Sun & its Energy
energy is the primary source of energy for all surface phenomena and life
on Earth. Combined with the material of the Earth (including the molecules
held close by the Earth's gravitational force called the atmosphere),
this energy provides for the immense diversity of life forms that are
found on the Earth. We will now look in detail at solar energy and its
interplay with the constituents of the Earth's atmosphere.
of the Sun
courtesy of NASA.
is a medium, yellow star, consisting primarily of hydrogen at temperatures
high enough to cause nuclear fusion. Nuclear
is a nuclear reaction in which hydrogen nuclei fuse together to form helium
nuclei and release energy. In this state, some 120 million tons of matter--mostly
hydrogen--are converted into helium on the sun every minute, with some
of the mass being converted into energy. The size of the sun determines
its temperature and the amount of energy radiated.
energy from the sun comes to Earth in the form of radiation. The term
"radiation" simply denotes the fact that the energy travels as rays, that
is, in straight lines. In general, the terms "solar energy" and "solar
radiation" simply refer to energy from the sun. Electromagnetic energy
is produced when electric charges change their potential energy. It is
characterized by the property that it is pure energy, not requiring any
matter (or medium) for its existence or movement. Electromagnetic energy
can therefore travel through space (which is a vacuum), traveling at a
speed that is the same for all forms of electromagnetic energy and is
equal to the speed of light, 3 x 108 m/sec (or 186,000 miles
radiates energy equally in all directions, and the Earth intercepts and
receives part of this energy. The power flux reaching the top of the Earth's
atmosphere is about 1400 Watts/m2. This measure simply means
that on the average, one square meter on the side of the Earth facing
the sun receives energy from the sun equal to that from fourteen 100 Watt
light bulbs every second!
is in a relatively stable state, and as far as we can tell, will continue
to be so for about another three billion years. The sun and other stars
do show periods of slightly higher than normal activity, detectable in
our sun by an increase in sunspot activity. During sunspot activity, more
energy reaches the Earth. The sun spends about a quarter of its time in
a state with very few sunspots. It is suspected that the sun dimmed about
ten times in the last 100,000 years causing "Little Ice Ages" (extended
periods of unusually cold temperatures) of about a couple of centuries
each. The last such quiescent state occurred in the late seventeenth century.
The sun has also shone with considerable above-average brightness at least
twice in our geological era: about 5,000 years ago, around the time of
the beginning of the ancient civilizations of China, Minoa, Sumeria, and
the Indus Valley; and about 1,000 years ago, when the temperatures of
Northern England rose high enough to allow vineyards to flourish there.
Spectrum - Basic Science
region of electromagnetic energy distinguished by wavelength and frequency
is called the electromagnetic spectrum. The propagation of the energy
along the rays is in the form of a wave
with the amount of energy alternating between high and low values, as
in a water wave. Thus we say that light, heat, etc., travel in the form
of waves. Wavelength
can be defined as the distance between two successive peaks (or troughs)
in waves of energy, while frequency is measured by counting the number
of peaks that pass a given point every second.
In the diagrams
of the spectra in this section, we use two different scales in measuring
wavelengths. The first is microns or micrometers (µm), which is
equal to 10-6 meters. The other is nanometers (nm), equal to
10-9 meters. In discussing small ranges of the spectrum, we
use units of nm, and in discussing the overall spectrum or larger regions,
we revert to µm.
is measured in units of cycles per second, or hertz (Hz). One cycle per
second is equal to one hertz.
of decreasing frequency (and increasing wavelength), the various regions
of the electromagnetic spectrum are: gamma
Electromagnetic energy from the sun consists mostly of a small amount
of ultraviolet, all visible light, and some infrared.
electromagnetic spectrum is depicted in Figure 2. Table 1 gives the same
information, as well as some technological applications.
