Elastic and Inelastic Scattering of Light
KRS Murthy
What
is Intensity of Light?
I
have already discussed a “ray of light” as the most basic
electromagnetic form of light. A beam of light is made up of many rays
of light. The intensity of a beam of light depends and is
proportional to the number of rays of light passing through a unit
cross section. In other words. A beam of light will become less
intense as it moves forward, due to different kind of losses, and
more importantly dispersion that increases with the distance
travelled by the beam made up of the rays of light. The number of
rays of light in the beam in a unit cross section reduces, due to the
spreading of the beam, in addition to other losses due to scattering
by air or dust particles, and also absorption.
The
Medium
Depending
on the medium in which the beam of light is traversing, the index of
diffraction plays a role in how the different wavelengths, and
corresponding frequencies, of light. For example, in the visible
light part of the electromagnetic spectrum, the smaller wavelengths
(correspondingly higher frequencies) have relatively more energy. The
violet rays have higher energy than indigo, which in turn relatively
have higher energy than blue. Red has the least energy in the visible
light part of the spectrum. Based on the energy, and the index of
diffraction of the medium, the different colors of light travel at
different speeds, and thus get separated from each other, in the
temporal dimension.
When
the index of the medium is 1, the light has zero diffraction.
However, air also has index higher than 1, which depends on the
number and type of particles of matter per unit area or volume, and
the state of matter, in the air, lower with closer to unity and
higher proportionately higher in the refractive index. In addition to
the index, loss due to scattering and absorption. Keep in mind that
the scattering of light by a particle depends on the size and
composition of the particle, and also the angle of incident of light
on the particle. Scattering can happen in all directions in the three
dimensional space, with respect to the angle of incidence in the
three dimensional space.
Scattering
is Necessary in Nature
The
most vital and essential nature of light is scattering, that enables
us to see objects, near and far, and also distinguish the distances
of different objects in our field of view, and similar for animals.
If you see a beam of light coming through your windows and other
openings on an easy Sunday morning, thank the dust particles in the
air in your house. If you didn't know, most of the dust particles in
your house, even when practically sealed tightly from outside dust to
enter your house, are the dead cells on the skins of you body, of
your coinhabitants, and animals. Off course, the animals shed their
body hair also, as even the human beings, albeit to a lower extent.
Elastic
and Inelastic Scattering of Light
To
explain the difference between the elastic and inelastic scattering,
let us take the analogy of a rubber ball bounced on the a flat even
surface. When you bounce the ball it bounces back, even though with
loss due friction, and few other physical loss. If the incident
energy and momentum is same as the bouncing energy and momentum, it
is elastic bouncing; in the case of scattering it is elastic
scattering. If the returning energy is different, its is inelastic
scattering.
When
light is incident on a particle or a molecule, both elastic and
inelastic scattering could happen, depending on the size and shape of
the particle or molecule, current energy, vibrational, rotational and
a variety of other static and dynamic properties of the particle or
molecule.
Let
us discuss different types of elastic and inelastic light scattering
modes, first the general overview, followed by current understanding
of the physics, and my interpretation going to the root of the basics
and fundamentals of nature of light and its interaction with matter
to include the elementary particles, electric and magnetic field
interactions, and the way it affects at atomic and molecular levels.
It
is also important to note and remember my explanation of intensity of
incident light, compared to a single ray of light. Most of the
scattering discoveries and their interpretation, as well as their
engineering, technological, industrial and commercial applications
are all based on the physical effects in terms of intensity, and not
very much at individual ray of light detail.
My
interpretation delves and dives into the interaction at individual
ray level focused on interaction of individual light ray with
elementary particles like the electrons, which play a pivotal and
primary role in all interactions, effects, and behaviour in nature.
No comments:
Post a Comment