Geology Reference
In-Depth Information
techniques and scientific principles governing the meth-
odologies used must be learnt and practiced first at home
before going to the field. It is also hoped that practices
and approaches and the results described in this chapter
(and elsewhere in this topic) can provide some intuition
to remote sensing modelers of sea ice.
radiation used presently in active and passive microwave
remote sensing systems are rather large and easily com-
prehensible. The wavelengths of 38-75 mm for C‐band
microwaves in the frequency range of 4-8 GHz (giga-
hertz) used in SAR systems (section 7.2) are certainly
graspable. The wavelengths involved in passive microwave
remote sensing systems range from about 8 to 17 mm
for the frequency range of 18-40 GHz in the designated
K/K
a
‐band (Table 7.3).
Figure 6.1a shows a simple sinusoidal wave with ampli-
tude of
AC
(from trough to crest) traveling from the left
to the right. In such a wave, the distance between two
successive troughs (
AA
′) or crests troughs (
CC
′), or a
corresponding distance, such as
BB
′, is called a wave-
length. Two points are in the “same phase” when they are
in the same relative position and moving in the same
direction, such as
A
and
A
′,
B
and
B
′, or
C
and
C
′ in
Figure 6.1a. The term
phase
is, therefore, meant for the
relative position of any two points on the wave and is one
of the most important parameter of an EM wave.
For successive waves, the phase angle (also called
phase shift) between two waveforms of the same fre-
quency is the angular difference between their starting
points. Figure 6.1b illustrates three waves with one at
phase shift ¼
λ
(quarter‐wave or
π
/2 radian) and another
½
λ
(half‐wave or
π
radian) behind the first one. Two
waveforms are said to be in phase when they have the
same frequency and there is no phase difference between
them. Two waveforms are said to be out of phase when
they have the same frequency and there is some amount
of phase shift between them.
Polarization is another characteristic of EM waves,
not only pertinent to the optical frequencies or wave-
lengths used in optical techniques for revealing the
microstructure of polycrystalline ice but also extremely
important for microwave remote sensing purposes. It is
defined as the plane of the electrical field in an electro-
magnetic wave. Only a cursory description will be made
here as a refresher because this subject is covered in any
physics textbook, for example,
Serway
[1990, Chapter 38,
Section 38.6]. Slightly detailed presentation on this topic
6.1. Relevant Optical pROpeRties
6.1.1. Polarized Light
Light is an EM wave in the visible range from violet to
red with wavelengths (universally denoted by the Greek
letter
λ
) from about 400 to 800 nm (nanometer is 10
−9
meter) and frequencies (denoted by the Greek letter
ν
) in
the range of about 750-380 THz (terahertz for 10
12
Hz).
The frequency is the number of vibrations executed in a
unit time (one second). The wavelength is, therefore,
defined as the ratio (
c
/
ν
) of the speed
c
of the wave and its
frequency. Light with one frequency is called “mono-
chromatic.” Within the visible range, however, the sensi-
tivity of the human eye varies in a measurable way. The
sensitivity of daylight‐adapted “normal” healthy human
eye is highest around the wavelength of about 550 nm
(green). Depending on the dilation of the pupil or level
of dark adaptation, the sensitivity of human eyes shifts
toward the red.
In vacuum, the speed of light is 2.99792458 × 10
8
m/s.
Speed of light or any EM wave depends, however, on the
medium through which the wave is propagating. It is also
dependent on wavelength and hence frequency, except
for vacuum. Thus a monochromatic light propagating in
vacuum can indeed be described not only by a single
frequency but also by a single wavelength. But, the wave-
length of the same monochromatic wave varies depending
on the medium. Due to the medium‐dependency of wave-
length, it is appropriate to identify EM waves only by
their frequency.
The wavelengths of EM waves in the visible range, as
mentioned above, are extremely small and incomprehen-
sible to think about or perceive, but the wavelengths of
(a)
(b)
λ
/2
λ
=
c
/
υ
λ
=
c
/
υ
C
C
C
C
′
Monochromatic
wave of
frequncy
ν
321
B
B
A
A
A
A
′
λ
/2
λ
/4
Figure 6.1 (a,b)
(a) One wave and (b) three waves, numbered 1-3, of the same monochromatic light with
wavelength
λ
but different phases:
λ
/4 between 1 and 2 and
λ
/2 between 1 and 3.
