3 Theoretical Models of Light Scattering and Absorption
43
When light enters a medium other than a vacuum, it slows down. The frequency
of the oscillation is maintained, but the wavelength changes. This is quantified by
the traveling wave equation:
v = f λ
(3.5)
where v represents velocity, f represents frequency, and λ represents wavelength.
When light encounters a molecule, it can either be scattered or absorbed. The
origin of the scatter is the charged particles within each atom. These particles vibrate
in time with the electric vector of the incident light (electromagnetic radiation).
Because electrons are orders of magnitude lighter than the nuclei, the electrons
vibrate far more vigorously than do the nuclei and are the main source of scattered
light. In “elastic” scatter, the accelerating (vibrating) charges emit radiation that has
the same wavelength as the incident light, but it is emitted in all directions. There
are multiple charged particles, and therefore multiple sources of scattering, within a
single atom or molecule. The emissions from the various charged particles interfere
with each other and give rise to a scattering pattern that is dependent upon the relative
placement of the vibrating charges within the molecule. (There is also such thing as
“inelastic scatter,” in which the wavelength of the emitted radiation is not the same
as that of the incident radiation. In the field of vibrational spectroscopy, inelastic
scattering is primarily encountered in Raman spectroscopy [4]).
Turning to absorption, when light transfers energy to a material, it does so in
discrete quantities of energy called “photons.” A photon is sometimes envisioned
as a “particle” of light, but this is a departure from the wave model of light we are
describing here. We picture a beam of light as a wave that contains a certain amount
of radiant energy, which can be sub-divided into a certain number of photons. The
amount of energy in one photon is given by:
E = hf
(3.6)
Where E is the energy, h is Plank’s constant, and f is the frequency. When SI units
are used throughout, E is expressed in Joules, f is expressed in hertz, and h = 6.63
× 10
–34 J·s.
3.4 Reflection and Refraction of Light at a Surface
In this section, we consider the case of a beam of light encountering a surface. When
light is traveling through a medium, such as air or glass, it travels in a straight line.
The speed of light is not uniform; it varies based upon the refractive index of the
medium in which it is traveling. When light encounters a glass window, the surfaces
of the glass are locations at which the light makes the transition from one medium
to another (air to glass and glass to air) and the refractive index changes.
43
When light enters a medium other than a vacuum, it slows down. The frequency
of the oscillation is maintained, but the wavelength changes. This is quantified by
the traveling wave equation:
v = f λ
(3.5)
where v represents velocity, f represents frequency, and λ represents wavelength.
When light encounters a molecule, it can either be scattered or absorbed. The
origin of the scatter is the charged particles within each atom. These particles vibrate
in time with the electric vector of the incident light (electromagnetic radiation).
Because electrons are orders of magnitude lighter than the nuclei, the electrons
vibrate far more vigorously than do the nuclei and are the main source of scattered
light. In “elastic” scatter, the accelerating (vibrating) charges emit radiation that has
the same wavelength as the incident light, but it is emitted in all directions. There
are multiple charged particles, and therefore multiple sources of scattering, within a
single atom or molecule. The emissions from the various charged particles interfere
with each other and give rise to a scattering pattern that is dependent upon the relative
placement of the vibrating charges within the molecule. (There is also such thing as
“inelastic scatter,” in which the wavelength of the emitted radiation is not the same
as that of the incident radiation. In the field of vibrational spectroscopy, inelastic
scattering is primarily encountered in Raman spectroscopy [4]).
Turning to absorption, when light transfers energy to a material, it does so in
discrete quantities of energy called “photons.” A photon is sometimes envisioned
as a “particle” of light, but this is a departure from the wave model of light we are
describing here. We picture a beam of light as a wave that contains a certain amount
of radiant energy, which can be sub-divided into a certain number of photons. The
amount of energy in one photon is given by:
E = hf
(3.6)
Where E is the energy, h is Plank’s constant, and f is the frequency. When SI units
are used throughout, E is expressed in Joules, f is expressed in hertz, and h = 6.63
× 10
–34 J·s.
3.4 Reflection and Refraction of Light at a Surface
In this section, we consider the case of a beam of light encountering a surface. When
light is traveling through a medium, such as air or glass, it travels in a straight line.
The speed of light is not uniform; it varies based upon the refractive index of the
medium in which it is traveling. When light encounters a glass window, the surfaces
of the glass are locations at which the light makes the transition from one medium
to another (air to glass and glass to air) and the refractive index changes.