9
Surussavadee
April 2014
Interdisciplinary Graduate School of Earth System
Science and Andaman Natural Disaster Management
www.essand.psu.ac.th
Electromagnetic Wave Models
Electromagnetic radiation is generated when an electrical
charge is accelerated.
The wavelength of electromagnetic radiation (λ) depends upon
the length of time that the charged particle is accelerated and
its frequency (f) depends on the number of accelerations per
second.
Wavelength is formally defined as the mean distance between
maximums (or minimums) of a roughly periodic pattern and has
units of meter.
Frequency is the number of wavelengths that pass a point per
unit time. A wave that sends one crest by every second
(completing one cycle) is said to have a frequency of one cycle
per second or one hertz, abbreviated 1 Hz.
10
Surussavadee
April 2014
Interdisciplinary Graduate School of Earth System
Science and Andaman Natural Disaster Management
www.essand.psu.ac.th
Electromagnetic Wave Models
= λ
v = velocity [m/s]
λ = wavelength [m]
f = frequency [Hz]
v in vacuum is 3 × 10 [m/s]
The longer the wavelength
the lower the frequency;
the shorter the wavelength,
the higher the frequency.
11
Surussavadee
April 2014
Interdisciplinary Graduate School of Earth System
Science and Andaman Natural Disaster Management
www.essand.psu.ac.th
Electromagnetic Wave Models
The sun is the main source of energy for the Earth.
The electromagnetic energy from the Sun travels in 8 minutes
across the intervening 93 million miles (150 million km) of
space to the Earth.
The Sun produces a continuous spectrum of electromagnetic
radiation ranging from very short, extremely high frequency
gamma and cosmic waves to long, very low frequency radio
waves
The Earth approximates a 300 K (27ºC)
blackbody and has a dominant wavelength at
approximately 9.7 μm.
12
Surussavadee
April 2014
Interdisciplinary Graduate School of Earth System
Science and Andaman Natural Disaster Management
www.essand.psu.ac.th
Blackbody Radiation
The main source of the earth’s energy is the sun.
When the atmosphere and terrestrial surface absorb
electromagnetic energy their physical temperatures tend to
rise.
At the same time both the atmosphere and surface radiate
energy in a process called thermal emission. The amount of
thermal radiation is a function of wavelength and physical
temperature.
Planck’s radiation law characterizes the thermal radiation of a
blackbody, which is any material that absorbs all incident
radiation without any reflection. It is hence a perfect absorber
and also a perfect emitter. Such blackbody spectral radiation
has intensity: