All objects above the **temperature** of **absolute
zero** (-273.15° Celsius) **radiate** **energy** to
their surrounding environment. This energy, or radiation,
is emitted as **electromagnetic
waves** that travel at the **speed
of light**. Many different types of radiation
have been identified. Each of these types is defined
by its **wavelength**.
The wavelength of electromagnetic radiation can vary
from being infinitely short to infinitely long (**Figure
6f-1**).

Figure 6f-1: Some
of the various types of electromagnetic radiation
as defined by wavelength. Visible light has a spectrum
that ranges from 0.40 to 0.71 micrometers (µm). |

Visible** light** is
a form of electromagnetic radiation that can be perceived
by our eyes. Light has a **wavelength** of
between 0.40 to 0.71 micrometers (µm). **Figure
6f-1** illustrates that various spectral color bands
that make up light. The Sun emits only a portion (44
%) of its radiation in zone. Solar radiation spans a
spectrum from approximately 0.1 to 4.0 micrometers. The
band from 0.1 to 0.4 micrometers is called **ultraviolet
radiation**. About 7% of the Sun's emission is
in this wavelength band. About 48% of the Sun's radiation
falls in the region between 0.71 to 4.0 micrometers.
This band is called the near (0.71 to 1.5 micrometers)
and far **infrared** (1.5
to 4.0 micrometers).

The amount of electromagnetic radiation
emitted by a body is directly related to its temperature.
If the body is a perfect emitter (**black body**), the amount of radiation
given off is proportional to the 4th power of its temperature
as measured in **Kelvin** units.
This natural phenomenon is described by the **Stefan-Boltzmann
Law**. The following simple equation describes
this law mathematically:

According to the Stephan-Boltzmann equation, a small increase in the temperature of a radiating body results in a large amount of additional radiation being emitted.

In general, good emitters of radiation
are also good absorbers of radiation at specific **wavelength** bands.
This is especially true of gases and is responsible for
the Earth's **greenhouse
effect**. Likewise, weak emitters of radiation
are also weak absorbers of radiation at specific wavelength
bands. This fact is referred to as **Kirchhoff's
Law**. Some objects in nature have almost completely
perfect abilities to absorb and emit radiation. We call
these objects **black
bodies**. The radiation characteristics of the
Sun and the Earth are very close to being black bodies.

The **wavelength** of **maximum
emission** of any body is inversely proportional
to its absolute temperature. Thus, the higher the
temperature, the shorter the wavelength of maximum
emission. This phenomenon is often called **Wien's
Law**. The following equation describes this
law:

Wien's law suggests that as the temperature of a body increases, the wavelength of maximum emission becomes smaller. According to the above equation the wavelength of maximum emission for the Sun (5800 Kelvins) is about 0.5 micrometers, while the wavelength of maximum emission for the Earth (288 Kelvins) is approximately 10.0 micrometers.

A graph that describes the quantity of
radiation that is emitted from a body at particular wavelengths
is commonly called a **spectrum**.
The following two **graphs** describe
the spectrums for the Sun and Earth.

## SUN

Figure 6f-2: Spectrum
of the Sun. The Sun emits most of its radiation in
a wavelength band between 0.1 and 4.0 micrometers
(µm). |

## EARTH

Figure 6f-3: Spectrum
of the Earth. The Earth emits most of its radiation
in a wavelength band between 0.5 and 30.0 micrometers
(µm). |

The above graphs illustrate two important points concerning the relationship between the temperature of a body and its emissions of electromagnetic radiation:

- The amount of radiation emitted from a body increases
exponentially with a linear rise in temperature (see
above
).*Stephan-Boltzmann's Law* - The average wavelength of electromagnetic emissions
becomes shorter with increasing temperature (see
above
).*Wien's Law*

Finally, the amount of radiation passing
through a specific area is inversely proportional to
the square of the distance of that area from the energy
source. This phenomenon is called the **Inverse
Square Law**.** **Using
this law we can model the effect that distance traveled
has on the intensity of emitted radiation from a body
like the Sun. **Figure 6f-4** suggests that the intensity
of radiation emitted by a body quickly diminishes with
distance in a nonlinear fashion.

Figure 6f-4: Diagram
illustrating the diffusion of radiation due to the
Inverse Square Law. |

Mathematically, the **Inverse
Square Law** is described
by the equation:

## Intensity = I/d^{2}

where **I** is the intensity of the radiation at 1d (see above **diagram**)
and **d** is
the distance traveled.

Let us try this equation
out. For example, what would be the intensity of emitted
radiation traveling **two** units of distance if
the intensity at 1d = 90,000 units?

## Intensity = 90,000/2^{2}

## Intensity = 90,000/4

## Intensity = 22,500 units

What would be the
intensity of emitted radiation traveling **three** units
of distance if the intensity at 1d = 90,000 units?

## Intensity = 90,000/3^{2}

## Intensity = 90,000/9

## Intensity = 10,000 units

What would be the intensity of
emitted radiation traveling **four** units of distance
if the intensity at 1d = 90,000 units?

## Intensity = 90,000/4^{2}

## Intensity = 90,000/16

## Intensity = 5625 units

**Note** that
the decrease in intensity with distance is not linear
when graphed (**Figure 6f-5**)!

**Figure 6f-5:** Reduction
in intensity of radiation with distance traveled.