Biophotonics_Concepts_to_Applications

l¼Qd ð 2 : 41 Þ

A molecule that has a permanent dipole moment is called apolar molecule.
Examples of polar molecules are water (H 2 O), ammonia (NH 3 ), hydrogen chloride
(HCl), and the amino acids arginine, lysine, and tyrosine. Symmetric nonpolar
molecules such as oxygen, nitrogen, carbon dioxide (CO 2 ), methane (CH 4 ),
ammonium (NH 4 ), carbon tetrachloride (CCl 4 ), and the amino acids glycine and
tryptophan have no permanent dipole moments.
The interaction of the oscillating electricfield of a lightwave and the dipole
moment of a molecule is a key effect that can help describe light-tissue interactions.
In addition, an energy exchange between two oscillating dipoles is used in
fluorescence microscopy. When an external electricfield interacts with either polar
or nonpolar molecules, thefield can distort the electron distribution around the
molecule. In both types of molecules this action generates a temporary induced
dipole momentμindthat is proportional to the electricfield E. This induced dipole
moment is given by

lind¼aE ð 2 : 42 Þ

where the parameterαis called thepolarizabilityof the molecule. Thus when a
molecule is subjected to the oscillating electricfield of a lightwave, the total dipole
momentμTis given by

lT¼lþlind¼QdþaE ð 2 : 43 Þ

As is shown in Fig.2.13, the resultant electricfield Edipoleat a distance r from
the dipole in any direction has a magnitude

Edipole=k

Qd

r^3

ð 2 : 44 Þ

where k = 1/4πε 0 = 9.0× 109 Nm^2 /C^2.

d

-Q

+Q

Distance r from dipole

Dipole

moment μ

Edipole = k Qd

r^3

Fig. 2.13 A dipole is defined
as two charges +Q and−Q
separated by a distance d

2.7 Lightwave-Molecular Dipole Interaction 47