electromagnetic radiation having λ between 400 and 700 nanometers (nm). A significant
amount of the incoming energy is in the infrared region above 700 nm. Some of the
inbound electromagnetic radiation is reflected directly from the atmosphere, clouds in the
atmosphere, and even Earth’s surface (snow is especially effective at reflecting visible
light). But most of the outbound energy is in the longer wavelength infrared region. As
discussed in Chapter 8, so-called greenhouse gases, such as carbon dioxide and methane
in the atmosphere, reabsorb this radiation and delay its eventual exit from Earth. This is
a good thing because it is what keeps the atmosphere tolerably warm. But if it happens
to excess, it may result in global warming, probably the most challenging environmental
problem of our time.
Electromagnetic radiation with wavelengths shorter than 400 nm cannot be seen
and is called ultraviolet radiation (the wavelengths of X-rays and gamma rays are even
shorter than those of ultraviolet). Infrared and visible radiation impinging on matter
mainly serve to warm it up. But, ultraviolet photons are sufficiently energetic that they
can “excite” the valence electrons of molecules to higher levels. This can result in effects
such as splitting molecules apart and cause chemical reactions to occur. Such reactions
are photochemical reactions. They are responsible for effects such as the formation
and destruction of ozone in the stratosphere and the formation of noxious photochemical
smog at ground level.
Exposed to the energetic ultraviolet radiation from the sun, we would quickly perish
because of its adverse effects on flesh. Fortunately, the atmosphere above us, thin though
it is, absorbs the most damaging ultraviolet radiation, although a little bit gets through,
and it is a good idea to minimize direct exposure to sunlight.
Direct and Indirect Solar Energy
From the discussion above, it is seen that a lot of energy comes from the sun. Most
of it is absorbed by the atmosphere, but a significant fraction reaches Earth’s surface
directly. We certainly use that energy because it keeps us and other living organisms
warm enough to sustain life. Photovoltaic cells that convert solar energy directly to
electricity, enable use of solar energy as a power source.
Living organisms use solar energy. Chlorophyll in plants capture the energy of
photons of visible light and use it to perform photosynthesis,
hν
6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2 (6.2.4)in which carbon dioxide from the atmosphere is combined with water to produce glucose
carbohydrate,C 6 H 12 O 6 , and molecular oxygen. The glucose is converted by plants and
other organisms into cellulose and other biomolecules making up biomass. Whereas
there is no usable energy to be obtained from the carbon dioxide and water reactants
of the photosynthesis reaction, there is a lot of chemical energy to be had from the
glucose and the biomolecules made from it. Aerobic respiration, essentially the reverse
of photosynthesis,
Chap. 6. Energy Relationships 139