The Electromagnetic Spectrum
When the various forms of radiant
energy are placed along a scale of
wavelengths, the resulting continuum
is called the electromagnetic spectrum.
Although each form of radiant
energy differs from its neighbors by an
extremely small amount, it is useful
to divide this spectrum into the generalized
categories shown in Figure
All radiations are believed to be
the result of electromagnetic oscillations.
In the case of radio waves, the
wavelengths are extremely long, being
on the order of 1010 nm, and are the
result of long electrical oscillations.
The fact that such energy permeates
our environment can easily be demonstrated
by turning on a radio or
television receiver in any part of the
technologically developed world. This
form of radiant energy is not believed
to have any direct effect upon the human body. Radio waves are customarily
characterized by their frequency,
expressed in hertz (cycles per second).
The portion of the electromagnetic
spectrum that we sense as heat
is called the infrared region. The origin
of this type of radiant energy, which
is shorter in wavelength than radio
waves, is believed to be the excitation
of electrons by thermal disturbance.
When these electrons absorb energy
from their surroundings, they are
placed in an elevated state of activity.
When they suddenly return to their
normal state, electromagnetic radiation
is given off. It has been shown that any
object at a temperature greater than
_273º C will give off this type of radiation.
Thus, all the objects we come into
contact with give off some infrared
energy. In general, the hotter an object,
the more total energy it produces and
the shorter the peak wavelength.
If an object is heated to a high
enough temperature, the wavelength
of the energy emitted will become
short enough to stimulate the retina
of the human eye and cause the sensation
of vision. It is this region of
the electromagnetic spectrum that is
termed light. Notice that it occupies
only a narrow section of the spectrum
between approximately 380 and
720 nm. Because the sensitivity of the
human visual system is so low at these
limits, 400 and 700 nm are generally
considered to be more realistic values.
Objects with very high temperatures
produce ultraviolet energy, which is
shorter than 400 nm in wavelength.
To produce radiant energy shorter
in wavelength than about 10 nm
requires that fast-moving electrons
bombard the object. When these rapidly
moving electrons strike the object, the
sudden stopping produces extremely
short wave energy called X-radiation, or
more commonly, X-rays. Still-shorter
wavelengths can be produced if the electron
bombardment intensifi es, as occurs
in a cyclotron. In addition, when radioactive
material decomposes, it emits
energy shorter in wavelength than
X-rays. In these two cases, the energy
is referred to as gamma rays, which
are usually 0.000001 nm (10_6 nm) in
wavelength and shorter. These forms
of electromagnetic energy are the most
energetic, penetrating radiation known.
Thus it can be seen that the wave
theory of radiant energy provides a
most useful system for classifying all
the known forms of radiation.
by
Basic
Photographic
Materials
and Processes
THIRD EDITION
Nanette Salvaggio
Dr. Leslie Stroebel and
Dr. Richard Zakia, Editors
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