Chemistry Reference
In-Depth Information
Fig. 10.1
The
instantaneous electric,
E
y
and magnetic
H
x
field
strength vectors of a light
wave as a function of
position along the axis of
propagation (from Calvert
and Pitts [
82
])
Our concept of light is that it also consists of packets of energy that travel in waves. In each packet,
there is a range of energies. These energies cannot be represented by one wavelength, but rather by a
whole spectrum of wavelengths. The energy of each particular wavelength in the wave-packet is a
discrete unit, a
. Electromagnetic radiation is described in terms of a transverse plane wave
involving associated electric and magnetic fields. Experimental data suggest that the electric vector
E
and magnetic vector
H
which describe the respective field strengths are aligned in planes at right
angles to one another, with both planes perpendicular to the direction of propagation of the wave. This
was illustrated by Calvin and Pitts [
82
], as shown in Fig.
10.1
.
A convenient model for the variation of the field strength as a function of time
quantum
x
along the axis of propagation is one that can be described in Cartesian coordinates by the sinusoidal
functions in the following equations:
K
and distance
E
y
¼ A
sin 2
pðx=l vtÞ
1
=
2
H
z
¼ðemÞ
A
sin 2
pðx=l vtÞ
In these equations,
E
y
is the electric field strength vector lying in the
xy
plane and increasing along
the
y
-axis,
H
z
is the magnetic field strength vector lying in the
xz
-plane and increasing along the
z
-
2
),
axis,
A
is the amplitude of the electric vector (the
intensity
of the wave is proportional to
A
e
is the
dielectric constant, and
m
is the magnetic permeability of the medium through which the light wave is
e
m
transported. In a vacuum,
and they are approximately unity in air. The length of the wave, that
is, the distance between adjacent maxima in the vectors measured at any instant along the direction of
wave propagation (the
=
x
l
v
is the frequency or number of complete cycles of vector
position change per second. The relationship between
axis) is
, while
l
and
v
is:
C=v ¼ l
where
C
is the velocity of the radiation. The frequency
v
is independent of the medium through which
the radiation travels. Wavelength
of the medium.
Ordinary light is not polarized. It consists of many electromagnetic vectors that are undulating in
fixed, though randomly oriented with respect to each other, planes. When the light is polarized in a
plane, it is believed that all the waves have their electric vectors oriented in the same direction. When
the light is polarized elliptically, then it is believed that two plane waves of equal wavelength and
frequency and with identical directions of propagation have the electric vectors perpendicular to one
another and out of phase, as shown in Fig.
10.1
.
The degree of polarization of light
l
and velocity
C
, on the other hand, depend on
e
and
m
p
is usually expressed by the equation
p ¼
I
jj
I
?
