Principle
Radiation is an essential element of the sun-Earth-atmosphere
system. Radiation obeys certain laws of physics. These laws
provide us with the knowledge that (a) warmer bodies radiate
more energy than cooler bodies, (b) warmer bodies radiate at
shorter wavelengths than cooler bodies, (c) good emitters are
good absorbers at a particular wavelength and (d) poor emitters
are poor absorbers at the same wavelength. |
Figure 1 -
Electromagnetic Spectrum
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Definition
- Electromagnetic radiation, or simply radiation, is an
advancing disturbance in electric and magnetic field existing
in space or in the media.
- The entire range of radiative wavelengths and their associated
frequencies is called the electromagnetic
(EM) spectrum.
Various types of radiation exist, including
cosmic rays (extremely high energy from the sun, mostly absorbed
by the atmosphere), gamma rays (very high energy from radioactive
decay), x-rays (very high energy which can penetrate through
the body for a photograph), ultraviolet radiation (high energy
which allows for formation of ozone and sunburns, can be very
dangerous), visible radiation (medium energy to which the human
eye is sensitive), infrared radiation (lower energy which heats
the lower portion ofthe atmosphere), microwave radiation (lower
energy used in microwave ovens to heat food), and radio waves
(lowest energy used to transmit information and, in meteorology,
used in radar technology).
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Figure 2 -
An Electromagnetic Waves
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Electromagnetic
Waves
- Electromagnetic waves travel at
the speed of light and are described by their
wavelength and frequency.
The speed of light is 300 million
meters per second. The wavelength of an electromagnetic
wave is measured as the distance from one crest of the
wave to the next. The frequency is defined as the rate
at which wave crests pass a fixed point. The frequency
equals the speed of light divided by the wavelength.
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Figure 3 -
Physics of Radiation
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Properties
of Radiation
- Warmer bodies radiate at shorter wavelengths than cooler
bodies.
The wavelength which is radiated
by an object depends on the object's temperature.
The warmer the object, the shorter the wavelength.
Hence, the sun radiates at shorter wavelengths (e.g.,
visible) than the earth (e.g., infrared).
- Warmer bodies radiate more energy than cooler bodies.
The energy emitted from an object
is proportional to the fourth root of the object's
temperature. As a result, the sun (temperature ~6000
degrees Celsius) radiates 160,000 times more energy
than the earth (temperature ~25 degrees Celsius)!
- Good emitters are good absorbers at a particular wavelength.
Poor emitters are poor absorbers at the same wavelength.
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Figure 4 -
Reflection, Absorption, Scattering, Refraction, and Transmission
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Radiative
Transfer
- Radiation can be absorbed, reflected, scattered,
refracted, or transmitted.
Absorbed: energy is retained by
a substance.
Reflected: the surface returns a
portion of the energy.
Scattered: the electromagnetic waves
are changed from propagating in one direction to all
directions.
Refracted: the electromagnetic waves
are changed from propagating in one direction to another
direction.
Transmitted: energy passes through
space or the media.
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