Range (in m, µm,
Role in Nature
to 10-10 m
10-8 to 10-4 µm
10-5 to 10-1 nm
to 3x1018 Hz
to 10-8 m
10-8 to 10-2 µm
10-5 to 10 nm
to 3x1016 Hz
diagnosis (lower frequencies)
to 4x10-7 m
10-2 to 0.4 µm
10-5 to 400 nm
to 0.75x1016 Hz
Promotes production of
Vitamin D in human skin;
photosynthesis in plants
to 8x10-7 m
to 0.8 µm
400 to 800 nm
to 0.375x1016 Hz
(Eyes respond to this range)
to 10-3 m
800 to 106 nm
to 3x1011 Hz
to 106 m
to 300 Hz
Regions of the entire electromagnetic spectrum and general applications.
that the regions are not strictly delineated.
specialized sensory organs that only detect some parts of the spectrum.
For example, the eye detects visible light, and even distinguishes different
wavelengths within the spectrum of visible light as color! The skin perceives
radiation from the infrared region of the spectrum as heat. Note that
sound is not a form of electromagnetic energy. Because sound is really
the energy of the motion of molecules through a medium (mechanical energy),
it cannot travel through a vacuum. As we already noted, electromagnetic
energy has no need for a medium through which to travel, and can therefore
travel through space from the sun to reach the Earth.
molecules absorb different regions of electromagnetic energy preferentially.
For example, the water molecule preferentially absorbs certain wavelengths
in the microwave region of the electromagnetic spectrum. This preference
is the basis of the efficient cooking of food by microwave ovens. Calcium,
a primary constituent of bones, absorbs energy in the x-ray region more
strongly than do the water or carbon in the cells of ordinary tissue,
allowing for the use of x-rays to generate images that show unevenness
such as broken bones or tumors. The chlorophyll molecule in green plants
absorbs mostly ultraviolet (and also some blue violet, and red light)
and uses this energy for photosynthesis. Most of the green light in sunlight
is reflected by leaves, making them appear green to our eyes.
range of electromagnetic energy emitted by the sun is known as the solar
spectrum, and lies mainly in three regions: ultraviolet, visible, and
infrared. The solar spectrum extends from about 0.29 µm (or 290
nm) in the longer wavelengths of the ultraviolet region, to over 3.2 µm
(3,200 nm) in the far infrared. Small amounts of radio waves are also
given off by the sun and other stars. In fact, if the sun's image is made
from its radio waves, it appears 10% larger than if its image is made
from visible light. There are some "cooler" stars that give off mostly
radio waves and no visible radiation.
range of energy given off by a star depends upon the temperature and size
of the star. Smaller, hotter stars (called "white dwarfs") give
off more energy in the blue region and appear "whiter" than
our yellow sun. Rigel, a star in the constellation Sirius, is a white
dwarf. Larger, cooler stars, called "red giants," emit more
light in the red region, and are exemplified by Antares and Betelgeuse.
Note that even a "cool" star still has a temperature of a million
degrees or so.
the sun does emit ultraviolet radiation, the majority of solar energy
comes in the form of "light" and "heat," in the visible and infrared regions
of the electromagnetic spectrum. As shown in Table 2, visible light spans
the relatively narrow range of 0.4 to 0.9 µm (or 400 to 700 nm).
Light is special to humans and many other animals due to the evolution
of the eye, a sensory organ that detects this part of the solar spectrum.
As noted earlier, our eyes even recognize parts of the visible light spectrum
as the sensations of color. Thus 400 nm radiation is perceived by the
eye as violet, and 600 nm radiation is perceived as red.
are all familiar with the rainbow of colors--the range of different wavelengths
that make up sunlight. The best way to visualize this concept, and the
most common scientific demonstration, is the image of a glass prism splitting
up white light into the colors. When raindrops act as prisms, we see a
rainbow. Often, when the sun is bright, various transparent objects such
as beveled edges of glass windows or glass pieces of a chandelier transmit
light as a spectrum. This phenomenon occurs because different wavelengths
of light (or different colors) travel through glass at different speeds,
causing them to bend at different angles. Figure 3 shows the spectrum
(violet, blue, green, yellow, orange, and red) going from the shortest
wavelengths (highest frequency) to the longest wavelengths (lowest frequency).
