7.22 Light Absorption by Biological Matter
247
Gustav Mie studied elastic light scattering from spherical particles whose
size is larger than the wavelength of the light. 35 In this case, there is little or no
frequency dependence of the scattering intensity. Scattering of sunlight from
water droplets in clouds is a good example, as is scattering of light by large
smoke particles.
Raman considered what could be learned about materials from the inelastic
scattering of light. The loss or gain of energy comes from the available
mechanisms for energy transfer in the material, which in turn depends on
molecular structure.
Brillouin scattering of light arises from the charge density variations in
a material, such as would occur if an acoustical wave were present, or
temperature variations caused changes in density.
All of these kinds of scattering are used by biochemists to study organic
molecules.
15. Electron spin flip: A photon (which carries a unit of angular momentum) can
cause a valence electron spin to flip, i.e. the electron’s magnetic moment vector
is changed from being aligned with an external magnetic field to being antialigned with that field. The Pauli exclusion principle impedes this flip if there
is a second electron already occupying the first electron’s molecular orbital, but
with opposite spin direction. If a flip occurs, there may be an accompanying
change in the molecular bonding. When an electron in a 1 T magnetic field
is forced to flip spin by absorbing a photon, that light will have a microwave
frequency.
16. Nuclear spin flip: Like electrons, protons have an intrinsic spin, as do some
nuclei. Associated with their spin S is a magnetic dipole field of strength μ. For
a proton, μ = (|e| /(mc))S. If the proton or other nucleus is put in a magnetic
field, either from local fields of other nuclei and electrons, or from an external
magnetic field, the ‘north’ direction of the nucleus will tend to align with the
magnetic field. Like electrons, protons can only have two measurable directions
of their spin: Either aligned or anti-aligned with the direction of a magnetic field
(so the spin projection is either +(1/2) ¯
h or −(1/2) ¯
h). However, some nuclei
have greater spins, allowing a range of projections from s ¯
h to −s ¯
h. When a
dipole is twisted away from the direction of an external magnetic field B, it
requires an energy (μ · B). For the proton, the change in the energy will be
2 μB. If the change is caused by a photon absorption, then hf = 2 μB so
the electromagnetic radiation being absorbed will have a frequency of f =
2 μB/ h = (eB/(2πmc)) . With magnetic field strengths in the tesla range, a
proton spin-flip will radiate in the radio region of the electromagnetic spectrum.
17. Dispersion, Diffraction, and Interference: As light passes through transparent
materials, the variation of the index of refraction cause colors to separate,
35 In the first decade of the 1900s, Mie studied the solution to Maxwell’s equations when a light
wave interacts with a spherical dielectric ball.
247
Gustav Mie studied elastic light scattering from spherical particles whose
size is larger than the wavelength of the light. 35 In this case, there is little or no
frequency dependence of the scattering intensity. Scattering of sunlight from
water droplets in clouds is a good example, as is scattering of light by large
smoke particles.
Raman considered what could be learned about materials from the inelastic
scattering of light. The loss or gain of energy comes from the available
mechanisms for energy transfer in the material, which in turn depends on
molecular structure.
Brillouin scattering of light arises from the charge density variations in
a material, such as would occur if an acoustical wave were present, or
temperature variations caused changes in density.
All of these kinds of scattering are used by biochemists to study organic
molecules.
15. Electron spin flip: A photon (which carries a unit of angular momentum) can
cause a valence electron spin to flip, i.e. the electron’s magnetic moment vector
is changed from being aligned with an external magnetic field to being antialigned with that field. The Pauli exclusion principle impedes this flip if there
is a second electron already occupying the first electron’s molecular orbital, but
with opposite spin direction. If a flip occurs, there may be an accompanying
change in the molecular bonding. When an electron in a 1 T magnetic field
is forced to flip spin by absorbing a photon, that light will have a microwave
frequency.
16. Nuclear spin flip: Like electrons, protons have an intrinsic spin, as do some
nuclei. Associated with their spin S is a magnetic dipole field of strength μ. For
a proton, μ = (|e| /(mc))S. If the proton or other nucleus is put in a magnetic
field, either from local fields of other nuclei and electrons, or from an external
magnetic field, the ‘north’ direction of the nucleus will tend to align with the
magnetic field. Like electrons, protons can only have two measurable directions
of their spin: Either aligned or anti-aligned with the direction of a magnetic field
(so the spin projection is either +(1/2) ¯
h or −(1/2) ¯
h). However, some nuclei
have greater spins, allowing a range of projections from s ¯
h to −s ¯
h. When a
dipole is twisted away from the direction of an external magnetic field B, it
requires an energy (μ · B). For the proton, the change in the energy will be
2 μB. If the change is caused by a photon absorption, then hf = 2 μB so
the electromagnetic radiation being absorbed will have a frequency of f =
2 μB/ h = (eB/(2πmc)) . With magnetic field strengths in the tesla range, a
proton spin-flip will radiate in the radio region of the electromagnetic spectrum.
17. Dispersion, Diffraction, and Interference: As light passes through transparent
materials, the variation of the index of refraction cause colors to separate,
35 In the first decade of the 1900s, Mie studied the solution to Maxwell’s equations when a light
wave interacts with a spherical dielectric ball.