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230
the instrument. Calculations are normally performed by the instrument
software (but manual calculations are possible).
Light scattering is caused by interactions between light (electric component
only) and electrons in the molecule or particle being observed. The amplitude
of the electric component of the incident light E is given by:
E = E
0
cos 2
πν
t
Where
ν
is the frequency (
ν
= c
λ
) and t is time. The molecules are thus
subjected to an oscillating electric field. The molecules are polarizible since
the electron cloud is influenced (polarized) as illustrated (highly simplified and
exaggerated) below:
An oscillating dipole
45
(frequency
ν
) and a corresponding oscillating dipole
moment (
µ
) are introduced. The dipole moment is given by:
µ
=
α
E
where
α
is the polarizability of the molecule. An oscillating dipole is a source
of electromagnetic radiation, and light (of the same wavelength) is emitted in
all directions:
45
Permanent dipoles are common in organic molecules because different atoms have
different electronegativities. Example: CH
3
F has a permanent
δ
- at the F atom and a
δ
+ at the
CH
3
.
Particle/
macromolecul
e
θ
Incident light
(I
0
)
Scattered
light (i
θ
)