Transcript The Electromagnetic Spectrum

The Electromagnetic
Spectrum
EG5503
(GIS & Earth Observation)
Lecture Topics
What is the Electromagnetic Spectrum?
 The concept of wavelength
 Properties of EMR waves
 EMR and the Sun-Atmosphere system
 How does remote sensing exploit EMR

What is the Electromagnetic Spectrum?
The term radiation covers a wide variety
of natural phenomena
 All radiation involves the exchange of
energy
 The energy associated with
electromagnetic radiation is called
radiant energy
 Radiant energy may exist in the
absence of matter

What is the Electromagnetic Spectrum?
All types of EMR are transmitted, or
propagated, as waves
 In common with all waves, the two most
fundamental properties of
electromagnetic waves are length and
frequency
 The longer the wavelength the lower the
frequency and vice versa

The concept of wavelength
Wavelength is usually measured in
metres (the SI unit of length),
micrometres (1µm=10-6m) and
nanometres (1nm=10-9m)
 The SI unit of frequency is hertz (cycles
per second)
 The electromagnetic spectrum may be
defined as the entire range of radiation
wavelengths

Electromagnetic
spectrum with
enhanced detail for
visible region of the
spectrum
Note the large range of
wavelengths
encompassed in the
spectrum - it is over
twenty orders of
magnitude!
Properties of EMR radiation

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Transfer energy from place to place
Can be emitted and absorbed by matter
Do not need a material medium to travel through
Travel at 3 X 108 metres per second in a
vacuum
Can be polarised (made to vibrate in a plane)
Can be reflected and refracted
Can be diffracted (e.g. using a prism)
Carry no electric charge
EMR and the Sun-atmosphere system
In order to understand how we can
measure the physical environment with
remote sensing, we must first understand
solar radiation
 The amount of energy received by a
surface perpendicular to the Sun’s rays at
the Earth’s outer atmosphere is called the
solar constant (about 1370 J m-2 s-1
average)

EMR and the Sun-atmosphere system
About 50% of incoming solar radiation is
lost by the atmosphere: scattered (30%)
and absorbed (20%)
 Scattering involves the absorption and reemission of energy by particles
 Absorption (unlike scattering) involves
energy exchange

EMR and the Sun-atmosphere system
Wavelengths less than and greater than
0.8µm (800nm) are often referred to as
shortwave and longwave radiation
respectively
 The shortwave solar radiation consists of
ultraviolet and visible
 The terrestrial longwave component is
known as infrared

EMR and the Sun-atmosphere system
Just under 50% of the radiation
reaching the Earth’s surface is in the
visible range
 Components of visible light are referred
to as colours
 Each colour behaves differently and
white light can be separated out by use
of a prism
 Colour separation occurs because of
differential refraction

EMR and the Sun-atmosphere system
The human eye cannot see infrared
radiation
 Infrared radiation is absorbed by water
vapour and carbon dioxide in the
troposphere
 The atmosphere’s relative transparency
to incoming solar (SW) radiation, and
ability to absorb/re-emit outgoing
infrared (LW) radiation is the natural
greenhouse effect

Remote Sensing and EMR
Remote sensing exploits the different
characteristics of the electromagnetic
spectrum
 Satellites use channels - a channel
corresponds to a specific waveband, or
portion of the electromagnetic spectrum
 The European geostationary weather
satellite METEOSAT for example has 3
channels
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CHANNEL
Visible
Infrared
Water Vapour
SPECTRAL RANGE
0.45 to 1µm
10.5 to 12.5 µm
5.7 to 7.1 µm
USE
Daytime imaging
Temperature estimation and Imaging
Tropospheric humidity estimation
Any
Questions ?