| MadSci Network: Physics |
Hi,
I think you should read the light scattering... I made a summary about
this topic.
The Light scattering [1, 2, 3, 5]
Light is electromagnetic radiation in the frequency range from
approximately 1013 Hz (infrared) to 1017 Hz (ultraviolet) or the
wavelength range from 3 nm to 30,000 nm. The conversion between frequency
and wavelength of light can be made using the speed of light (c=3x108
m/sn in vacuo).
c=lambda.v (1)
Visible light is the part of the electromagnetic radiation to which the
human eye is sensitive. When white light, which contains a range of
wavelengths, is separated by wavelength, each wavelength is perceived to
correspond to a different color. As light propagates it has the
characteristic of both a transverse wave (a light wave) as well as a
particle (a photon). As a wave, light have wave-like properties such as
frequency, wavelength, and interference; as a particle, light have
particle-like properties such as momentum, velocity, and position. In
most of the discussions we discuss situations where the dimension of a
light beam is larger than that of the particulate material though the
sizes of the particles can be much smaller, as well as larger than the
wavelength of light. In these cases, descriptions from geometric optics
are no longer sufficient to describe the behavior of scattered light, and
more sophisticated theories have to be used. When a light beam
illuminates a piece of matter having a dielectric constant different from
unity, light will be absorbed or scattered, or both, depending on the
wavelength of light and the optical properties of the material. The net
result of the absorption and scattering caused by the material is known
as the extinction of incident light,
Extinction=Absorption+scattering (2)
When light interacts with the electrons bound in the material so as to re-
radiate light, scattering is observed. The detected scattering is from
particle(s) in a scattering volume, the cross section between the beam
and the detection cone. The absorbed light energy that becomes the
excitation energy of particles will be dissipated mostly through thermal
degradation (i. e., converted to heat) or lost through a radiative decay
producing fluorescence or phosphorescence depending on the electronic
structure of the material. Because many materials exhibit strong
absorption in the infrared and ultraviolet regions, which greatly reduces
scattering intensity, most light scattering measurements are performed
using visible light. Scattering is only observed when a material is in
itself heterogeneous, either due to local density fluctuations in the
pure material or due to the optical heterogeneity for dispersed particles
in a medium. In a perfectly homogeneous and isotropic material the
radiation scattered by individual molecules interferes destructively, and
so no scattering is observed.
Scattering intensity from a unit volume of material that is illuminated
by a unit flux of light is a function of the complex refractive index
ratio between the material and its surrounding medium along with various
other properties of the material. In the regime of Rayleigh scattering,
where particle dimension is much smaller than the wavelength of light,
the scattering intensity, is inversely proportional to the fourth power
of the wavelength when the same material is illuminated by light of
different wavelengths but having the same intensity; i. e., the shorter
the wavelength, the stronger the scattering:
Is~Io/(lambda^4) (3)
In fact, this wavelength dependence on scattering power was the first
observed scattering phenomenon in nature. Attempts to explain natural
scattering phenomena can be traced back to early in the eleventh century
and an Arabic physicist Ibn-al-Haitham, known as Alhazen of Basra,
through the Italian Renaissance and the famous Italian painter,
architect, musician, and engineer Leonardo Da Vinci, to Sir Issac, Newton
in the seventeenth century. The first systematic studies of scattering
effects and the development of explanations took place in the 1860’ s by
JohnTyndall who analyzed suspensions and aerosols. It was perhaps when
John Strutt (Lord Rayleigh) observed and studied scattering from natural
phenomena in the 1870’ s , that a solid foundation of light scattering,
as a branch of the science of the nature of light, was laid. Scattering
theories were further developed by the grand masters of physics such as
Ludvig Lorenz, Gustav Mie , Peter Debye, and Albert Einstein around the
turn of the century along with the establishment of Maxwell’s
electromagnetic theory. During the twentieth century, light scattering
theories, involving time-averaged scattering intensity and intensity
fluctuation, were further developed. With the establishment of quantum
mechanics, light scattering became a mature field of science enabling the
