248
7 Light in Biology and Medicine
generating dispersion and rainbow effects. The separation of colors by a prism
demonstrates dispersion.
Light passing opaque objects whose size is comparable to the wavelength
of the light will produce diffraction patterns. The location of the diffraction
patterns will depend on the color of the light, and so color fringes will be
generated from white light by such structures. The iridescence of pearls is
due to diffraction from a fine-scale structures near the surface of the pearl.
Interference of light reflecting from closely spaced layers in a material can also
show colors. Oil slicks on water and the iridescence of bird feathers and insect
wings are further examples.
Far up in the electromagnetic spectrum, and therefore not affecting visible light
color, gamma rays will also be absorbed by materials in ways similar to atomic and
molecular absorption. Such absorption tends to be dominated by:
1. Nuclear rotations: The absorbed gamma ray may cause the nucleus to change its
rotational state.
2. Nuclear vibrations: The absorbed gamma ray may cause the nucleus to change
its vibrational state. With heavy nuclei, the protons may undergo collective
vibrational motion relative to the neutrons.
3. Nuclear excitations: An incoming gamma ray into a nucleus can cause individual
protons to be excited into unoccupied proton quantum states at higher energy than
their state before the disturbance. These resonant absorptions can be relatively
sharp in photon frequencies, due to the lifetime of the excited states. For example,
iron-57 ( 57 Fe) has an excited state at 14.4125 keV above its ground state, and this
level has an energy width of only 4.66 neV wide, corresponding to a lifetime of
141 ns.
4. Nuclear transformations: A gamma ray absorbed by a nucleus can cause some
nuclei to undergo a nuclear transformation. Nuclear fragments can be knocked
out (such as a proton or an alpha particle), or the gamma ray can induce a weak
interaction, changing the nuclear atomic number, and cause the emission of an
electron or positron together with an antineutrino or neutrino.
The nucleus of most atoms can be excited into rotational, vibrational, and
excitational modes by gamma ray energy photons. In the process, isolated nuclei
are kicked by the gamma ray. Rudolf Mössbauer discovered that if the nuclei are
anchored in a crystal, some gamma absorption will be effectively recoilless because
the whole crystal can take up the gamma ray’s momentum. The sharpness in energy
of the nuclear states lets us change the gamma absorption by a small motion of
the crystal, because the Doppler effect changes how the nuclei absorb the gammas.
Since the nuclear state energies are affected by the molecular electrons in s-states
(no angular-momentum states), the position of the gamma absorption frequencies
are correspondingly affected. Such measurements for organic molecules give
information about those molecules’ configurations. An application to biochemical
research will be given in Sect. 8.7.2.
7 Light in Biology and Medicine
generating dispersion and rainbow effects. The separation of colors by a prism
demonstrates dispersion.
Light passing opaque objects whose size is comparable to the wavelength
of the light will produce diffraction patterns. The location of the diffraction
patterns will depend on the color of the light, and so color fringes will be
generated from white light by such structures. The iridescence of pearls is
due to diffraction from a fine-scale structures near the surface of the pearl.
Interference of light reflecting from closely spaced layers in a material can also
show colors. Oil slicks on water and the iridescence of bird feathers and insect
wings are further examples.
Far up in the electromagnetic spectrum, and therefore not affecting visible light
color, gamma rays will also be absorbed by materials in ways similar to atomic and
molecular absorption. Such absorption tends to be dominated by:
1. Nuclear rotations: The absorbed gamma ray may cause the nucleus to change its
rotational state.
2. Nuclear vibrations: The absorbed gamma ray may cause the nucleus to change
its vibrational state. With heavy nuclei, the protons may undergo collective
vibrational motion relative to the neutrons.
3. Nuclear excitations: An incoming gamma ray into a nucleus can cause individual
protons to be excited into unoccupied proton quantum states at higher energy than
their state before the disturbance. These resonant absorptions can be relatively
sharp in photon frequencies, due to the lifetime of the excited states. For example,
iron-57 ( 57 Fe) has an excited state at 14.4125 keV above its ground state, and this
level has an energy width of only 4.66 neV wide, corresponding to a lifetime of
141 ns.
4. Nuclear transformations: A gamma ray absorbed by a nucleus can cause some
nuclei to undergo a nuclear transformation. Nuclear fragments can be knocked
out (such as a proton or an alpha particle), or the gamma ray can induce a weak
interaction, changing the nuclear atomic number, and cause the emission of an
electron or positron together with an antineutrino or neutrino.
The nucleus of most atoms can be excited into rotational, vibrational, and
excitational modes by gamma ray energy photons. In the process, isolated nuclei
are kicked by the gamma ray. Rudolf Mössbauer discovered that if the nuclei are
anchored in a crystal, some gamma absorption will be effectively recoilless because
the whole crystal can take up the gamma ray’s momentum. The sharpness in energy
of the nuclear states lets us change the gamma absorption by a small motion of
the crystal, because the Doppler effect changes how the nuclei absorb the gammas.
Since the nuclear state energies are affected by the molecular electrons in s-states
(no angular-momentum states), the position of the gamma absorption frequencies
are correspondingly affected. Such measurements for organic molecules give
information about those molecules’ configurations. An application to biochemical
research will be given in Sect. 8.7.2.