Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

REVIEW OF PHYSICAL PRINCIPLES 37
The relationship between the peak magnetic and electric field intensities H0 and
ε0 depends on the magnetic permeability μ and the electrical permittivity ∈ of the
medium through which the electromagnetic wave is propagating. This relationship
is given by
√ √
H0 μ = ε0 ∈, (2.55)
where ∈ is the permittivity of the medium. Permittivity is a measure of the capacity
for storing energy in a medium that is in an electric field. The permittivity of free
space is ∈0= 8.85 × 10 −12 C 2 /N · m 2 , and the permittivity of any other medium is
the product of the relative permittivity, k∈ and the permittivity of free space, ∈0:
∈= k∈× ∈0. The greater the value of ∈, the greater is its interaction with the ε field
and the greater is its ability to store energy. Permittivity is frequency dependent and
generally decreases with increasing frequency. If the wave is traveling through free
space, then
√ √
H0 μ0 = ε0 ∈0. (2.56)
Radio waves, microwaves (radar), infrared radiation, visible light, ultraviolet light,
and X-rays are all electromagnetic radiations. They are qualitatively alike but differ
in wavelength to form a continuous electromagnetic spectrum.
All these radiations are transmitted through the atmosphere (which may be considered,
for this purpose, as free space) at a speed very close to 3 × 10 8 m/s. Since
the speed of all electromagnetic waves in free space is a constant, Eq. (2.47), when
applied to electromagnetic waves in free space, becomes
c = 3 × 10 8 m/s = f × λ. (2.57)
Specifying either the frequency or wavelength of an electromagnetic wave in free
space is equivalent to specifying both. Free-space wavelengths may range from 5 ×
10 6 m for 60-Hz electric waves through visible light (green light has a wavelength of
about 500 nanometers, or nm, and a frequency of 6 × 10 14 Hz) to short-wavelength
X- and gamma radiation (whose wavelengths are on the order of 10 nm or less).
There is no sharp cutoff in wavelength at either end of the spectrum nor is there a
sharp dividing line between the various portions of the electromagnetic spectrum.
Each portion blends into the next, and the lines of demarcation, shown in Figure
2-17, are arbitrarily placed to show the approximate wavelength span of the regions
of the electromagnetic spectrum.
Figure 2-17. The electromagnetic spectrum.