100 PART 1^ |^ EXPLORING THE SKY
thickness of a sheet of household plastic wrap. It is awkward to
describe such short distances in millimeters, so scientists give the
wavelength of light using nanometer (nm) units, equal to one-
billionth of a meter (10−9 m). Another unit that astronomers
commonly use is called the angstrom (Å) (named after the
Swedish astronomer Anders Jonas Ångström). One angstrom is
10 −10 m, one-tenth of a nanometer. Th e wavelength of visible
light ranges from about 400 to 700 nm (4000 to 7000 Å). Just
as you sense the wavelength of sound as pitch, you sense the
wavelength of light as color. Light near the short-wavelength end
of the visible spectrum (400 nm) looks violet to your eyes, and
light near the long-wavelength end (700 nm) looks red.
Figure 6-3 shows that the visible spectrum makes up only a
small part of the entire electromagnetic spectrum. Beyond the
red end of the visible spectrum lies infrared radiation, where
wavelengths range from 700 nm to about 1 mm. Your eyes are
not sensitive to this radiation, but your skin senses it as heat.
A “heat lamp” warms you by giving off infrared radiation.
Beyond the infrared part of the electromagnetic spectrum lie
radio waves. Th e radio radiation used for AM radio transmissions
has wavelengths of a few kilometers down to a few hundred
meters, while FM, television, military, government, cell phone,
and ham radio transmissions have wavelengths that range down
to a few centimeters. Microwave transmission, used for radar and
some long-distance telephone communications as well as for
cooking food in a microwave oven, has wavelengths from a few
centimeters down to about 1 mm.
Th e boundaries between the sections of the spectrum are not
sharply defi ned. Long-wavelength infrared radiation blends
smoothly into the shortest microwave radio waves. Similarly,
there is no natural division between the short-wavelength infra-
red and the long-wavelength part of the visible spectrum.
Now look at the other end of the electromagnetic spectrum
in Figure 6-3 and notice that electromagnetic waves shorter than
violet are called ultraviolet. Electromagnetic waves that are even
shorter are called X-rays, and the shortest are gamma rays. Again,
the boundaries between these wavelength ranges are defi ned only
by conventional usage, not by natural divisions.
Recall the formula for the energy of a photon. Extremely
short-wavelength photons such as X-rays and gamma rays have
high energies and can be dangerous. Even ultraviolet photons have
enough energy to do you harm. Small doses of ultraviolet produce
a suntan, and larger doses cause sunburn and skin cancers.
Contrast this to the lower-energy infrared photons. Individually
they have too little energy to aff ect skin pigment, a fact that
explains why you can’t get a tan from a heat lamp. Only by con-
centrating many low-energy photons in a small area, as in a micro-
wave oven, can you transfer signifi cant amounts of energy.
Astronomers are interested in electromagnetic radiation
from space because it carries clues to the nature of stars, planets,
and other celestial objects. Earth’s atmosphere is opaque to most
electromagnetic radiation, as shown in the graph at the bottomlight does not require a medium, and so it can travel through a
perfect vacuum. Th ere is no sound on the moon, but there is
plenty of sunlight. Th ere is a Common Misconception that
radio waves are related to sound. Actually, radio waves are a type
of light, electromagnetic radiation that your radio converts into
sound so you can listen. Radio communication works just fi ne
between astronauts standing on the moon.
Although electromagnetic radiation can behave as a wave, it
can also behave as a fl ood of particles. A particle of electromag-
netic radiation is called a photon, and you can think of a photon
as a bundle of waves.
Th e amount of energy a photon carries depends inversely on
its wavelength. Th at is, shorter-wavelength photons carry more
energy, and longer-wavelength photons carry less. A simple for-
mula expresses this relationship:
E hc__
Here h is Planck’s constant ( 6.63 × 10−34 joule s), c is the speed of
light (3.00 × 10^8 m/s), and is the wavelength in meters. Th is book
will not use this formula for calculations; the important point is the
inverse relationship between the energy E and the wavelength .
As gets smaller, E gets larger. A photon of long wavelength carries
a very small amount of energy, but a photon with a very short
wavelength can carry much more energy.
The Electromagnetic Spectrum
A spectrum is an array of electromagnetic radiation displayed in
order of wavelength. You are most familiar with the spectrum of
visible light, which you see in rainbows. Th e colors of the visible
spectrum diff er in wavelength, with red having the longest wave-
length and violet the shortest. Th e visible spectrum is shown at
the top of ■ Figure 6-3.
Th e average wavelength of visible light is about 0.0005 mm.
You could put roughly 50 light waves end to end across the
Motion at the speed of lightWavelength1, 2, 3, 4, 5...■ Figure 6-2
All electromagnetic waves travel at the speed of light. The wavelength is the
distance between successive peaks. The frequency of the wave is the number
of peaks that pass you in one second.