On either side of the visible spectrum are the ultraviolet (shorter wavelength
than violet) and infrared (longer wavelength than red). These wavelengths
are mostly absorbed by the glass and are, of course, outside the range
of wavelengths that our vision can detect.
eye effectively perceives and distinguishes visible light, infrared (wavelengths
longer than red) is perceived as heat when it is absorbed by the skin
and converted into energy of the molecules of the skin. Infrared plays
an important role in the temperature of the Earth and its atmosphere,
and in turn, the climate of the Earth. We will discuss this role in more
detail in the section pertaining to the interaction solar energy with
now discuss how much energy is available in the different wavelength regions
of the solar spectrum.
Distribution in the Solar Spectrum
energy can be discussed in terms of its energy distribution, or the spread
of energy over a range of wavelengths. This distribution of energy is
also known as the spectral distribution. The measure of radiation may
be quantified in terms of the amount of energy falling per second (measured
in Watts) per unit area (in square meters, m2) in each band
of 1 µm wavelength.
provides a broad range of energy, primarily concentrated around the visible
and infrared regions. This energy is an important feature of the background
conditions that led to the evolution of our life forms on Earth, and continue
to support this life. There is a small amount of high-energy radiation
like x-rays in the sun's energy but these do not penetrate below the topmost
layer of the atmosphere, and we do not consider them here.
In the ultraviolet
region of the solar spectrum around 0.28 µm wavelength, there is
less than 100 W/m2 in a 1 µm band of radiation. In a
1 µm band around the red wavelength of 0.6 µm, however, there
is over 2,000 W/m2. From 0.75 µm or so, there is infrared
radiation ranging from about 1,000 W/m2/µm at 0.8 µm
to about 100 W/m2/µm at 2.2 µm. This relatively
low level of energy persists far into the infrared region.
distribution (or range of energies) of the solar radiation that falls
on top of the Earth's atmosphere is represented in Figure 4. As this spectral
distribution is close to what the sun emits, we can say that this is the
sun's emission spectrum. The x-axis (or horizontal axis) represents the
range of wavelengths in the solar spectrum (measured in nanometers), while
the y-axis (or vertical axis) represents the amount of power (Watts) in
each micron-wide band of wavelength falling on each square meter just
outside of the Earth's atmosphere (measured in units of Watts/meter2/µm).
This figure shows that most of the energy coming from the sun is in the
visible region of the electromagnetic spectrum, making up what we call
sunlight (white light).
4: Solar spectral distribution entering
the lower parts of the atmosphere.
and Absorption Spectra - Basic Science
falls on a surface, it can either be reflected, transmitted, absorbed,
or varying degrees of all three. Different colored surfaces appear different
to the eye because of differences in the way they reflect and absorb light.
Stars are sources of radiation, giving off their own energy. Their color
appears to us through the light they emit. So, a bluish star gives off
more blue light than a yellow star like the sun. To see non-luminous objects,
we need light from some other source to fall on them, and the reflected
light reaches our eye. The colors of non-luminous objects are thus dependent
on what wavelengths of energy they reflect and what wavelengths they absorb.
consists of the full spectrum of colors. If white light falls on a "perfectly"
white surface, all of the light is reflected -causing all colors to reach
the eye - and the reflecting surface is perceived as white. On the other
hand, the perception of black is the absence of any color reaching the
eye, meaning that all light is absorbed. In any case, the incident amount
of energy (I), or the amount of energy falling on a particular
surface, is equal to the sum of the amount reflected (r) and the
amount absorbed (a).
following figures show schematically what happens when white light falls
on a perfectly white surface, on a perfectly black surface, and on a green
surface. In each case, Part A of the figure represents what happens when
a ray or beam of white light falls on the surface. Part B of all the diagrams
shows the spectrum of incident radiation and the spectrum of reflected
radiation, with the x-axis representing wavelength and the y-axis representing
energy intensity. The third part of each diagram set, labeled C, shows
what is known as the absorption spectrum, showing what wavelengths are
absorbed. Note that in part C, while the x-axis still represents wavelength,
the y-axis is now a measure of absorption and not energy. For simplicity
we assume that all light not reflected is absorbed, although some might
be transmitted. So what we label "absorption spectrum" below
is actually an "absorption + transmission spectrum."
In the case of the white surface, almost all incident light is reflected.
Thus the incident spectrum (I) and the reflected spectrum (r) are the
same. Because none of the light is absorbed, the absorption spectrum (a)
may be shown as a flat line close to zero.