240
7 Light in Biology and Medicine
in the material, and on the concentration of a solute, the ‘specific rotary power’ is
the rotation angle α divided by the optical length and the solute molar concentration:
[α] = α/(L [c]).
The reason for the polarization is the following. Unpolarized light may be
thought of as a combination of the two possible linearly-polarized light components
or a combination of the two possible circularly polarized light components. An
electromagnetic wave passing along a helical molecule with some electric dipoles
aligned along the helix will lose energy in distorting those dipoles for that component of the wave whose electric field has the same sense of rotation through space
as the molecule. That component of the electromagnetic wave with the opposite
sense of rotation will lose less energy. For example, unpolarized light scattered by a
sugar solution will become polarized, as can be seen by passing the scattered light
through a quarter-wave plate, and then viewing with a linearly-polarizing sheet.
Sugars have right-handed chirality, as does DNA. Right-handed chiral molecules
are also called dextrorotary, while left-handed ones, such as most cellular proteins,
are called levorotary.
7.22 Light Absorption by Biological Matter
The only way to make electromagnetic waves is through the acceleration of charges
or by forcing changes in the direction of intrinsic magnetic fields of particles.
The only way to detect them is through their absorption by charges accelerated
by the wave or by the wave rotating particles with intrinsic magnetic fields. 32 For
each enumerated technique we have for generating a certain frequency range of
electromagnetic waves, there is a parallel technique for absorbing that wave. But
the technology for generation can widely differ from that for absorption.
The molecules in cells which are identified or selected as light absorbers are
called ‘chromophores’. Chromophores which are indigenous to a particular tissue
are called endogenic. Those injected or ingested are called exogenic. Figure 7.18
shows the absorption coefficient for important materials found in tissue.
Light is scattered and absorbed by biological materials through a number of
important mechanisms, all of which involve the acceleration of charges or intrinsic
spin flips associated with the incoming electric field oscillations and, to a much
lesser extent, the magnetic field oscillations. Generally, light changes on entering a
material because of one or more of the following processes:
Light scattering: The energy absorbed by charges in the material can be given off
immediately as scattered light, often divided into incoherent scattering (such
as Rayleigh scattering) and coherent scattering, (such as Mie scattering).
32 If we find that nature has intrinsic magnetic charge, then these kinds of charges also are able to
emit and absorb electromagnetic waves. So far, such ‘magnetic monopoles’ have not been found.
Electrons, protons, and neutrons all have magnetic dipole fields.
7 Light in Biology and Medicine
in the material, and on the concentration of a solute, the ‘specific rotary power’ is
the rotation angle α divided by the optical length and the solute molar concentration:
[α] = α/(L [c]).
The reason for the polarization is the following. Unpolarized light may be
thought of as a combination of the two possible linearly-polarized light components
or a combination of the two possible circularly polarized light components. An
electromagnetic wave passing along a helical molecule with some electric dipoles
aligned along the helix will lose energy in distorting those dipoles for that component of the wave whose electric field has the same sense of rotation through space
as the molecule. That component of the electromagnetic wave with the opposite
sense of rotation will lose less energy. For example, unpolarized light scattered by a
sugar solution will become polarized, as can be seen by passing the scattered light
through a quarter-wave plate, and then viewing with a linearly-polarizing sheet.
Sugars have right-handed chirality, as does DNA. Right-handed chiral molecules
are also called dextrorotary, while left-handed ones, such as most cellular proteins,
are called levorotary.
7.22 Light Absorption by Biological Matter
The only way to make electromagnetic waves is through the acceleration of charges
or by forcing changes in the direction of intrinsic magnetic fields of particles.
The only way to detect them is through their absorption by charges accelerated
by the wave or by the wave rotating particles with intrinsic magnetic fields. 32 For
each enumerated technique we have for generating a certain frequency range of
electromagnetic waves, there is a parallel technique for absorbing that wave. But
the technology for generation can widely differ from that for absorption.
The molecules in cells which are identified or selected as light absorbers are
called ‘chromophores’. Chromophores which are indigenous to a particular tissue
are called endogenic. Those injected or ingested are called exogenic. Figure 7.18
shows the absorption coefficient for important materials found in tissue.
Light is scattered and absorbed by biological materials through a number of
important mechanisms, all of which involve the acceleration of charges or intrinsic
spin flips associated with the incoming electric field oscillations and, to a much
lesser extent, the magnetic field oscillations. Generally, light changes on entering a
material because of one or more of the following processes:
Light scattering: The energy absorbed by charges in the material can be given off
immediately as scattered light, often divided into incoherent scattering (such
as Rayleigh scattering) and coherent scattering, (such as Mie scattering).
32 If we find that nature has intrinsic magnetic charge, then these kinds of charges also are able to
emit and absorb electromagnetic waves. So far, such ‘magnetic monopoles’ have not been found.
Electrons, protons, and neutrons all have magnetic dipole fields.