Geology Reference
In-Depth Information
Box 6.3 Light and other forms of electromagnetic radiation
the light that we see, like other forms of electromagnetic
radiation, is an electromagnetic disturbance that propa-
gates energy through space, rather like the radiating ripples
on the surface of a pond disturbed by a stone. the source
of light excites simultaneous 'ripples' in electric and mag-
netic field strength, which spread out from the source at the
speed of light.
the essential characteristics of any electromagnetic
wave are the
frequency
of vibration of the electromagnetic
field (
v
) in hertz (hz = oscillations per second = s
−1
) and its
wavelength
(
λ
) in metres (Box 5.2). these are complemen-
tary properties related through the equation:
Photon energy
E
=
h
υ
10
-15
eV
γ
-rays (nuclear)
10
6
10
20
10
-10
X-RAYS
10
3
ULTRA-VIOLET
VISIBLE
10
15
10
0
10
-5
INFRA-RED
10
10
MICROWAVES
10
0
λ
v
=
c
RADIO WAVES
10
5
where
c
is the speed of light in m s
−1
(
c
= 2.997 × 10
8
m s
−1
in vacuum). the wavelength is the parameter normally
used to characterize the quality of visible light that we call
colour see Figure 6.3.1. Frequency, however, is the more
fundamental property: unlike wavelength and
c
, it is inde-
pendent of the refractive index of the medium through
which the light is passing.
Light energy is
quantized
: a light beam, though appar-
ently a continuous stream of waves, actually consists of
minute packets or 'quanta' of wave energy called photons,
resembling the wave pulses associated with the electron
(Figure 5.1). planck showed at the turn of the century that
each photon has a kinetic energy
E
q
, related to the freq-
uency of the light of which it forms a part:
10
5
Figure 6.3.1
the electromagnetic spectrum.
equal to the
energy difference
Δ
E
between the electron's
initial and final states. It follows that the light emitted by
atoms undergoing this transition has a frequency given by:
v
=
Eh Eh
=
∆
q
/
/
the corresponding wavelength is
λ=
hc
∆
Since the energy levels (and
Δ
E
s) in an atom depend on
the nuclear charge Z, the wavelengths of atomic spectra
vary predictably from one element to the next, and can be
used (when separated by a spectrometer into constituent
wavelengths) to identify the elements present in a com-
plex sample without separating the elements chemically.
the
intensity
of each wavelength 'peak' in the spectrum
provides a measure of the concentration in the sample of
the element to which it relates (Box 6.4).
Eh
q
=
v
where
h
is called planck's constant and has the value
6.626 × 10
−34
J s.
When an electron falls from a high energy level in an
atom to a lower one, it emits a quantum of energy in the
form of an electromagnetic photon, whose energy is exactly
which Δ
l
would be = 0), between 3d and 2 s states (for
which Δ
l
= 2), or between 3p and 3 s (for which Δ
n
= 0).
be generated in the electron shells of atoms. (γ-rays
have higher energies, but are produced in nuclei.) The
high energies indicate that X-rays arise from electron
transitions involving the deepest, most tightly bound
energy levels in the atom, in particular the K and L
shells. The energy level structure in these shells is
simple (Figure 5.7) owing to the restrictions that
apply to the value of quantum number
l
when
n
is
X-ray spectra
X-rays are electromagnetic waves of very short wave-
length (about 10
-8
to 10
-11
m) and high frequency (Box 6.3.).
They are the most energetic form of radiation that can
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