Nature And Laws Of Radiation

The behavior of electromagnetic radiation may be summed up in the following simplified statements:

Every item of matter with a temperature above absolute zero emits radiation.

Substances that emit the maximum amount of radiation in all wavelengths are known as black bodies. Such bodies will absorb all radiation incident upon them. A black body is thus a perfect radiator and absorber.

Substances absorb radiation of wavelengths, which they can emit.

The wavelengths at which energy is emitted by substances depend on their temperature—the higher the temperature, the shorter the wavelength.

Gases emit and absorb radiation only in certain wavelengths.

The amount of radiation absorbed by a gas is proportional to the number of molecules of the gas and the intensity of radiation of that wavelength.

Wavelength

The wavelength of electromagnetic radiation is given by the equation

where X is the wavelength, the shortest distance between consecutive crests in the wave trans; c is the constant equal to the velocity of light, 3 x 1010 cm-sec-1; and v is the frequency, the number of vibrations or cycles per second.

Planck's Law

Electromagnetic radiation consists of the flow of quanta or particles, and the energy content (E) of each quantum is proportional to the frequency given by the equation

where h is Planck's constant (having a value of 6.625 x 10-27 erg-sec-1) and v is the frequency. The equation indicates that the greater the frequency, the greater is the energy of the quantum.

Kirchoff's Law

Any gray object (other than a perfect black body) that receives radiation disposes of a part of it in reflection and transmission. The absorptivity, reflectivity, and transmissivity are each less than or equal to unity.

This law states that the absorptivity a of an object for radiation of a specific wavelength is equal to its emissivity e for the same wavelength. The equation of the law is a (X) = e (X). (2.3)

Stefan-Boltzman Law

This law states that the intensity of radiation emitted by a radiating body is proportional to the fourth power of the absolute temperature of that body:

Flux = cTa4

where o is the Stefan-Boltzman constant (5.67 x 10-5 erg-cm-2-sec-1-K-4) and Ta is the absolute temperature of the body.

Wein's Law

The wavelength of maximum intensity of emission from a black body is inversely proportional to the absolute temperature (T) of the body. Thus,

Wavelength (A) of maximum intensity (^m) = 2897 T-1. (2.5)

For the sun the wavelength of the maximum emission is near 0.5 |j,m and is in the visible portion of the electromagnetic spectrum.

Lambert's Law

This law states the permeability of the atmosphere to solar radiation. The intensity of solar radiation on a vertical irradiation at the earth's surface is given by the equation where Io represents the solar constant, q is the transmission factor for the layer thickness 1 (solar angle 90°), and m represents distance of the air transversed. When the transmission factor q is replaced by the extinction coefficient a (a = In-q), the equation takes the form

About 95 percent of the sun's radiation is contained between 0.3 and 2.4 ^m, 1.2 percent in wavelengths < 0.3 pm, and 3.6 percent in wavelengths > 2.4 ^m (Iqbal, 1983). A systematic division of solar radiation according to frequency and wavelength is given in Tables 2.1 to 2.3. An approximation of energy content in various segments of shortwave radiation is given in Table 2.2. A more detailed picture of the energy content and nature of the solar radiation spectrum is given in Table 2.3.

EARTH'S ANNUAL GLOBAL MEAN RADIATIVE ENERGY BUDGET

The global annual mean energy budget is determined by the net radiation flow of energy through the top of the atmosphere and at the earth's surface.

Solar Radiation and Its Role in Plant Growth TABLE 2.1. Broad bands of the solar spectrum

Color	X (|xm) Irradiance W m-2		% of solar constant
Ultraviolet	< 0.4

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