is the uptake of energy
To absorb simply means to take up energy. Thus the
term absorption, although giving the impression that it's a highly complex
process, is simply the process of absorbing energy from a source of
energy. As an example, a goalkeeper catching a forcefully kicked ball
absorbs the energy with his or her body. Hopefully the goalkeeper will
deftly do so with the hands, rather than with another body part, but either way
the goalkeeper absorbs the energy.
||A molecule of H2O
The water molecule, with its chemical formula H2O, consists of an oxygen atom (depicted here in blue color) and two atoms of hydrogen (drawn in red). In reality the atoms are
Molecules and radiation particles, called photons, are similar to the goalkeeper and the
ball of our previous example, but on a much, much smaller scale. Although
molecules can absorb photons, they can only do so when the photons have a
certain amount of energy.
Referring back to our example, suppose a photon is about to hit a molecule, just
as the ball is going to be caught by the goalkeeper. Whereas the skilled goalkeeper is
able to catch the ball no matter whether it has been kicked softly or
forcefully, molecules are only able to absorb photons when the photons contain
certain energy values the molecules prefer.
|Molecules are able to take up photons only at certain energy values
The molecules and photons represent a system which changes its state only if exactly the right amount of energy has been provided.
The molecule will neither take up a photon which has too much energy nor one that has insufficient energy.
How can we visualize
"Rats! Missed again!"
Miriam is miffed because she did not succeed in sinking the ball in the cup while playing miniature golf. Most of us who ever have played this game
know the situation, with the hump and, exactly on top of it, the hole.
If the ball has been hit too softly it will roll up the hump a short way - as a rule irritatingly only up to about an inch off the hole - and only too often change its mind and return down the side of the hump.
The golf ball has too little energy to be taken up.
If the ball has been hit too hard it will skip over the hole and roll down again on the other side of the hump. Again, the ball has not lost its energy, but
rather profits from rolling down the other side of the hump, rolling even
further away on this attempt to hole out. This misfortune had just befallen Miriam for the third time and now, mumbling, she is grubbing for a second golf ball in her trouser pockets.
The golf ball has too much energy to be taken up.
Her friend, of course, manages to sink the ball on her first attempt. The ball
plops into the hole with a slight "toc!" and stays there. The balk has taken up
the ball's energy of motion, thus absorbing the energy. The ball is no
longer rolling, but instead is lying in the trough.
|A spectrum shows how absorption depends on
energy of the radiation.
We can draw an absorption spectrum for the hump on the miniature golf course. The spectrum shows us that the hump "absorbs" the golf ball only if
the golf ball hits it with a certain
energy. In the case of the golf ball there
is a small range of energy levels that will
yield a successful shot into the cup.
An amount of energy which is either too high or too low does not yield an uptake.
Similarly to the hump at the miniature golf course, water molecules absorb the energy of radiation particles only at
certain values. In the case of water, absorption can occur at a large
number of discrete energy values, more than most other molecules. It is as if their were many humps of very different heights, for which different amounts of energy are needed in order to be able to actually sink the golf ball into the hole.
This is relatively simple to understand when we look at what a molecule does with the energy.
|Radiative energy sets molecules
Energy absorbed does not simply vanish. It must change something or be
transferred elsewhere. A goalkeeper will lean forward and strengthen his
or her stand so that the energy of the ball is transferred to the ground through
the goalkeeper. Let us imagine the goalkeeper is not standing on the sward with both his feet, but
is instead on the street wearing roller blades. Certainly it would require more effort not to
lose his or her balance, and he or she would likely trundle backwards a bit.
Molecules float freely in space. They are not in contact with anything else,
and thus retain all of their energy until they collide with another molecule.
For them the configuration of the atoms within the molecules loses its balance,
and they begin to swing.
Molecules are able to swing, or oscillate, in many ways. Each
different mode of oscillation requires a very specific amount of energy to
stimulate the oscillation. That is why molecules absorb at different radiation energies.
Water molecules absorb so well in the range of infrared radiation radiated by the earth that only a small band remains transparent for the photons.
This energy range is called the "atmospheric window." In the chapter concerning the
greenhouse effect, we will see that the absorption bands of most of the so-called "greenhouse gases" are much smaller
than that for water, absorbing radiation in far fewer wavelengths.
Water absorbs at both the left and the right of the atmospheric window5.
coming from the sun and
re-emitted by the earth hits molecules in the air.
If the energy of this radiation
matches the energy necessary to create particular oscillations of the molecules (to "excite" them), the radiation energy is taken up (=absorbed)
and stored in the air itself.