by high frequency and longer waves by lower frequency.

Kirchoff’s law: relates emissivity to absorption of radiation at a particular wavelength.

α λ = ε λ

αλ is the fractional amount of energy that is absorbed at a given wave length while ελ is emissivity at that wavelength. This law shows that strong absorbs of radiation are also strong emitters.

Wien’s law states that wavelength of maximum radiation is inversely proportional to the absolute temperature. Thus the higher the temperature, the shorter the wave length at which maximum radiation occurs:

    λmax = 2897 / T

where T is the temperature in degree kelvin. The higher the temperature of a black body, the shorter the wavelength of maximum emission.

According to this ,

For Sun

λmax = 2897 / 6000 = 0.48 μm

For Earth.

λmax = 2897/288 = 10 μm 

At the temperature of the sun’s surface, the maximum radiation is in the range from 0.4 to 0.7 micro meter. 

This is precisely the range of radiation that the human eye perceives, and we call it the visible range. This is very good example of evolutionary processes. Human eyes have evolved to take maximum advantage of that part of solar radiation that is in greatest abundance. 

Reference: Climatology – second edition – (John E Oliver, John J Hidore)