about 22 pounds (10 kilograms). A large
male can weigh more than 44 pounds
(20 kilograms).
Raccoons have a pattern of dark fur
around their eyes that looks like a mask.
They also have bushy tails that have
dark bands, or rings. Their bodies are
covered with shaggy, coarse fur that is
iron-gray to blackish in color. They are
nocturnal omnivores, which means they
are active at night and eat both plants
and animals. Raccoons eat crabs, frogs,
fishes, rodents, fruit, and plants, including
crops. In cities they tend to eat garbage
or anything else they can find.
Raccoons live high in hollow trees or at
the tops of buildings in cities. In captivity
raccoons can live up to 20 years, but
most do not live more than 5 years in
the wild. Many are killed by humans for
their fur or because they are considered
pests.
#More to explore
Mammal
Radar
Radar is a system that uses waves of
energy to sense objects. It can find a
faraway object and tell how fast it is
moving. Radar is very useful because it
can sense objects even at night and
through thick clouds.
How RadarWorks
A radar system is made up of a transmitter,
a receiver, an antenna, and a display.
The transmitter creates radio waves,
which are invisible streams of energy.
The antenna sends the waves out into
the air. When the waves hit an object,
they reflect, or bounce back, to the
antenna.
The waves then pass to the receiver,
which makes sense of them. By measuring
how long it takes the waves to
return, the receiver can tell how far away
the object is. The receiver can also tell
where a moving object is headed and at
what speed. Sometimes it can tell how
big the object is, too. The receiver sends
this information to a display, or screen,
for people to see.
Shaggy fur keeps a raccoon warm in the
winter.
The word
radar is short
for radio
detection and
ranging.
BRITANNICA STUDENT ENCYCLOPEDIA Radar 15
travel through empty space, like in outer
space. They can also travel through air
and even solid substances, such as glass.
Through empty space, the waves travel
at a speed of about 186,282 miles per
second (299,792 kilometers per second).
Like all waves, electromagnetic waves
have peaks and valleys. The distance
from the peak of one wave to the peak
of the next wave is called a wavelength.
Different kinds of electromagnetic waves
have different wavelengths.
From longest to shortest wavelength,
the types of electromagnetic radiation
are radio waves, infrared rays, visible
light, ultraviolet rays, X-rays, and
gamma rays. Radio waves are used for
sending signals, including television and
radio broadcasts. Microwaves are short
radio waves that can be used to cook
food in microwave ovens. Infrared
radiation, or heat, is given off by warm
objects. Infrared radiation from the sun
heats Earth. Visible light is all the
wavelengths of electromagnetic
radiation that the human eye can see.
Different wavelengths of visible light
are seen as different colors. Ultraviolet
radiation is used by the skin to make a
nutrient called vitamin D. X-rays are
used to make a picture of something
inside an object, such as bones inside
the body. Gamma rays are used in
medicine to kill cancer cells.
The shorter the wavelength, the more
energy the radiation carries. The types
with the most energy—ultraviolet radiation,
X-rays, and gamma rays—can be
harmful to the body.
Mechanical Radiation
Unlike electromagnetic radiation,
mechanical radiation cannot travel
through empty space. It can travel only
through a substance, such as air, water,
or solid objects. A common example of
mechanical radiation is sound. Sound is
produced by the back-and-forth motion
of a vibrating object. This motion produces
sound waves that travel away from
the object. Sound travels more slowly
than light does. This is why during a
storm, the sound of thunder follows a
flash of lightning. An earthquake produces
another form of mechanical radiation.
The shock waves created by an
earthquake travel through Earth and
cause the ground to shake.
Nuclear Radiation
Nuclear radiation comes from the
nucleus, or center, of atoms. It is produced
when the nucleus changes in
some way. The nucleus of some types of
By sending
sound waves
through the
body, doctors
can check on
the health of a
growing baby
inside its
mother.
Electromagnetic radiation travels in waves.
A wavelength is the distance from the peak
of one wave to the peak of the next wave.
Short wavelengths have a shorter distance
between peaks than long wavelengths.
BRITANNICA STUDENT ENCYCLOPEDIA Radiation 17
atoms is unstable. An unstable nucleus
can naturally change into another type
of nucleus. When it does this, it shoots
off nuclear radiation in the form of
small particles or gamma rays. Atoms
that give off radiation in this way are
called radioactive. Nuclear radiation
carries a lot of energy, so it can be dangerous.
Cosmic Rays
Cosmic rays are a type of radiation made
up of tiny parts of atoms that speed
through outer space. Cosmic rays can
come from the sun, but most come from
farther away in the galaxy. Some cosmic
rays travel almost at the speed of light.
Because they move so fast, they have