numerical computation of scattering problems and experimental
applications in studies of liquid and macromolecule solutions. In the
past few decades the use of light scattering as a tool in many branches
of material studies has flourished and has penetrated into different
fields of sciences both theoretically and experimentally. To a large
extent, this is associated with the development and commercialization of
several new technologies, especially the laser and microelectronic
devices, including the computer. As an example, the photon correlation
experiment was pioneered in the 1950’s, yet the popularity of this
technology was made possible only after coherent light sources from
lasers became available at low cost in the 1970’ s. Resolving the
distribution of Brownian motion in a macromolecule solution or a particle
suspension by photon correlation spectroscopy was practical only when
microelectronics the computation became fast enough so that a user did
not have to wait hours for the result. Also, using the T-matrix or
discrete dipole moment methods to compute scattering patterns from
irregularly shaped particles was not feasible until the clock-time of a
computer exceeded a few hundred megahertz. Used as a general term, “light
scattering” can be encountered in several branches of the physical
sciences, such as optics, physical chemistry, material science, fluid
dynamics and solid physics. There are many ways to use light scattering
phenomena to study various properties of materials. Each has its own
terminology, expertise, applications, and techniques of measurement, and
each involves different disciplines in the physical sciences. In the
measurement of scattering intensity fluctuations as a function of time,
there is transient light
scattering, dynamic light scattering and diffusing wave light scattering.
There is static light scattering and turbidity measurements based on time-
averaged scattering intensity, and there is phase Doppler analysis for
phase analysis of scattered light. For measurements performed under an
additional applied field, there is electrophoretic light scattering,
electric field light scattering, and forced Rayleigh scattering, etc. For
measurement of optically anisotropic material, there is circular
dichroism light scattering. For measurements other than in liquid and air
there is surface Raman scattering, surface evanescent scattering and
solid scattering, etc. Depending on whether the frequency of scattered
light to be detected is the same as that of the incident light, a light
scattering experiment may be described as elastic (ELS), quasi-elastic
(QELS), or as inelastic light scattering (IELS). In ELS, the scattering
signal to be detected is the time-averaged light intensity and thus its
frequency deviation from the incident light is often not measured. The
intensity of scattered light is a function of the optical properties of
the particles and the medium, the dimension and mass of the particles,
sample concentration, and observation angle and distance. Information
regarding particles is obtained through their static status in an ELS
measurement. In QELS, the frequency of scattered light to be detected is
slightly different from that of the incident light, typically in the
range of a few to a few hundred. The frequency difference comes from the
translational and rotational motions of the particles and its value is
directly related to the particles' motions. QELS is often employed in
studying the motions of particles and other information regarding the
particles may also be revealed through their motions. In IELS, the
scattered frequency differs by an amount much larger than a few hundred
from that of the incident light due to the involvement of other forms of
energy, such as the vibrational and rotational energy of scatterers in
Raman scattering and photon-phonon interaction in Brillouin scattering.
IELS scattering signals are extremely weak for scatterers of large mass
when compared to signals from ELS or QELS and thus not many applications
have been found in particle characterization. IELS is often used in the
structural study of molecules and liquids.
References
[1] Xu Renliang, Particle characterization: Light scattering Methods,
(Hingham, MA, USA: Kluwer academic publishers), 2000
[2] Michael F. Drenski and et al., Simultaneous multiple sample light
scattering for analysis of polymer solutions, Journal of Applied Polymer
Science, Vol.92, 2724-2732 (2004)
[3] Brown, W., Ed. Light Scattering, Principles and Development; Oxford
Science: Oxford, 1996
[4] Berne and Pecora, ‘Dynamic Light scattering’ John Wiley: New York,
1975
[5] Van de Hulst, H.C. Light scattering by Small Particles; Wiley: New
York, 1957
Best Wishes,
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