|1. What is
the electromagnetic spectrum?
The electromagnetic spectrum consists of all the different wavelengths
of electromagnetic radiation, including light, radio waves, and X-rays
(see chart at the bottom of this page). It is a continuum
of wavelengths from zero to infinity. We name regions of the spectrum
rather arbitrarily, but the names give us a general sense of the energy;
for example, ultraviolet light has shorter wavelengths than radio light.
The only region in the entire electromagnetic spectrum that our eyes are
sensitive to is the visible region.
- Gamma rays have the shortest wavelengths,
of less than 0.01 nanometers (about the size of an atomic nucleus).
This is the highest frequency and most energetic region of the electromagnetic
spectrum. Gamma rays can result from nuclear reactions taking place
in objects such as pulsars, quasars, and black holes.
- X-rays range in wavelength from 0.01 to
10 nanometers (about the size of an atom). They are generated, for example,
by super-heated gas from exploding stars and quasars, where temperatures
are near a million to ten million degrees.
- Ultraviolet radiation has wavelengths of
10 to 310 nanometers (about the size of a virus). Young, hot stars produce
a lot of ultraviolet light and bathe interstellar space with this energetic
- Visible light covers the range of wavelengths
from 400 to 700 nanometers (from the size of a molecule to a protozoan).
The Sun emits most of its radiation in the visible range, which our
eyes perceive as the colors of the rainbow. Our eyes are sensitive only
to this small portion of the electromagnetic spectrum.
- Infrared wavelengths span from 710 nanometers
to 1 millimeter (from the width of a pinpoint to the size of small plant
seeds). At a temperature of 37 degrees C, our bodies radiate with a
peak intensity near 900 nanometers.
- Radio waves are longer than 1 millimeter.
Since these are the longest waves, they have the lowest energy and are
associated with the lowest temperatures. Radio wavelengths are found
everywhere: in the background radiation of the universe, in interstellar
clouds, and in the cool remnants of supernova explosions, to name a
few. Radio stations use radio wavelengths of electromagnetic radiation
to send signals that our radios then translate into sound. These wavelengths
are typically a few feet long in the FM band and up to 300 yards or
more in the AM band. Radio stations transmit electromagnetic radiation,
not sound. The radio station encodes a pattern on the electromagnetic
radiation it transmits, and then our radios receive the electromagnetic
radiation, decode the pattern and translate the pattern into sound.
New instrumentation and computer techniques of the late 20th century
allow scientists to measure the universe in many regions of the electromagnetic
spectrum. We build devices that are sensitive to the light that our eyes
cannot see. Then, so that we can "see" these regions of the
electromagnetic spectrum, computer image-processing techniques assign
arbitrary color values to the light.
Light is a disturbance of electric and magnetic fields
that travels in the form of a wave. Imagine throwing a pebble into a still
pond and watching the circular ripples moving outward. Like those ripples,
each light wave has a series of high points known as crests, where the
electric field is highest, and a series of low points known as troughs,
where the electric field is lowest. The wavelength is the distance between
two wave crests, which is the same as the distance between two troughs.
The number of waves that pass through a given point in one second is called
the frequency, measured in units of cycles per second called Hertz. The
speed of the wave therefore equals the frequency times the wavelength.
|3. What is
the relationship between frequency and wavelength?
Wavelength and frequency of light are closely related.
The higher the frequency, the shorter the wavelength. Because all light
waves move through a vacuum at the same speed, the number of wave crests
passing by a given point in one second depends on the wavelength. That
number, also known as the frequency, will be larger for a short-wavelength
wave than for a long-wavelength wave. The equation that relates wavelength
and frequency is:
For electromagnetic radiation, the speed is equal to the speed of
light, c, and the equation becomes:
|4. What is
the relationship between wavelength, frequency, and energy?
The energy of a wave is directly proportional to its
frequency, but inversely proportional to its wavelength. In other words,
the greater the energy, the larger the frequency and the shorter (smaller)
the wavelength. Given the relationship between wavelength and frequency
described above, it follows that short wavelengths are more energetic
than long wavelengths.