Atmosphere and Electromagnetic Radiation
This page contains discussions related to atmospheric absorptions of incoming radiations from celestial sources. The discussion is from the yahoogroup
http://groups.yahoo.com/groups/Astronomy_Activities_2009/
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The discussion started like this :-
One question asked by some students had been to do with the reason why most of the radiation other than Optical and Radio, is absorbed by the atmosphere - why there are those specific absorptions are scattering that are responsible for this. If this could be discussed in some amount of detail, it would be very interesting. -- Posted by Rathnasree, Nehru Planetarium, New Delhi.
As answered by Larry Stringer of Hilltop Observatory, Texas
Most of the gases that make up our atmosphere are referred to
as "selective absorbers". What I mean by that is, each gas absorbs only certain
wavelengths of light. The reason for this of course is that the electrons that
orbit the nucleus of an atom have what we call fixed orbital shells. The orbital
shell for each atom is different. For example an atom like hydrogen, its
electrons, orbit at set distances and are different than those for atoms of say
nitrogen.
Each orbital shell is associated with a given energy level; the greater the
distance from the nucleus of the atom, the greater the energy level. In
observing atoms, you will find that electrons will jump to a higher shell when
excited by the absorption of energy. The photon must have the exact amount of
energy to move the electron from, say, shell one to shell two. If the incoming
light (photons) doesn't have enough energy to move the electron to shell two,
then no action will take place so the atom will not stay in this excited, state
for any length of time. When the potential energy is emitted, and the electron
returns to a stable state or its "ground state" (as some people call it) which
is the lowest energy level or orbital distance that it can achieve. If you
recall your physics class you will remember that the amount of energy carried by
a photon depends on the carrier wavelength. Thus the atoms that comprise a gas
can only absorb, or emit, particular wavelengths of energy.
So to make a long story short, when you look at the results of selective
absorption, you will find that there is very little
absorption for the atmosphere as a whole in the shortwave end of the light
spectrum, especially in the visible light band, which is the band of maximum
emission for the Sun. So in a nut shell, the atmosphere absorbs far more
emmissions in the longwave band of the electromagnetic spectrum which is the
region of maximum emission (10µm) for the Earth.
Further discussed by Tanmoy Lashkar from Cambridge
With regard to the question on selective absorption by the
atmosphere, there is something that I'd like to point out - most of the
absorption occurs due to the excitations of molecules as a whole, and not atomic
ones. The reason is that atomic excitations require a lot of energy (~ few
electron volts) and writing E = h*f, where h is Planck's constant and f is the
frequency of the radiation absorbed, we find that the radiation absorbed mostly
corresponds to the Ultraviolet and X-rays.
Large sections of the spectrum are absorbed rather by molecules like water
vapour present in the atmosphere. Infrared radiation causes vibrational
excitations and microwave radiation (wavelength ~ millimeters) causes rotational
excitations of water vapour, Carbon-di-oxide, ozone and other molecules present
in the atmosphere.
If you go to the first reference link below, there is a graph showing the
wavelengths that are absorbed by the Earth's atmosphere. The blue bands
correspond to absorption. The region from 1 Angstrom to 1000 Angstrom
corresponds to absorption by atoms, often getting ionised in the process. This
is followed by the window of visible and near UV, where there is no absorptoin.
The bands from 1000 Angstrom through to infrared region are vibrational
molecular excitations, followed by rotational ones. This band has deep windows
where no molecules absorb, which allows ground-based infrared telescopes to
function.
This is followed by the Radio window from hundreds of Gigahertz (centimeter
wavelengths) to about 10 MHz (wavelengths ~ tens of meters). The atmosphere is
transparent in this region, allowing radio waves from distant galaxies to reach
us. This is where Radio telescopes like the GMRT operate. With longer
wavelengths, the ionosphere kicks in. The ionosphere is a region of ionised gas
starting about 85 km above the earth's surface, that absorbs strongly any
radiation passing through it below the plasma frequency, which is a
characteristic frequency depending upon the density of free electrons. This
varies with time of day and year. This is also the atmospheric layer responsible
for long distance high-frequency (HF) radio propagation.
If one wishes to observe in any region of the spectrum which the atmosphere does
not allow through, the only way to do so is to place observatories either on
mountain tops (Mt. Palomar, Mt. Wilson, Kitt Peak, Mauna Kea) or in sapce
(Spitzer Space Telescope (infrared), Chandra X-ray observatory, XMM-Newton X-ray
observatory, Compton Gamma ray observatory).
(The discussion is ongoing. Please do join the group
http://groups.yahoo.com/groups/Astronomy_Activities_2009/ and post with your
inputs)