January 10, 2010 / Vol. 8, No. 1 / CHINESE OPTICS LETTERS 111
Light propagation and reflection-refraction event in
absorbing media
Jianqi Shen (!!!ïïïjjj)
∗
, Haitao Yu (uuu°°°777), and Jindeng Lu (©©©???)
College of Science, University of Shanghai for Science and Technology, Shanghai 200093, China
∗
E-mail: shenjq@online.sh.cn
Received February 12, 2009
The light propagation within an absorbing medium and the reflection and refraction at the interface of two
absorbing media are studied. By using the unit vectors denoting the planes of constant field amplitude and
constant phase respectively, the light propagation and attenuation are described by the effective refractive
indices which depend on both the complex refractive index of the medium and the angle between the unit
vectors. With the expression for the light propagation, the corresponding Snell’s law and the expression
of Fresnel coefficients are obtained, which can be applied to describe the reflection-refraction event at the
interface b etween an arbitrary combination of transparent and absorbing media.
OCIS co des: 260.2110, 120.5700, 120.5710, 080.1510.
doi: 10.3788/COL20100801.0111.
Geometrical optics approximation (GOA) of light scat-
tering by large particles with the size much larger than
the wavelength of the incident light is considered as a
good method by avoiding difficulties in Lorenz-Mie cal-
culation. Studies on the GOA of light scattering have
been of interest to many researchers during the past sev-
eral decades
[1−12]
. Most of the publications treat the
light scattering by transparent particles. As far as we
know, the GOA calculations of light scattering by ab-
sorbing particles are rare due to the difficulties in calcu-
lating the propagation of light in the absorbing medium
and its Fresnel coefficients at the interface of the me-
dia. This problem was recently investigated by Yang
et al.
[13,14]
by introducing the effective refractive in-
dex, and they presented the formulae for calculating the
Fresnel coefficients at the interface of the transparent-
absorbing media, which was based on the assumption of
the plane of constant amplitude being parallel to the in-
terface.
The assumption of Yang et al. is reasonable for the
reflection-refraction event while the incident light hits the
absorbing particle suspended in air, as p = 0 in Fig. 1.
However, for the high order reflection-refraction events
(i.e., p = 1,2,···) in which the light within the particle
is reflected and refracted at the interface, the plane of
constant amplitude is usually unparallel to the interface.
Besides, if one would calculate the coated particles con-
taining absorbing core and shell, he has to account for
the interface between absorbing-absorbing media.
In this letter, we study the light propagation in an ab-
sorbing medium and the reflection-refraction event of the
light at the interface of absorbing-absorbing media. The
media are assumed to be uniform and isotropic. The pa-
rameters such as the permeability ε, the permittivity µ,
and the conductivity σ are time-independent. The time
dependence of the light wave is expressed in the form exp
(iωt), where ω is the angular frequency and t is the time.
Substituting the time-harmonic wave (i.e., E (r, t) =
E (r) exp (iωt) and H (r, t) = H (r) exp (iωt)) into the
Maxwell equations (in SI units)
[15,16]
, we may obtain
½
∇ × E (r) = −iωµH (r)
∇ × H (r) = iωˆεE (r)
, (1)
½
∇
2
E (r) + k
2
0
ˆm
2
E (r) = 0
∇
2
H (r) + k
2
0
ˆm
2
H (r) = 0
, (2)
where ˆε = ε −i
σ
ω
is the equivalent complex permeability,
k
0
is the wave number in vacuum. The complex refrac-
tive index ˆm is defined by ˆm
2
= (ω/k
0
)
2
ˆεµ = c
2
ˆεµ (c
is the light speed in vacuum), whose real and imaginary
parts m
re
and m
im
are obtained as
ˆm = m
re
− i · m
im
,
m
re
= c
v
u
u
t
1
2
Ã
r
µ
2
ε
2
+
µ
2
σ
2
ω
2
+ µε
!
m
im
= c
v
u
u
t
1
2
Ã
r
µ
2
ε
2
+
µ
2
σ
2
ω
2
− µε
!
. (3)
Fig. 1. Schematic illustration of the reflection-refraction
events of the light in an absorbing sphere.
1671-7694/2010/010111-04
c
° 2010 Chinese Optics Letters