Electromagnetic waves can travel in
a vacuum.
Energy that moves in the form of EM waves is called radiation
(RAY-dee-AY-shuhn). Radiation is different from the transfer of energy
through a medium by a mechanical wave. A mechanical wave must
vibrate the medium as it moves, and this uses some of the wave’s energy.
Eventually, every mechanical wave will give up all of its energy to the
medium and disappear. An EM wave can travel without a material
medium—that is, in a vacuum or space empty of matter—and does
not lose energy as it moves. In theory, an EM wave can travel forever.
EM waves are also called
rays. The words radiation
and radiate come from the
Latin word radius, which
means “ray“ or “spoke
of a wheel.“
How EM Waves Travel in a Vacuum
Because they do not need a medium, EM waves can pass through outer
space, which is a near vacuum. Also, because they do not give up energy
in traveling, EM waves can cross the great distances that separate stars
and galaxies. For example, rays from the Sun travel about 150 million
kilometers (93 million mi) to reach Earth. Rays from the most distant
galaxies travel for billions of years before reaching Earth.
Usually, EM waves spread outward in all directions from the
source of the disturbance. The waves then travel until something
interferes with them. The farther the waves move from their source,
the more they spread out. As they spread out, there are fewer waves in
a given area and less energy is transferred. Only a very small part of
the energy radiated from the Sun is transferred to Earth. But that
energy is still a great amount—enough to sustain life on the planet.
The Speed of EM Waves in a Vacuum
In a vacuum, EM waves travel at a constant speed, and they travel very
fast—about 300,000 kilometers (186,000 mi) per second. In 1 second,
an EM wave can travel a distance greater than 7 times the distance
around Earth. Even at this speed, rays from the Sun take about 8 minutes to reach Earth. This constant speed is called the
speed of light. The vast distances of space are
often measured in units of time traveled at this
speed. For example, the Sun is about 8 lightminutes away from Earth. The galaxy
shown in the photograph is 60 million
light-years from Earth.
The light and other EM
waves from this galaxy
took approximately 60
million years to reach
Earth.
How are EM waves used to measure distances in space?
Chapter 15: Electromagnetic Waves 507